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Compaction returns back to zram the number of migrated objects, which is
quite uninformative -- we have objects of different sizes so user space
cannot obtain any valuable data from that number. Change compaction to
operate in terms of pages and return back to compaction issuer the
number of pages that were freed during compaction. So from now on we
will export more meaningful value in zram<id>/mm_stat -- the number of
freed (compacted) pages.
This requires:
(a) a rename of `num_migrated' to 'pages_compacted'
(b) a internal API change -- return first_page's fullness_group from
putback_zspage(), so we know when putback_zspage() did
free_zspage(). It helps us to account compaction stats correctly.
Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Acked-by: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
210 lines
7.4 KiB
Plaintext
210 lines
7.4 KiB
Plaintext
zram: Compressed RAM based block devices
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----------------------------------------
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* Introduction
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The zram module creates RAM based block devices named /dev/zram<id>
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(<id> = 0, 1, ...). Pages written to these disks are compressed and stored
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in memory itself. These disks allow very fast I/O and compression provides
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good amounts of memory savings. Some of the usecases include /tmp storage,
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use as swap disks, various caches under /var and maybe many more :)
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Statistics for individual zram devices are exported through sysfs nodes at
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/sys/block/zram<id>/
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* Usage
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Following shows a typical sequence of steps for using zram.
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1) Load Module:
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modprobe zram num_devices=4
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This creates 4 devices: /dev/zram{0,1,2,3}
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num_devices parameter is optional and tells zram how many devices should be
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pre-created. Default: 1.
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2) Set max number of compression streams
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Compression backend may use up to max_comp_streams compression streams,
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thus allowing up to max_comp_streams concurrent compression operations.
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By default, compression backend uses single compression stream.
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Examples:
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#show max compression streams number
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cat /sys/block/zram0/max_comp_streams
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#set max compression streams number to 3
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echo 3 > /sys/block/zram0/max_comp_streams
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Note:
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In order to enable compression backend's multi stream support max_comp_streams
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must be initially set to desired concurrency level before ZRAM device
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initialisation. Once the device initialised as a single stream compression
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backend (max_comp_streams equals to 1), you will see error if you try to change
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the value of max_comp_streams because single stream compression backend
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implemented as a special case by lock overhead issue and does not support
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dynamic max_comp_streams. Only multi stream backend supports dynamic
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max_comp_streams adjustment.
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3) Select compression algorithm
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Using comp_algorithm device attribute one can see available and
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currently selected (shown in square brackets) compression algortithms,
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change selected compression algorithm (once the device is initialised
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there is no way to change compression algorithm).
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Examples:
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#show supported compression algorithms
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cat /sys/block/zram0/comp_algorithm
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lzo [lz4]
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#select lzo compression algorithm
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echo lzo > /sys/block/zram0/comp_algorithm
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4) Set Disksize
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Set disk size by writing the value to sysfs node 'disksize'.
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The value can be either in bytes or you can use mem suffixes.
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Examples:
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# Initialize /dev/zram0 with 50MB disksize
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echo $((50*1024*1024)) > /sys/block/zram0/disksize
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# Using mem suffixes
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echo 256K > /sys/block/zram0/disksize
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echo 512M > /sys/block/zram0/disksize
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echo 1G > /sys/block/zram0/disksize
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Note:
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There is little point creating a zram of greater than twice the size of memory
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since we expect a 2:1 compression ratio. Note that zram uses about 0.1% of the
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size of the disk when not in use so a huge zram is wasteful.
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5) Set memory limit: Optional
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Set memory limit by writing the value to sysfs node 'mem_limit'.
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The value can be either in bytes or you can use mem suffixes.
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In addition, you could change the value in runtime.
