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![Mauro Carvalho Chehab](/assets/img/avatar_default.png)
Link: https://lore.kernel.org/r/6b2ddb36983e81e7028de6e5fd0c643c2fb4c6c9.1583136624.git.mchehab+huawei@kernel.org Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org> Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
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ReStructuredText
673 lines
29 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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====================
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The SCSI Tape Driver
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====================
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This file contains brief information about the SCSI tape driver.
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The driver is currently maintained by Kai Mäkisara (email
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Kai.Makisara@kolumbus.fi)
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Last modified: Tue Feb 9 21:54:16 2016 by kai.makisara
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Basics
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======
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The driver is generic, i.e., it does not contain any code tailored
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to any specific tape drive. The tape parameters can be specified with
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one of the following three methods:
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1. Each user can specify the tape parameters he/she wants to use
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directly with ioctls. This is administratively a very simple and
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flexible method and applicable to single-user workstations. However,
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in a multiuser environment the next user finds the tape parameters in
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state the previous user left them.
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2. The system manager (root) can define default values for some tape
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parameters, like block size and density using the MTSETDRVBUFFER ioctl.
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These parameters can be programmed to come into effect either when a
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new tape is loaded into the drive or if writing begins at the
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beginning of the tape. The second method is applicable if the tape
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drive performs auto-detection of the tape format well (like some
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QIC-drives). The result is that any tape can be read, writing can be
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continued using existing format, and the default format is used if
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the tape is rewritten from the beginning (or a new tape is written
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for the first time). The first method is applicable if the drive
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does not perform auto-detection well enough and there is a single
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"sensible" mode for the device. An example is a DAT drive that is
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used only in variable block mode (I don't know if this is sensible
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or not :-).
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The user can override the parameters defined by the system
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manager. The changes persist until the defaults again come into
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effect.
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3. By default, up to four modes can be defined and selected using the minor
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number (bits 5 and 6). The number of modes can be changed by changing
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ST_NBR_MODE_BITS in st.h. Mode 0 corresponds to the defaults discussed
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above. Additional modes are dormant until they are defined by the
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system manager (root). When specification of a new mode is started,
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the configuration of mode 0 is used to provide a starting point for
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definition of the new mode.
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Using the modes allows the system manager to give the users choices
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over some of the buffering parameters not directly accessible to the
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users (buffered and asynchronous writes). The modes also allow choices
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between formats in multi-tape operations (the explicitly overridden
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parameters are reset when a new tape is loaded).
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If more than one mode is used, all modes should contain definitions
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for the same set of parameters.
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Many Unices contain internal tables that associate different modes to
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supported devices. The Linux SCSI tape driver does not contain such
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tables (and will not do that in future). Instead of that, a utility
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program can be made that fetches the inquiry data sent by the device,
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scans its database, and sets up the modes using the ioctls. Another
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alternative is to make a small script that uses mt to set the defaults
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tailored to the system.
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The driver supports fixed and variable block size (within buffer
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limits). Both the auto-rewind (minor equals device number) and
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non-rewind devices (minor is 128 + device number) are implemented.
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In variable block mode, the byte count in write() determines the size
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of the physical block on tape. When reading, the drive reads the next
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tape block and returns to the user the data if the read() byte count
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is at least the block size. Otherwise, error ENOMEM is returned.
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In fixed block mode, the data transfer between the drive and the
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driver is in multiples of the block size. The write() byte count must
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be a multiple of the block size. This is not required when reading but
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may be advisable for portability.
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Support is provided for changing the tape partition and partitioning
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of the tape with one or two partitions. By default support for
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partitioned tape is disabled for each driver and it can be enabled
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with the ioctl MTSETDRVBUFFER.
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By default the driver writes one filemark when the device is closed after
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writing and the last operation has been a write. Two filemarks can be
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optionally written. In both cases end of data is signified by
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returning zero bytes for two consecutive reads.
