356 lines
13 KiB
Plaintext
356 lines
13 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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There are several ways to configure and manage zram device(-s):
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a) using zram and zram_control sysfs attributes
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b) using zramctl utility, provided by util-linux (util-linux@vger.kernel.org).
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In this document we will describe only 'manual' zram configuration steps,
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IOW, zram and zram_control sysfs attributes.
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In order to get a better idea about zramctl please consult util-linux
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documentation, zramctl man-page or `zramctl --help'. Please be informed
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that zram maintainers do not develop/maintain util-linux or zramctl, should
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you have any questions please contact util-linux@vger.kernel.org
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Following shows a typical sequence of steps for using zram.
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WARNING
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=======
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For the sake of simplicity we skip error checking parts in most of the
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examples below. However, it is your sole responsibility to handle errors.
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zram sysfs attributes always return negative values in case of errors.
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The list of possible return codes:
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-EBUSY -- an attempt to modify an attribute that cannot be changed once
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the device has been initialised. Please reset device first;
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-ENOMEM -- zram was not able to allocate enough memory to fulfil your
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needs;
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-EINVAL -- invalid input has been provided.
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If you use 'echo', the returned value that is changed by 'echo' utility,
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and, in general case, something like:
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echo 3 > /sys/block/zram0/max_comp_streams
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if [ $? -ne 0 ];
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handle_error
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fi
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should suffice.
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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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Regardless the value passed to this attribute, ZRAM will always
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allocate multiple compression streams - one per online CPUs - thus
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allowing several concurrent compression operations. The number of
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allocated compression streams goes down when some of the CPUs
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become offline. There is no single-compression-stream mode anymore,
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unless you are running a UP system or has only 1 CPU online.
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To find out how many streams are currently available:
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cat /sys/block/zram0/max_comp_streams
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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 algorithms,
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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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For the time being, the `comp_algorithm' content does not necessarily
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show every compression algorithm supported by the kernel. We keep this
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list primarily to simplify device configuration and one can configure
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a new device with a compression algorithm that is not listed in
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`comp_algorithm'. The thing is that, internally, ZRAM uses Crypto API
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and, if some of the algorithms were built as modules, it's impossible
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to list all of them using, for instance, /proc/crypto or any other
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method. This, however, has an advantage of permitting the usage of
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custom crypto compression modules (implementing S/W or H/W compression).
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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 attributes. 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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mem_used_max WO reset the `mem_used_max' counter (see later)
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mem_limit WO specifies the maximum amount of memory ZRAM can use
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to store the compressed data
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writeback_limit WO specifies the maximum amount of write IO zram can
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write out to backing device as 4KB unit
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writeback_limit_enable RW show and set writeback_limit feature
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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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compact WO trigger memory compaction
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debug_stat RO this file is used for zram debugging purposes
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backing_dev RW set up backend storage for zram to write out
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idle WO mark allocated slot as idle
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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 the number of failed reads
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failed_writes the number of failed writes
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invalid_io the number of non-page-size-aligned I/O requests
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notify_free Depending on device usage scenario it may account
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a) the number of pages freed because of swap slot free
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notifications or b) the number of pages freed because of
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REQ_DISCARD requests sent by bio. The former ones are
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sent to a swap block device when a swap slot is freed,
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which implies that this disk is being used as a swap disk.
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The latter ones are sent by filesystem mounted with
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discard option, whenever some data blocks are getting
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discarded.
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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 uncompressed size of data stored in this disk.
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This excludes same-element-filled pages (same_pages) since
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no memory is allocated for them.
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Unit: bytes
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compr_data_size compressed size of data stored in this disk
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mem_used_total the amount of memory allocated for this disk. This
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includes allocator fragmentation and metadata overhead,
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allocated for this disk. So, allocator space efficiency
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can be calculated using compr_data_size and this statistic.
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Unit: bytes
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mem_limit the maximum amount of memory ZRAM can use to store
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the compressed data
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mem_used_max the maximum amount of memory zram have consumed to
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store the data
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same_pages the number of same element filled pages written to this disk.
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No memory is allocated for such pages.