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Examples:
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# limit /dev/zram0 with 50MB memory
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echo $((50*1024*1024)) > /sys/block/zram0/mem_limit
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# Using mem suffixes
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echo 256K > /sys/block/zram0/mem_limit
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echo 512M > /sys/block/zram0/mem_limit
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echo 1G > /sys/block/zram0/mem_limit
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# To disable memory limit
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echo 0 > /sys/block/zram0/mem_limit
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6) Activate:
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mkswap /dev/zram0
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swapon /dev/zram0
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mkfs.ext4 /dev/zram1
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mount /dev/zram1 /tmp
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7) Add/remove zram devices
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zram provides a control interface, which enables dynamic (on-demand) device
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addition and removal.
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In order to add a new /dev/zramX device, perform read operation on hot_add
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attribute. This will return either new device's device id (meaning that you
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can use /dev/zram<id>) or error code.
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Example:
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cat /sys/class/zram-control/hot_add
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1
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To remove the existing /dev/zramX device (where X is a device id)
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execute
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echo X > /sys/class/zram-control/hot_remove
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8) Stats:
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Per-device statistics are exported as various nodes under /sys/block/zram<id>/
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A brief description of exported device attritbutes. For more details please
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read Documentation/ABI/testing/sysfs-block-zram.
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Name access description
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---- ------ -----------
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disksize RW show and set the device's disk size
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initstate RO shows the initialization state of the device
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reset WO trigger device reset
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num_reads RO the number of reads
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failed_reads RO the number of failed reads
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num_write RO the number of writes
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failed_writes RO the number of failed writes
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invalid_io RO the number of non-page-size-aligned I/O requests
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max_comp_streams RW the number of possible concurrent compress operations
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comp_algorithm RW show and change the compression algorithm
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notify_free RO the number of notifications to free pages (either
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slot free notifications or REQ_DISCARD requests)
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zero_pages RO the number of zero filled pages written to this disk
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orig_data_size RO uncompressed size of data stored in this disk
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compr_data_size RO compressed size of data stored in this disk
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mem_used_total RO the amount of memory allocated for this disk
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mem_used_max RW the maximum amount memory zram have consumed to
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store compressed data
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mem_limit RW the maximum amount of memory ZRAM can use to store
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the compressed data
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pages_compacted RO the number of pages freed during compaction
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(available only via zram<id>/mm_stat node)
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compact WO trigger memory compaction
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WARNING
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=======
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per-stat sysfs attributes are considered to be deprecated.
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The basic strategy is:
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-- the existing RW nodes will be downgraded to WO nodes (in linux 4.11)
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-- deprecated RO sysfs nodes will eventually be removed (in linux 4.11)
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The list of deprecated attributes can be found here:
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Documentation/ABI/obsolete/sysfs-block-zram
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Basically, every attribute that has its own read accessible sysfs node
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(e.g. num_reads) *AND* is accessible via one of the stat files (zram<id>/stat
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or zram<id>/io_stat or zram<id>/mm_stat) is considered to be deprecated.
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User space is advised to use the following files to read the device statistics.
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File /sys/block/zram<id>/stat
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Represents block layer statistics. Read Documentation/block/stat.txt for
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details.
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File /sys/block/zram<id>/io_stat
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The stat file represents device's I/O statistics not accounted by block
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layer and, thus, not available in zram<id>/stat file. It consists of a
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single line of text and contains the following stats separated by
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whitespace:
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failed_reads
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failed_writes
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invalid_io
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notify_free
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File /sys/block/zram<id>/mm_stat
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The stat file represents device's mm statistics. It consists of a single
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line of text and contains the following stats separated by whitespace:
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orig_data_size
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compr_data_size
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mem_used_total
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mem_limit
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mem_used_max
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zero_pages
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num_migrated
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9) Deactivate:
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swapoff /dev/zram0
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umount /dev/zram1
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10) Reset:
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Write any positive value to 'reset' sysfs node
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echo 1 > /sys/block/zram0/reset
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echo 1 > /sys/block/zram1/reset
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This frees all the memory allocated for the given device and
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resets the disksize to zero. You must set the disksize again
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before reusing the device.
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Nitin Gupta
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ngupta@vflare.org
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