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Writing filemarks without the immediate bit set in the SCSI command block acts
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as a synchronization point, i.e., all remaining data form the drive buffers is
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written to tape before the command returns. This makes sure that write errors
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are caught at that point, but this takes time. In some applications, several
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consecutive files must be written fast. The MTWEOFI operation can be used to
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write the filemarks without flushing the drive buffer. Writing filemark at
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close() is always flushing the drive buffers. However, if the previous
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operation is MTWEOFI, close() does not write a filemark. This can be used if
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the program wants to close/open the tape device between files and wants to
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skip waiting.
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If rewind, offline, bsf, or seek is done and previous tape operation was
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write, a filemark is written before moving tape.
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The compile options are defined in the file linux/drivers/scsi/st_options.h.
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4. If the open option O_NONBLOCK is used, open succeeds even if the
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drive is not ready. If O_NONBLOCK is not used, the driver waits for
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the drive to become ready. If this does not happen in ST_BLOCK_SECONDS
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seconds, open fails with the errno value EIO. With O_NONBLOCK the
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device can be opened for writing even if there is a write protected
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tape in the drive (commands trying to write something return error if
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attempted).
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Minor Numbers
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=============
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The tape driver currently supports up to 2^17 drives if 4 modes for
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each drive are used.
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The minor numbers consist of the following bit fields::
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dev_upper non-rew mode dev-lower
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20 - 8 7 6 5 4 0
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The non-rewind bit is always bit 7 (the uppermost bit in the lowermost
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byte). The bits defining the mode are below the non-rewind bit. The
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remaining bits define the tape device number. This numbering is
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backward compatible with the numbering used when the minor number was
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only 8 bits wide.
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Sysfs Support
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=============
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The driver creates the directory /sys/class/scsi_tape and populates it with
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directories corresponding to the existing tape devices. There are autorewind
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and non-rewind entries for each mode. The names are stxy and nstxy, where x
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is the tape number and y a character corresponding to the mode (none, l, m,
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a). For example, the directories for the first tape device are (assuming four
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modes): st0 nst0 st0l nst0l st0m nst0m st0a nst0a.
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Each directory contains the entries: default_blksize default_compression
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default_density defined dev device driver. The file 'defined' contains 1
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if the mode is defined and zero if not defined. The files 'default_*' contain
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the defaults set by the user. The value -1 means the default is not set. The
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file 'dev' contains the device numbers corresponding to this device. The links
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'device' and 'driver' point to the SCSI device and driver entries.
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Each directory also contains the entry 'options' which shows the currently
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enabled driver and mode options. The value in the file is a bit mask where the
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bit definitions are the same as those used with MTSETDRVBUFFER in setting the
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options.
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A link named 'tape' is made from the SCSI device directory to the class
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directory corresponding to the mode 0 auto-rewind device (e.g., st0).
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Sysfs and Statistics for Tape Devices
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=====================================
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The st driver maintains statistics for tape drives inside the sysfs filesystem.
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The following method can be used to locate the statistics that are
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available (assuming that sysfs is mounted at /sys):
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1. Use opendir(3) on the directory /sys/class/scsi_tape
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2. Use readdir(3) to read the directory contents
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3. Use regcomp(3)/regexec(3) to match directory entries to the extended
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regular expression "^st[0-9]+$"
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4. Access the statistics from the /sys/class/scsi_tape/<match>/stats
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directory (where <match> is a directory entry from /sys/class/scsi_tape
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that matched the extended regular expression)
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The reason for using this approach is that all the character devices
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pointing to the same tape drive use the same statistics. That means
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that st0 would have the same statistics as nst0.
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The directory contains the following statistics files:
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1. in_flight
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- The number of I/Os currently outstanding to this device.
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2. io_ns
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- The amount of time spent waiting (in nanoseconds) for all I/O
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to complete (including read and write). This includes tape movement
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commands such as seeking between file or set marks and implicit tape
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movement such as when rewind on close tape devices are used.
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3. other_cnt
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- The number of I/Os issued to the tape drive other than read or
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write commands. The time taken to complete these commands uses the
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following calculation io_ms-read_ms-write_ms.
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4. read_byte_cnt
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- The number of bytes read from the tape drive.
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5. read_cnt
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- The number of read requests issued to the tape drive.
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6. read_ns
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- The amount of time (in nanoseconds) spent waiting for read
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requests to complete.
