Kernel-3.10.0-957.el7_zra

published: updated: Kernel_3.10.0-957.el7_内核文档

zram: Compressed RAM based block devices

  • Introduction

The zram module creates RAM based block devices named /dev/zram
( = 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/

  • Usage

Following shows a typical sequence of steps for using zram.

  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.

  1. Set max number of compression streams
    Compression backend may use up to max_comp_streams compression streams,
    thus allowing up to max_comp_streams concurrent compression operations.
    By default, compression backend uses single compression stream.

    Examples:
    #show max compression streams number
    cat /sys/block/zram0/max_comp_streams

    #set max compression streams number to 3
    echo 3 > /sys/block/zram0/max_comp_streams

Note:
In order to enable compression backend’s multi stream support max_comp_streams
must be initially set to desired concurrency level before ZRAM device
initialisation. Once the device initialised as a single stream compression
backend (max_comp_streams equals to 1), you will see error if you try to change
the value of max_comp_streams because single stream compression backend
implemented as a special case by lock overhead issue and does not support
dynamic max_comp_streams. Only multi stream backend supports dynamic
max_comp_streams adjustment.

  1. Select compression algorithm
    Using comp_algorithm device attribute one can see available and
    currently selected (shown in square brackets) compression algortithms,
    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

  2. 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.

  1. 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

  2. Activate:
    mkswap /dev/zram0
    swapon /dev/zram0

    mkfs.ext4 /dev/zram1
    mount /dev/zram1 /tmp

  3. 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) 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

  1. Stats:
    Per-device statistics are exported as various nodes under /sys/block/zram/

A brief description of exported device attritbutes. 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
num_reads RO the number of reads
failed_reads RO the number of failed reads
num_write RO the number of writes
failed_writes RO the number of failed writes
invalid_io RO the number of non-page-size-aligned I/O requests
max_comp_streams RW the number of possible concurrent compress operations
comp_algorithm RW show and change the compression algorithm
notify_free RO the number of notifications to free pages (either
slot free notifications or REQ_DISCARD requests)
zero_pages RO the number of zero filled pages written to this disk
orig_data_size RO uncompressed size of data stored in this disk
compr_data_size RO compressed size of data stored in this disk
mem_used_total RO the amount of memory allocated for this disk
mem_used_max RW the maximum amount memory zram have consumed to
store compressed data
mem_limit RW the maximum amount of memory ZRAM can use to store
the compressed data
num_migrated RO the number of objects migrated migrated by compaction

WARNING

per-stat sysfs attributes are considered to be deprecated.
The basic strategy is:
– the existing RW nodes will be downgraded to WO nodes (in linux 4.11)
– deprecated RO sysfs nodes will eventually be removed (in linux 4.11)

The list of deprecated attributes can be found here:
Documentation/ABI/obsolete/sysfs-block-zram

Basically, every attribute that has its own read accessible sysfs node
(e.g. num_reads) AND is accessible via one of the stat files (zram/stat
or zram/io_stat or zram/mm_stat) is considered to be deprecated.

User space is advised to use the following files to read the device statistics.

File /sys/block/zram/stat

Represents block layer statistics. Read Documentation/block/stat.txt for
details.

File /sys/block/zram/io_stat

The stat file represents device’s I/O statistics not accounted by block
layer and, thus, not available in zram/stat file. It consists of a
single line of text and contains the following stats separated by
whitespace:
failed_reads
failed_writes
invalid_io
notify_free

File /sys/block/zram/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
compr_data_size
mem_used_total
mem_limit
mem_used_max
zero_pages
num_migrated

  1. Deactivate:
    swapoff /dev/zram0
    umount /dev/zram1

  2. 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.

Nitin Gupta
ngupta@vflare.org

oosTech by Sam Lee