mirror of
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
synced 2024-08-26 19:00:25 +00:00
![Minchan Kim](/assets/img/avatar_default.png)
This patch adds document and kconfig for using of writeback feature. Link: http://lkml.kernel.org/r/1498459987-24562-10-git-send-email-minchan@kernel.org Signed-off-by: Minchan Kim <minchan@kernel.org> Cc: Juneho Choi <juno.choi@lge.com> Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
246 lines
9.3 KiB
Text
246 lines
9.3 KiB
Text
zram: Compressed RAM based block devices
|
|
----------------------------------------
|
|
|
|
* Introduction
|
|
|
|
The zram module creates RAM based block devices named /dev/zram<id>
|
|
(<id> = 0, 1, ...). Pages written to these disks are compressed and stored
|
|
in memory itself. These disks allow very fast I/O and compression provides
|
|
good amounts of memory savings. Some of the usecases include /tmp storage,
|
|
use as swap disks, various caches under /var and maybe many more :)
|
|
|
|
Statistics for individual zram devices are exported through sysfs nodes at
|
|
/sys/block/zram<id>/
|
|
|
|
* Usage
|
|
|
|
There are several ways to configure and manage zram device(-s):
|
|
a) using zram and zram_control sysfs attributes
|
|
b) using zramctl utility, provided by util-linux (util-linux@vger.kernel.org).
|
|
|
|
In this document we will describe only 'manual' zram configuration steps,
|
|
IOW, zram and zram_control sysfs attributes.
|
|
|
|
In order to get a better idea about zramctl please consult util-linux
|
|
documentation, zramctl man-page or `zramctl --help'. Please be informed
|
|
that zram maintainers do not develop/maintain util-linux or zramctl, should
|
|
you have any questions please contact util-linux@vger.kernel.org
|
|
|
|
Following shows a typical sequence of steps for using zram.
|
|
|
|
WARNING
|
|
=======
|
|
For the sake of simplicity we skip error checking parts in most of the
|
|
examples below. However, it is your sole responsibility to handle errors.
|
|
|
|
zram sysfs attributes always return negative values in case of errors.
|
|
The list of possible return codes:
|
|
-EBUSY -- an attempt to modify an attribute that cannot be changed once
|
|
the device has been initialised. Please reset device first;
|
|
-ENOMEM -- zram was not able to allocate enough memory to fulfil your
|
|
needs;
|
|
-EINVAL -- invalid input has been provided.
|
|
|
|
If you use 'echo', the returned value that is changed by 'echo' utility,
|
|
and, in general case, something like:
|
|
|
|
echo 3 > /sys/block/zram0/max_comp_streams
|
|
if [ $? -ne 0 ];
|
|
handle_error
|
|
fi
|
|
|
|
should suffice.
|
|
|
|
1) Load Module:
|
|
modprobe zram num_devices=4
|
|
This creates 4 devices: /dev/zram{0,1,2,3}
|
|
|
|
num_devices parameter is optional and tells zram how many devices should be
|
|
pre-created. Default: 1.
|
|
|
|
2) Set max number of compression streams
|
|
Regardless the value passed to this attribute, ZRAM will always
|
|
allocate multiple compression streams - one per online CPUs - thus
|
|
allowing several concurrent compression operations. The number of
|
|
allocated compression streams goes down when some of the CPUs
|
|
become offline. There is no single-compression-stream mode anymore,
|
|
unless you are running a UP system or has only 1 CPU online.
|
|
|
|
To find out how many streams are currently available:
|
|
cat /sys/block/zram0/max_comp_streams
|
|
|
|
3) Select compression algorithm
|
|
Using comp_algorithm device attribute one can see available and
|
|
currently selected (shown in square brackets) compression algorithms,
|
|
change selected compression algorithm (once the device is initialised
|
|
there is no way to change compression algorithm).
|
|
|
|
Examples:
|
|
#show supported compression algorithms
|
|
cat /sys/block/zram0/comp_algorithm
|
|
lzo [lz4]
|
|
|
|
#select lzo compression algorithm
|
|
echo lzo > /sys/block/zram0/comp_algorithm
|
|
|
|
For the time being, the `comp_algorithm' content does not necessarily
|
|
show every compression algorithm supported by the kernel. We keep this
|
|
list primarily to simplify device configuration and one can configure
|
|
a new device with a compression algorithm that is not listed in
|
|
`comp_algorithm'. The thing is that, internally, ZRAM uses Crypto API
|
|
and, if some of the algorithms were built as modules, it's impossible
|
|
to list all of them using, for instance, /proc/crypto or any other
|
|
method. This, however, has an advantage of permitting the usage of
|
|
custom crypto compression modules (implementing S/W or H/W compression).
|
|
|
|
4) Set Disksize
|
|
Set disk size by writing the value to sysfs node 'disksize'.
|
|
The value can be either in bytes or you can use mem suffixes.
|
|
Examples:
|
|
# Initialize /dev/zram0 with 50MB disksize
|
|
echo $((50*1024*1024)) > /sys/block/zram0/disksize
|
|
|
|
# Using mem suffixes
|
|
echo 256K > /sys/block/zram0/disksize
|
|
echo 512M > /sys/block/zram0/disksize
|
|
echo 1G > /sys/block/zram0/disksize
|
|
|
|
Note:
|
|
There is little point creating a zram of greater than twice the size of memory
|
|
since we expect a 2:1 compression ratio. Note that zram uses about 0.1% of the
|
|
size of the disk when not in use so a huge zram is wasteful.
|
|
|
|
5) Set memory limit: Optional
|
|
Set memory limit by writing the value to sysfs node 'mem_limit'.