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7. write_byte_cnt
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- The number of bytes written to the tape drive.
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8. write_cnt
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- The number of write requests issued to the tape drive.
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9. write_ns
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- The amount of time (in nanoseconds) spent waiting for write
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requests to complete.
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10. resid_cnt
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- The number of times during a read or write we found
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the residual amount to be non-zero. This should mean that a program
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is issuing a read larger thean the block size on tape. For write
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not all data made it to tape.
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.. Note::
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The in_flight value is incremented when an I/O starts the I/O
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itself is not added to the statistics until it completes.
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The total of read_cnt, write_cnt, and other_cnt may not total to the same
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value as iodone_cnt at the device level. The tape statistics only count
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I/O issued via the st module.
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When read the statistics may not be temporally consistent while I/O is in
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progress. The individual values are read and written to atomically however
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when reading them back via sysfs they may be in the process of being
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updated when starting an I/O or when it is completed.
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The value shown in in_flight is incremented before any statstics are
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updated and decremented when an I/O completes after updating statistics.
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The value of in_flight is 0 when there are no I/Os outstanding that are
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issued by the st driver. Tape statistics do not take into account any
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I/O performed via the sg device.
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BSD and Sys V Semantics
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=======================
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The user can choose between these two behaviours of the tape driver by
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defining the value of the symbol ST_SYSV. The semantics differ when a
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file being read is closed. The BSD semantics leaves the tape where it
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currently is whereas the SYS V semantics moves the tape past the next
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filemark unless the filemark has just been crossed.
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The default is BSD semantics.
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Buffering
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=========
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The driver tries to do transfers directly to/from user space. If this
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is not possible, a driver buffer allocated at run-time is used. If
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direct i/o is not possible for the whole transfer, the driver buffer
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is used (i.e., bounce buffers for individual pages are not
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used). Direct i/o can be impossible because of several reasons, e.g.:
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- one or more pages are at addresses not reachable by the HBA
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- the number of pages in the transfer exceeds the number of
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scatter/gather segments permitted by the HBA
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- one or more pages can't be locked into memory (should not happen in
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any reasonable situation)
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The size of the driver buffers is always at least one tape block. In fixed
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block mode, the minimum buffer size is defined (in 1024 byte units) by
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ST_FIXED_BUFFER_BLOCKS. With small block size this allows buffering of
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several blocks and using one SCSI read or write to transfer all of the
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blocks. Buffering of data across write calls in fixed block mode is
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allowed if ST_BUFFER_WRITES is non-zero and direct i/o is not used.
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Buffer allocation uses chunks of memory having sizes 2^n * (page
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size). Because of this the actual buffer size may be larger than the
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minimum allowable buffer size.
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NOTE that if direct i/o is used, the small writes are not buffered. This may
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cause a surprise when moving from 2.4. There small writes (e.g., tar without
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-b option) may have had good throughput but this is not true any more with
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2.6. Direct i/o can be turned off to solve this problem but a better solution
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is to use bigger write() byte counts (e.g., tar -b 64).
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Asynchronous writing. Writing the buffer contents to the tape is
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started and the write call returns immediately. The status is checked
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at the next tape operation. Asynchronous writes are not done with
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direct i/o and not in fixed block mode.
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Buffered writes and asynchronous writes may in some rare cases cause
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problems in multivolume operations if there is not enough space on the
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tape after the early-warning mark to flush the driver buffer.
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Read ahead for fixed block mode (ST_READ_AHEAD). Filling the buffer is
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attempted even if the user does not want to get all of the data at
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this read command. Should be disabled for those drives that don't like
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a filemark to truncate a read request or that don't like backspacing.
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Scatter/gather buffers (buffers that consist of chunks non-contiguous
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in the physical memory) are used if contiguous buffers can't be
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allocated. To support all SCSI adapters (including those not
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supporting scatter/gather), buffer allocation is using the following
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three kinds of chunks:
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1. The initial segment that is used for all SCSI adapters including
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those not supporting scatter/gather. The size of this buffer will be
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(PAGE_SIZE << ST_FIRST_ORDER) bytes if the system can give a chunk of
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this size (and it is not larger than the buffer size specified by
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ST_BUFFER_BLOCKS). If this size is not available, the driver halves
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the size and tries again until the size of one page. The default
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settings in st_options.h make the driver to try to allocate all of the
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buffer as one chunk.
