Documentation/dax: Update Usage section
Update the Usage section to reflect the new individual dax selection functionality. Signed-off-by: Ira Weiny <ira.weiny@intel.com> Acked-by: Randy Dunlap <rdunlap@infradead.org> Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com> Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
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Ira Weiny
Darrick J. Wong
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712b2698e4
commit
83d9088659
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@ -20,8 +20,144 @@ Usage
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If you have a block device which supports DAX, you can make a filesystem
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on it as usual. The DAX code currently only supports files with a block
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size equal to your kernel's PAGE_SIZE, so you may need to specify a block
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size when creating the filesystem. When mounting it, use the "-o dax"
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option on the command line or add 'dax' to the options in /etc/fstab.
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size when creating the filesystem.
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Currently 3 filesystems support DAX: ext2, ext4 and xfs. Enabling DAX on them
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is different.
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Enabling DAX on ext4 and ext2
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-----------------------------
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When mounting the filesystem, use the "-o dax" option on the command line or
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add 'dax' to the options in /etc/fstab. This works to enable DAX on all files
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within the filesystem. It is equivalent to the '-o dax=always' behavior below.
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Enabling DAX on xfs
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-------------------
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Summary
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-------
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1. There exists an in-kernel file access mode flag S_DAX that corresponds to
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the statx flag STATX_ATTR_DAX. See the manpage for statx(2) for details
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about this access mode.
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2. There exists a persistent flag FS_XFLAG_DAX that can be applied to regular
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files and directories. This advisory flag can be set or cleared at any
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time, but doing so does not immediately affect the S_DAX state.
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3. If the persistent FS_XFLAG_DAX flag is set on a directory, this flag will
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be inherited by all regular files and subdirectories that are subsequently
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created in this directory. Files and subdirectories that exist at the time
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this flag is set or cleared on the parent directory are not modified by
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this modification of the parent directory.
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4. There exist dax mount options which can override FS_XFLAG_DAX in the
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setting of the S_DAX flag. Given underlying storage which supports DAX the
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following hold:
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"-o dax=inode" means "follow FS_XFLAG_DAX" and is the default.
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"-o dax=never" means "never set S_DAX, ignore FS_XFLAG_DAX."
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"-o dax=always" means "always set S_DAX ignore FS_XFLAG_DAX."
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"-o dax" is a legacy option which is an alias for "dax=always".
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This may be removed in the future so "-o dax=always" is
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the preferred method for specifying this behavior.
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NOTE: Modifications to and the inheritance behavior of FS_XFLAG_DAX remain
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the same even when the filesystem is mounted with a dax option. However,
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in-core inode state (S_DAX) will be overridden until the filesystem is
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remounted with dax=inode and the inode is evicted from kernel memory.
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5. The S_DAX policy can be changed via:
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a) Setting the parent directory FS_XFLAG_DAX as needed before files are
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created
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b) Setting the appropriate dax="foo" mount option
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c) Changing the FS_XFLAG_DAX flag on existing regular files and
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directories. This has runtime constraints and limitations that are
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described in 6) below.
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6. When changing the S_DAX policy via toggling the persistent FS_XFLAG_DAX flag,
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the change in behaviour for existing regular files may not occur
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immediately. If the change must take effect immediately, the administrator
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needs to:
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a) stop the application so there are no active references to the data set
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the policy change will affect
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b) evict the data set from kernel caches so it will be re-instantiated when
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the application is restarted. This can be achieved by:
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i. drop-caches
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ii. a filesystem unmount and mount cycle
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iii. a system reboot
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Details
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-------
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There are 2 per-file dax flags. One is a persistent inode setting (FS_XFLAG_DAX)
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and the other is a volatile flag indicating the active state of the feature
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(S_DAX).
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FS_XFLAG_DAX is preserved within the filesystem. This persistent config
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setting can be set, cleared and/or queried using the FS_IOC_FS[GS]ETXATTR ioctl
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(see ioctl_xfs_fsgetxattr(2)) or an utility such as 'xfs_io'.
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New files and directories automatically inherit FS_XFLAG_DAX from
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their parent directory _when_ _created_. Therefore, setting FS_XFLAG_DAX at
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directory creation time can be used to set a default behavior for an entire
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sub-tree.
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To clarify inheritance, here are 3 examples:
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Example A:
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mkdir -p a/b/c
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xfs_io -c 'chattr +x' a
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mkdir a/b/c/d
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mkdir a/e
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dax: a,e
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no dax: b,c,d
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Example B:
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mkdir a
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xfs_io -c 'chattr +x' a
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mkdir -p a/b/c/d
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dax: a,b,c,d
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no dax:
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Example C:
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mkdir -p a/b/c
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xfs_io -c 'chattr +x' c
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mkdir a/b/c/d
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dax: c,d
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no dax: a,b
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The current enabled state (S_DAX) is set when a file inode is instantiated in
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memory by the kernel. It is set based on the underlying media support, the
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value of FS_XFLAG_DAX and the filesystem's dax mount option.
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statx can be used to query S_DAX. NOTE that only regular files will ever have
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S_DAX set and therefore statx will never indicate that S_DAX is set on
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directories.
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Setting the FS_XFLAG_DAX flag (specifically or through inheritance) occurs even
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if the underlying media does not support dax and/or the filesystem is
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overridden with a mount option.
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Implementation Tips for Block Driver Writers
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@ -94,7 +230,7 @@ sysadmins have an option to restore the lost data from a prior backup/inbuilt
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redundancy in the following ways:
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1. Delete the affected file, and restore from a backup (sysadmin route):
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This will free the file system blocks that were being used by the file,
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This will free the filesystem blocks that were being used by the file,
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and the next time they're allocated, they will be zeroed first, which
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happens through the driver, and will clear bad sectors.
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