|
|
The value can be either in bytes or you can use mem suffixes.
|
|
In addition, you could change the value in runtime.
|
|
Examples:
|
|
# limit /dev/zram0 with 50MB memory
|
|
echo $((50*1024*1024)) > /sys/block/zram0/mem_limit
|
|
|
|
# Using mem suffixes
|
|
echo 256K > /sys/block/zram0/mem_limit
|
|
echo 512M > /sys/block/zram0/mem_limit
|
|
echo 1G > /sys/block/zram0/mem_limit
|
|
|
|
# To disable memory limit
|
|
echo 0 > /sys/block/zram0/mem_limit
|
|
|
|
6) Activate:
|
|
mkswap /dev/zram0
|
|
swapon /dev/zram0
|
|
|
|
mkfs.ext4 /dev/zram1
|
|
mount /dev/zram1 /tmp
|
|
|
|
7) Add/remove zram devices
|
|
|
|
zram provides a control interface, which enables dynamic (on-demand) device
|
|
addition and removal.
|
|
|
|
In order to add a new /dev/zramX device, perform read operation on hot_add
|
|
attribute. This will return either new device's device id (meaning that you
|
|
can use /dev/zram<id>) or error code.
|
|
|
|
Example:
|
|
cat /sys/class/zram-control/hot_add
|
|
1
|
|
|
|
To remove the existing /dev/zramX device (where X is a device id)
|
|
execute
|
|
echo X > /sys/class/zram-control/hot_remove
|
|
|
|
8) Stats:
|
|
Per-device statistics are exported as various nodes under /sys/block/zram<id>/
|
|
|
|
A brief description of exported device attributes. For more details please
|
|
read Documentation/ABI/testing/sysfs-block-zram.
|
|
|
|
Name access description
|
|
---- ------ -----------
|
|
disksize RW show and set the device's disk size
|
|
initstate RO shows the initialization state of the device
|
|
reset WO trigger device reset
|
|
mem_used_max WO reset the `mem_used_max' counter (see later)
|
|
mem_limit WO specifies the maximum amount of memory ZRAM can use
|
|
to store the compressed data
|
|
max_comp_streams RW the number of possible concurrent compress operations
|
|
comp_algorithm RW show and change the compression algorithm
|
|
compact WO trigger memory compaction
|
|
debug_stat RO this file is used for zram debugging purposes
|
|
backing_dev RW set up backend storage for zram to write out
|
|
|
|
|
|
User space is advised to use the following files to read the device statistics.
|
|
|
|
File /sys/block/zram<id>/stat
|
|
|
|
Represents block layer statistics. Read Documentation/block/stat.txt for
|
|
details.
|
|
|
|
File /sys/block/zram<id>/io_stat
|
|
|
|
The stat file represents device's I/O statistics not accounted by block
|
|
layer and, thus, not available in zram<id>/stat file. It consists of a
|
|
single line of text and contains the following stats separated by
|
|
whitespace:
|
|
failed_reads the number of failed reads
|
|
failed_writes the number of failed writes
|
|
invalid_io the number of non-page-size-aligned I/O requests
|
|
notify_free Depending on device usage scenario it may account
|
|
a) the number of pages freed because of swap slot free
|
|
notifications or b) the number of pages freed because of
|
|
REQ_DISCARD requests sent by bio. The former ones are
|
|
sent to a swap block device when a swap slot is freed,
|
|
which implies that this disk is being used as a swap disk.
|
|
The latter ones are sent by filesystem mounted with
|
|
discard option, whenever some data blocks are getting
|
|
discarded.
|
|
|
|
File /sys/block/zram<id>/mm_stat
|
|
|
|
The stat file represents device's mm statistics. It consists of a single
|
|
line of text and contains the following stats separated by whitespace:
|
|
orig_data_size uncompressed size of data stored in this disk.
|
|
This excludes same-element-filled pages (same_pages) since
|
|
no memory is allocated for them.
|
|
Unit: bytes
|
|
compr_data_size compressed size of data stored in this disk
|
|
mem_used_total the amount of memory allocated for this disk. This
|
|
includes allocator fragmentation and metadata overhead,
|
|
allocated for this disk. So, allocator space efficiency
|
|
can be calculated using compr_data_size and this statistic.
|
|
Unit: bytes
|
|
mem_limit the maximum amount of memory ZRAM can use to store
|
|
the compressed data
|
|
mem_used_max the maximum amount of memory zram have consumed to
|
|
store the data
|
|
same_pages the number of same element filled pages written to this disk.
|
|
No memory is allocated for such pages.
|
|
pages_compacted the number of pages freed during compaction
|
|
|
|
9) Deactivate:
|
|
swapoff /dev/zram0
|
|
umount /dev/zram1
|
|
|
|
10) Reset:
|
|
Write any positive value to 'reset' sysfs node
|
|
echo 1 > /sys/block/zram0/reset
|
|
echo 1 > /sys/block/zram1/reset
|
|
|
|
This frees all the memory allocated for the given device and
|
|
resets the disksize to zero. You must set the disksize again
|
|
before reusing the device.
|
|
|
|
* Optional Feature
|
|
|
|
= writeback
|
|
|
|
With incompressible pages, there is no memory saving with zram.
|
|
Instead, with CONFIG_ZRAM_WRITEBACK, zram can write incompressible page
|
|
to backing storage rather than keeping it in memory.
|
|
User should set up backing device via /sys/block/zramX/backing_dev
|
|
before disksize setting.
|
|
|
|
Nitin Gupta
|
|
ngupta@vflare.org
|