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2. The scatter/gather segments to fill the specified buffer size are
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allocated so that as many segments as possible are used but the number
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of segments does not exceed ST_FIRST_SG.
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3. The remaining segments between ST_MAX_SG (or the module parameter
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max_sg_segs) and the number of segments used in phases 1 and 2
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are used to extend the buffer at run-time if this is necessary. The
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number of scatter/gather segments allowed for the SCSI adapter is not
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exceeded if it is smaller than the maximum number of scatter/gather
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segments specified. If the maximum number allowed for the SCSI adapter
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is smaller than the number of segments used in phases 1 and 2,
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extending the buffer will always fail.
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EOM Behaviour When Writing
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==========================
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When the end of medium early warning is encountered, the current write
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is finished and the number of bytes is returned. The next write
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returns -1 and errno is set to ENOSPC. To enable writing a trailer,
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the next write is allowed to proceed and, if successful, the number of
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bytes is returned. After this, -1 and the number of bytes are
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alternately returned until the physical end of medium (or some other
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error) is encountered.
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Module Parameters
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=================
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The buffer size, write threshold, and the maximum number of allocated buffers
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are configurable when the driver is loaded as a module. The keywords are:
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========================== ===========================================
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buffer_kbs=xxx the buffer size for fixed block mode is set
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to xxx kilobytes
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write_threshold_kbs=xxx the write threshold in kilobytes set to xxx
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max_sg_segs=xxx the maximum number of scatter/gather
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segments
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try_direct_io=x try direct transfer between user buffer and
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tape drive if this is non-zero
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========================== ===========================================
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Note that if the buffer size is changed but the write threshold is not
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set, the write threshold is set to the new buffer size - 2 kB.
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Boot Time Configuration
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=======================
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If the driver is compiled into the kernel, the same parameters can be
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also set using, e.g., the LILO command line. The preferred syntax is
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to use the same keyword used when loading as module but prepended
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with 'st.'. For instance, to set the maximum number of scatter/gather
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segments, the parameter 'st.max_sg_segs=xx' should be used (xx is the
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number of scatter/gather segments).
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For compatibility, the old syntax from early 2.5 and 2.4 kernel
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versions is supported. The same keywords can be used as when loading
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the driver as module. If several parameters are set, the keyword-value
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pairs are separated with a comma (no spaces allowed). A colon can be
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used instead of the equal mark. The definition is prepended by the
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string st=. Here is an example::
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st=buffer_kbs:64,write_threshold_kbs:60
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The following syntax used by the old kernel versions is also supported::
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st=aa[,bb[,dd]]
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where:
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- aa is the buffer size for fixed block mode in 1024 byte units
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- bb is the write threshold in 1024 byte units
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- dd is the maximum number of scatter/gather segments
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IOCTLs
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======
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The tape is positioned and the drive parameters are set with ioctls
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defined in mtio.h The tape control program 'mt' uses these ioctls. Try
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to find an mt that supports all of the Linux SCSI tape ioctls and
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opens the device for writing if the tape contents will be modified
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(look for a package mt-st* from the Linux ftp sites; the GNU mt does
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not open for writing for, e.g., erase).
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The supported ioctls are:
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The following use the structure mtop:
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MTFSF
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Space forward over count filemarks. Tape positioned after filemark.
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MTFSFM
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As above but tape positioned before filemark.
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MTBSF
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Space backward over count filemarks. Tape positioned before
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filemark.
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MTBSFM
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As above but ape positioned after filemark.
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MTFSR
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Space forward over count records.
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MTBSR
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Space backward over count records.
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MTFSS
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Space forward over count setmarks.
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MTBSS
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Space backward over count setmarks.
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MTWEOF
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Write count filemarks.
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MTWEOFI
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Write count filemarks with immediate bit set (i.e., does not
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wait until data is on tape)
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MTWSM
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Write count setmarks.
