131 lines
4.1 KiB
Plaintext
131 lines
4.1 KiB
Plaintext
zram: Compressed RAM based block devices
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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. 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) Stats:
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Per-device statistics are exported as various nodes under
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/sys/block/zram<id>/
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disksize
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num_reads
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num_writes
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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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zero_pages
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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_used_max
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8) Deactivate:
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swapoff /dev/zram0
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umount /dev/zram1
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9) 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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