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pages_compacted the number of pages freed during compaction
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huge_pages the number of incompressible pages
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File /sys/block/zram<id>/bd_stat
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The stat file represents device's backing device statistics. It consists of
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a single line of text and contains the following stats separated by whitespace:
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bd_count size of data written in backing device.
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Unit: 4K bytes
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bd_reads the number of reads from backing device
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Unit: 4K bytes
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bd_writes the number of writes to backing device
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Unit: 4K bytes
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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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* Optional Feature
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= writeback
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With CONFIG_ZRAM_WRITEBACK, zram can write idle/incompressible page
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to backing storage rather than keeping it in memory.
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To use the feature, admin should set up backing device via
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"echo /dev/sda5 > /sys/block/zramX/backing_dev"
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before disksize setting. It supports only partition at this moment.
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If admin want to use incompressible page writeback, they could do via
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"echo huge > /sys/block/zramX/write"
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To use idle page writeback, first, user need to declare zram pages
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as idle.
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"echo all > /sys/block/zramX/idle"
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From now on, any pages on zram are idle pages. The idle mark
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will be removed until someone request access of the block.
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IOW, unless there is access request, those pages are still idle pages.
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Admin can request writeback of those idle pages at right timing via
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"echo idle > /sys/block/zramX/writeback"
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With the command, zram writeback idle pages from memory to the storage.
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If there are lots of write IO with flash device, potentially, it has
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flash wearout problem so that admin needs to design write limitation
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to guarantee storage health for entire product life.
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To overcome the concern, zram supports "writeback_limit" feature.
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The "writeback_limit_enable"'s default value is 0 so that it doesn't limit
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any writeback. IOW, if admin want to apply writeback budget, he should
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enable writeback_limit_enable via
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$ echo 1 > /sys/block/zramX/writeback_limit_enable
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Once writeback_limit_enable is set, zram doesn't allow any writeback
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until admin set the budget via /sys/block/zramX/writeback_limit.
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(If admin doesn't enable writeback_limit_enable, writeback_limit's value
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assigned via /sys/block/zramX/writeback_limit is meaninless.)
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If admin want to limit writeback as per-day 400M, he could do it
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like below.
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$ MB_SHIFT=20
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$ 4K_SHIFT=12
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$ echo $((400<<MB_SHIFT>>4K_SHIFT)) > \
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/sys/block/zram0/writeback_limit.
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$ echo 1 > /sys/block/zram0/writeback_limit_enable
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If admin want to allow further write again once the bugdet is exausted,
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he could do it like below
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$ echo $((400<<MB_SHIFT>>4K_SHIFT)) > \
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/sys/block/zram0/writeback_limit
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If admin want to see remaining writeback budget since he set,
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$ cat /sys/block/zramX/writeback_limit
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If admin want to disable writeback limit, he could do
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$ echo 0 > /sys/block/zramX/writeback_limit_enable
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The writeback_limit count will reset whenever you reset zram(e.g.,
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system reboot, echo 1 > /sys/block/zramX/reset) so keeping how many of
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writeback happened until you reset the zram to allocate extra writeback
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budget in next setting is user's job.
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If admin want to measure writeback count in a certain period, he could
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know it via /sys/block/zram0/bd_stat's 3rd column.
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= memory tracking
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With CONFIG_ZRAM_MEMORY_TRACKING, user can know information of the
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zram block. It could be useful to catch cold or incompressible
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pages of the process with*pagemap.
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If you enable the feature, you could see block state via
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/sys/kernel/debug/zram/zram0/block_state". The output is as follows,
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300 75.033841 .wh.
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301 63.806904 s...
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302 63.806919 ..hi
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First column is zram's block index.
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Second column is access time since the system was booted
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Third column is state of the block.
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(s: same page
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w: written page to backing store
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h: huge page
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i: idle page)
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First line of above example says 300th block is accessed at 75.033841sec
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and the block's state is huge so it is written back to the backing
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storage. It's a debugging feature so anyone shouldn't rely on it to work
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properly.
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Nitin Gupta
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ngupta@vflare.org
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