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Diffstat (limited to 'Documentation/blockdev/zram.txt')
| -rw-r--r-- | Documentation/blockdev/zram.txt | 164 |
1 files changed, 77 insertions, 87 deletions
diff --git a/Documentation/blockdev/zram.txt b/Documentation/blockdev/zram.txt index 5bda5031c83d..4fced8a21307 100644 --- a/Documentation/blockdev/zram.txt +++ b/Documentation/blockdev/zram.txt @@ -59,34 +59,23 @@ 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 +Regardless the value passed to this attribute, ZRAM will always +allocate multiple compression streams - one per online CPUs - thus +allowing several concurrent compression operations. The number of +allocated compression streams goes down when some of the CPUs +become offline. There is no single-compression-stream mode anymore, +unless you are running a UP system or has only 1 CPU online. + +To find out how many streams are currently available: 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). +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: +Examples: #show supported compression algorithms cat /sys/block/zram0/comp_algorithm lzo [lz4] @@ -94,17 +83,27 @@ max_comp_streams adjustment. #select lzo compression algorithm echo lzo > /sys/block/zram0/comp_algorithm +For the time being, the `comp_algorithm' content does not necessarily +show every compression algorithm supported by the kernel. We keep this +list primarily to simplify device configuration and one can configure +a new device with a compression algorithm that is not listed in +`comp_algorithm'. The thing is that, internally, ZRAM uses Crypto API +and, if some of the algorithms were built as modules, it's impossible +to list all of them using, for instance, /proc/crypto or any other +method. This, however, has an advantage of permitting the usage of +custom crypto compression modules (implementing S/W or H/W compression). + 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 +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 + # 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 @@ -112,20 +111,20 @@ 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 +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 + # 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 + # To disable memory limit + echo 0 > /sys/block/zram0/mem_limit 6) Activate: mkswap /dev/zram0 @@ -162,41 +161,14 @@ 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 +mem_used_max WO reset the `mem_used_max' counter (see later) +mem_limit WO specifies the maximum amount of memory ZRAM can use + to store the compressed data 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<id>/mm_stat node) compact WO trigger memory compaction +debug_stat RO this file is used for zram debugging purposes -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<id>/stat -or zram<id>/io_stat or zram<id>/mm_stat) is considered to be deprecated. User space is advised to use the following files to read the device statistics. @@ -211,22 +183,40 @@ The stat file represents device's I/O statistics not accounted by block layer and, thus, not available in zram<id>/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 + failed_reads the number of failed reads + failed_writes the number of failed writes + invalid_io the number of non-page-size-aligned I/O requests + notify_free Depending on device usage scenario it may account + a) the number of pages freed because of swap slot free + notifications or b) the number of pages freed because of + REQ_DISCARD requests sent by bio. The former ones are + sent to a swap block device when a swap slot is freed, + which implies that this disk is being used as a swap disk. + The latter ones are sent by filesystem mounted with + discard option, whenever some data blocks are getting + discarded. File /sys/block/zram<id>/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 + orig_data_size uncompressed size of data stored in this disk. + This excludes same-element-filled pages (same_pages) since + no memory is allocated for them. + Unit: bytes + compr_data_size compressed size of data stored in this disk + mem_used_total the amount of memory allocated for this disk. This + includes allocator fragmentation and metadata overhead, + allocated for this disk. So, allocator space efficiency + can be calculated using compr_data_size and this statistic. + Unit: bytes + mem_limit the maximum amount of memory ZRAM can use to store + the compressed data + mem_used_max the maximum amount of memory zram have consumed to + store the data + same_pages the number of same element filled pages written to this disk. + No memory is allocated for such pages. + pages_compacted the number of pages freed during compaction 9) Deactivate: swapoff /dev/zram0 |