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 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 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. 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 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) 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/ 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 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 of memory zram have consumed to store the data (to reset this counter to the actual current value, write 1 to this attribute) mem_limit RW the maximum amount of memory ZRAM can use to store the compressed data pages_compacted RO the number of pages freed during compaction (available only via zram/mm_stat node) compact WO trigger memory 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 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. Nitin Gupta ngupta@vflare.org