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MTREW
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Rewind tape.
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MTOFFL
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Set device off line (often rewind plus eject).
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MTNOP
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Do nothing except flush the buffers.
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MTRETEN
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Re-tension tape.
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MTEOM
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Space to end of recorded data.
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MTERASE
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Erase tape. If the argument is zero, the short erase command
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is used. The long erase command is used with all other values
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of the argument.
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MTSEEK
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Seek to tape block count. Uses Tandberg-compatible seek (QFA)
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for SCSI-1 drives and SCSI-2 seek for SCSI-2 drives. The file and
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block numbers in the status are not valid after a seek.
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MTSETBLK
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Set the drive block size. Setting to zero sets the drive into
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variable block mode (if applicable).
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MTSETDENSITY
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Sets the drive density code to arg. See drive
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documentation for available codes.
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MTLOCK and MTUNLOCK
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Explicitly lock/unlock the tape drive door.
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MTLOAD and MTUNLOAD
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Explicitly load and unload the tape. If the
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command argument x is between MT_ST_HPLOADER_OFFSET + 1 and
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MT_ST_HPLOADER_OFFSET + 6, the number x is used sent to the
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drive with the command and it selects the tape slot to use of
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HP C1553A changer.
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MTCOMPRESSION
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Sets compressing or uncompressing drive mode using the
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SCSI mode page 15. Note that some drives other methods for
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control of compression. Some drives (like the Exabytes) use
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density codes for compression control. Some drives use another
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mode page but this page has not been implemented in the
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driver. Some drives without compression capability will accept
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any compression mode without error.
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MTSETPART
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Moves the tape to the partition given by the argument at the
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next tape operation. The block at which the tape is positioned
|
|
is the block where the tape was previously positioned in the
|
|
new active partition unless the next tape operation is
|
|
MTSEEK. In this case the tape is moved directly to the block
|
|
specified by MTSEEK. MTSETPART is inactive unless
|
|
MT_ST_CAN_PARTITIONS set.
|
|
MTMKPART
|
|
Formats the tape with one partition (argument zero) or two
|
|
partitions (argument non-zero). If the argument is positive,
|
|
it specifies the size of partition 1 in megabytes. For DDS
|
|
drives and several early drives this is the physically first
|
|
partition of the tape. If the argument is negative, its absolute
|
|
value specifies the size of partition 0 in megabytes. This is
|
|
the physically first partition of many later drives, like the
|
|
LTO drives from LTO-5 upwards. The drive has to support partitions
|
|
with size specified by the initiator. Inactive unless
|
|
MT_ST_CAN_PARTITIONS set.
|
|
MTSETDRVBUFFER
|
|
Is used for several purposes. The command is obtained from count
|
|
with mask MT_SET_OPTIONS, the low order bits are used as argument.
|
|
This command is only allowed for the superuser (root). The
|
|
subcommands are:
|
|
|
|
* 0
|
|
The drive buffer option is set to the argument. Zero means
|
|
no buffering.
|
|
* MT_ST_BOOLEANS
|
|
Sets the buffering options. The bits are the new states
|
|
(enabled/disabled) the following options (in the
|
|
parenthesis is specified whether the option is global or
|
|
can be specified differently for each mode):
|
|
|
|
MT_ST_BUFFER_WRITES
|
|
write buffering (mode)
|
|
MT_ST_ASYNC_WRITES
|
|
asynchronous writes (mode)
|
|
MT_ST_READ_AHEAD
|
|
read ahead (mode)
|
|
MT_ST_TWO_FM
|
|
writing of two filemarks (global)
|
|
MT_ST_FAST_EOM
|
|
using the SCSI spacing to EOD (global)
|
|
MT_ST_AUTO_LOCK
|
|
automatic locking of the drive door (global)
|
|
MT_ST_DEF_WRITES
|
|
the defaults are meant only for writes (mode)
|
|
MT_ST_CAN_BSR
|
|
backspacing over more than one records can
|
|
be used for repositioning the tape (global)
|
|
MT_ST_NO_BLKLIMS
|
|
the driver does not ask the block limits
|
|
from the drive (block size can be changed only to
|
|
variable) (global)
|
|
MT_ST_CAN_PARTITIONS
|
|
enables support for partitioned
|
|
tapes (global)
|
|
MT_ST_SCSI2LOGICAL
|
|
the logical block number is used in
|
|
the MTSEEK and MTIOCPOS for SCSI-2 drives instead of
|
|
the device dependent address. It is recommended to set
|
|
this flag unless there are tapes using the device
|
|
dependent (from the old times) (global)
|
|
MT_ST_SYSV
|
|
sets the SYSV semantics (mode)
|
|
MT_ST_NOWAIT
|
|
enables immediate mode (i.e., don't wait for
|
|
the command to finish) for some commands (e.g., rewind)
|
|
MT_ST_NOWAIT_EOF
|
|
enables immediate filemark mode (i.e. when
|
|
writing a filemark, don't wait for it to complete). Please
|
|
see the BASICS note about MTWEOFI with respect to the
|
|
possible dangers of writing immediate filemarks.
|
|
MT_ST_SILI
|
|
enables setting the SILI bit in SCSI commands when
|
|
reading in variable block mode to enhance performance when
|
|
reading blocks shorter than the byte count; set this only
|
|
if you are sure that the drive supports SILI and the HBA
|
|
correctly returns transfer residuals
|
|
MT_ST_DEBUGGING
|
|
debugging (global; debugging must be
|
|
compiled into the driver)
|
|
|
|
* MT_ST_SETBOOLEANS, MT_ST_CLEARBOOLEANS
|
|
Sets or clears the option bits.
|
|
* MT_ST_WRITE_THRESHOLD
|
|
Sets the write threshold for this device to kilobytes
|
|
specified by the lowest bits.
|
|
* MT_ST_DEF_BLKSIZE
|
|
Defines the default block size set automatically. Value
|
|
0xffffff means that the default is not used any more.
|
|
* MT_ST_DEF_DENSITY, MT_ST_DEF_DRVBUFFER
|
|
Used to set or clear the density (8 bits), and drive buffer
|
|
state (3 bits). If the value is MT_ST_CLEAR_DEFAULT
|
|
(0xfffff) the default will not be used any more. Otherwise
|
|
the lowermost bits of the value contain the new value of
|
|
the parameter.
|
|
* MT_ST_DEF_COMPRESSION
|
|
The compression default will not be used if the value of
|
|
the lowermost byte is 0xff. Otherwise the lowermost bit
|
|
contains the new default. If the bits 8-15 are set to a
|
|
non-zero number, and this number is not 0xff, the number is
|
|
used as the compression algorithm. The value
|
|
MT_ST_CLEAR_DEFAULT can be used to clear the compression
|
|
default.
|
|
* MT_ST_SET_TIMEOUT
|
|
Set the normal timeout in seconds for this device. The
|
|
default is 900 seconds (15 minutes). The timeout should be
|
|
long enough for the retries done by the device while
|
|
reading/writing.
|
|
* MT_ST_SET_LONG_TIMEOUT
|
|
Set the long timeout that is used for operations that are
|
|
known to take a long time. The default is 14000 seconds
|
|
(3.9 hours). For erase this value is further multiplied by
|
|
eight.
|
|
* MT_ST_SET_CLN
|
|
Set the cleaning request interpretation parameters using
|
|
the lowest 24 bits of the argument. The driver can set the
|
|
generic status bit GMT_CLN if a cleaning request bit pattern
|
|
is found from the extended sense data. Many drives set one or
|
|
more bits in the extended sense data when the drive needs
|
|
cleaning. The bits are device-dependent. The driver is
|
|
given the number of the sense data byte (the lowest eight
|
|
bits of the argument; must be >= 18 (values 1 - 17
|
|
reserved) and <= the maximum requested sense data sixe),
|
|
a mask to select the relevant bits (the bits 9-16), and the
|
|
bit pattern (bits 17-23). If the bit pattern is zero, one
|
|
or more bits under the mask indicate cleaning request. If
|
|
the pattern is non-zero, the pattern must match the masked
|
|
sense data byte.
|
|
|
|
(The cleaning bit is set if the additional sense code and
|
|
qualifier 00h 17h are seen regardless of the setting of
|
|
MT_ST_SET_CLN.)
|
|
|
|
The following ioctl uses the structure mtpos:
|
|
|
|
MTIOCPOS
|
|
Reads the current position from the drive. Uses
|
|
Tandberg-compatible QFA for SCSI-1 drives and the SCSI-2
|
|
command for the SCSI-2 drives.
|
|
|
|
The following ioctl uses the structure mtget to return the status:
|
|
|
|
MTIOCGET
|
|
Returns some status information.
|
|
The file number and block number within file are returned. The
|
|
block is -1 when it can't be determined (e.g., after MTBSF).
|
|
The drive type is either MTISSCSI1 or MTISSCSI2.
|
|
The number of recovered errors since the previous status call
|
|
is stored in the lower word of the field mt_erreg.
|
|
The current block size and the density code are stored in the field
|
|
mt_dsreg (shifts for the subfields are MT_ST_BLKSIZE_SHIFT and
|
|
MT_ST_DENSITY_SHIFT).
|
|
The GMT_xxx status bits reflect the drive status. GMT_DR_OPEN
|
|
is set if there is no tape in the drive. GMT_EOD means either
|
|
end of recorded data or end of tape. GMT_EOT means end of tape.
|
|
|
|
|
|
Miscellaneous Compile Options
|
|
=============================
|
|
|
|
The recovered write errors are considered fatal if ST_RECOVERED_WRITE_FATAL
|
|
is defined.
|
|
|
|
The maximum number of tape devices is determined by the define
|
|
ST_MAX_TAPES. If more tapes are detected at driver initialization, the
|
|
maximum is adjusted accordingly.
|
|
|
|
Immediate return from tape positioning SCSI commands can be enabled by
|
|
defining ST_NOWAIT. If this is defined, the user should take care that
|
|
the next tape operation is not started before the previous one has
|
|
finished. The drives and SCSI adapters should handle this condition
|
|
gracefully, but some drive/adapter combinations are known to hang the
|
|
SCSI bus in this case.
|
|
|
|
The MTEOM command is by default implemented as spacing over 32767
|
|
filemarks. With this method the file number in the status is
|
|
correct. The user can request using direct spacing to EOD by setting
|
|
ST_FAST_EOM 1 (or using the MT_ST_OPTIONS ioctl). In this case the file
|
|
number will be invalid.
|
|
|
|
When using read ahead or buffered writes the position within the file
|
|
may not be correct after the file is closed (correct position may
|
|
require backspacing over more than one record). The correct position
|
|
within file can be obtained if ST_IN_FILE_POS is defined at compile
|
|
time or the MT_ST_CAN_BSR bit is set for the drive with an ioctl.
|
|
(The driver always backs over a filemark crossed by read ahead if the
|
|
user does not request data that far.)
|
|
|
|
|
|
Debugging Hints
|
|
===============
|
|
|
|
Debugging code is now compiled in by default but debugging is turned off
|
|
with the kernel module parameter debug_flag defaulting to 0. Debugging
|
|
can still be switched on and off with an ioctl. To enable debug at
|
|
module load time add debug_flag=1 to the module load options, the
|
|
debugging output is not voluminous. Debugging can also be enabled
|
|
and disabled by writing a '0' (disable) or '1' (enable) to the sysfs
|
|
file /sys/bus/scsi/drivers/st/debug_flag.
|
|
|
|
If the tape seems to hang, I would be very interested to hear where
|
|
the driver is waiting. With the command 'ps -l' you can see the state
|
|
of the process using the tape. If the state is D, the process is
|
|
waiting for something. The field WCHAN tells where the driver is
|
|
waiting. If you have the current System.map in the correct place (in
|
|
/boot for the procps I use) or have updated /etc/psdatabase (for kmem
|
|
ps), ps writes the function name in the WCHAN field. If not, you have
|
|
to look up the function from System.map.
|
|
|
|
Note also that the timeouts are very long compared to most other
|
|
drivers. This means that the Linux driver may appear hung although the
|
|
real reason is that the tape firmware has got confused.
|