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diff --git a/Documentation/admin-guide/blockdev/zram.rst b/Documentation/admin-guide/blockdev/zram.rst new file mode 100644 index 000000000..c73b16930 --- /dev/null +++ b/Documentation/admin-guide/blockdev/zram.rst @@ -0,0 +1,440 @@ +======================================== +zram: Compressed RAM-based block devices +======================================== + +Introduction +============ + +The zram module creates RAM-based block devices named /dev/zram<id> +(<id> = 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 use cases 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<id>/ + +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 is set by the 'echo' utility, +and, in general case, something like:: + + echo 3 > /sys/block/zram0/max_comp_streams + if [ $? -ne 0 ]; then + 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 +======================================== + +Regardless of the value passed to this attribute, ZRAM will always +allocate multiple compression streams - one per online CPU - 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 have only 1 CPU online. + +To find out how many streams are currently available:: + + cat /sys/block/zram0/max_comp_streams + +3) Select compression algorithm +=============================== + +Using comp_algorithm device attribute one can see available and +currently selected (shown in square brackets) compression algorithms, +or change the 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 + +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 + + # 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 a read operation on the hot_add +attribute. This will return either the new device's device id (meaning that you +can use /dev/zram<id>) or an 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<id>/ + +A brief description of exported device attributes follows. 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 +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 +writeback_limit WO specifies the maximum amount of write IO zram + can write out to backing device as 4KB unit +writeback_limit_enable RW show and set writeback_limit feature +max_comp_streams RW the number of possible concurrent compress + operations +comp_algorithm RW show and change the compression algorithm +compact WO trigger memory compaction +debug_stat RO this file is used for zram debugging purposes +backing_dev RW set up backend storage for zram to write out +idle WO mark allocated slot as idle +====================== ====== =============================================== + + +User space is advised to use the following files to read the device statistics. + +File /sys/block/zram<id>/stat + +Represents block layer statistics. Read Documentation/block/stat.rst for +details. + +File /sys/block/zram<id>/io_stat + +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 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 + b) the number of pages freed because of + REQ_OP_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 mm_stat file represents the device's mm statistics. It consists of a single +line of text and contains the following stats separated by whitespace: + + ================ ============================================================= + orig_data_size uncompressed size of data stored in this disk. + 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 has 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 + huge_pages the number of incompressible pages + huge_pages_since the number of incompressible pages since zram set up + ================ ============================================================= + +File /sys/block/zram<id>/bd_stat + +The bd_stat file represents a device's backing device statistics. It consists of +a single line of text and contains the following stats separated by whitespace: + + ============== ============================================================= + bd_count size of data written in backing device. + Unit: 4K bytes + bd_reads the number of reads from backing device + Unit: 4K bytes + bd_writes the number of writes to backing device + Unit: 4K bytes + ============== ============================================================= + +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. + +Optional Feature +================ + +writeback +--------- + +With CONFIG_ZRAM_WRITEBACK, zram can write idle/incompressible page +to backing storage rather than keeping it in memory. +To use the feature, admin should set up backing device via:: + + echo /dev/sda5 > /sys/block/zramX/backing_dev + +before disksize setting. It supports only partitions at this moment. +If admin wants to use incompressible page writeback, they could do it via:: + + echo huge > /sys/block/zramX/writeback + +To use idle page writeback, first, user need to declare zram pages +as idle:: + + echo all > /sys/block/zramX/idle + +From now on, any pages on zram are idle pages. The idle mark +will be removed until someone requests access of the block. +IOW, unless there is access request, those pages are still idle pages. +Additionally, when CONFIG_ZRAM_MEMORY_TRACKING is enabled pages can be +marked as idle based on how long (in seconds) it's been since they were +last accessed:: + + echo 86400 > /sys/block/zramX/idle + +In this example all pages which haven't been accessed in more than 86400 +seconds (one day) will be marked idle. + +Admin can request writeback of those idle pages at right timing via:: + + echo idle > /sys/block/zramX/writeback + +With the command, zram will writeback idle pages from memory to the storage. + +Additionally, if a user choose to writeback only huge and idle pages +this can be accomplished with:: + + echo huge_idle > /sys/block/zramX/writeback + +If an admin wants to write a specific page in zram device to the backing device, +they could write a page index into the interface. + + echo "page_index=1251" > /sys/block/zramX/writeback + +If there are lots of write IO with flash device, potentially, it has +flash wearout problem so that admin needs to design write limitation +to guarantee storage health for entire product life. + +To overcome the concern, zram supports "writeback_limit" feature. +The "writeback_limit_enable"'s default value is 0 so that it doesn't limit +any writeback. IOW, if admin wants to apply writeback budget, they should +enable writeback_limit_enable via:: + + $ echo 1 > /sys/block/zramX/writeback_limit_enable + +Once writeback_limit_enable is set, zram doesn't allow any writeback +until admin sets the budget via /sys/block/zramX/writeback_limit. + +(If admin doesn't enable writeback_limit_enable, writeback_limit's value +assigned via /sys/block/zramX/writeback_limit is meaningless.) + +If admin wants to limit writeback as per-day 400M, they could do it +like below:: + + $ MB_SHIFT=20 + $ 4K_SHIFT=12 + $ echo $((400<<MB_SHIFT>>4K_SHIFT)) > \ + /sys/block/zram0/writeback_limit. + $ echo 1 > /sys/block/zram0/writeback_limit_enable + +If admins want to allow further write again once the budget is exhausted, +they could do it like below:: + + $ echo $((400<<MB_SHIFT>>4K_SHIFT)) > \ + /sys/block/zram0/writeback_limit + +If an admin wants to see the remaining writeback budget since last set:: + + $ cat /sys/block/zramX/writeback_limit + +If an admin wants to disable writeback limit, they could do:: + + $ echo 0 > /sys/block/zramX/writeback_limit_enable + +The writeback_limit count will reset whenever you reset zram (e.g., +system reboot, echo 1 > /sys/block/zramX/reset) so keeping how many of +writeback happened until you reset the zram to allocate extra writeback +budget in next setting is user's job. + +If admin wants to measure writeback count in a certain period, they could +know it via /sys/block/zram0/bd_stat's 3rd column. + +memory tracking +=============== + +With CONFIG_ZRAM_MEMORY_TRACKING, user can know information of the +zram block. It could be useful to catch cold or incompressible +pages of the process with*pagemap. + +If you enable the feature, you could see block state via +/sys/kernel/debug/zram/zram0/block_state". The output is as follows:: + + 300 75.033841 .wh. + 301 63.806904 s... + 302 63.806919 ..hi + +First column + zram's block index. +Second column + access time since the system was booted +Third column + state of the block: + + s: + same page + w: + written page to backing store + h: + huge page + i: + idle page + +First line of above example says 300th block is accessed at 75.033841sec +and the block's state is huge so it is written back to the backing +storage. It's a debugging feature so anyone shouldn't rely on it to work +properly. + +Nitin Gupta +ngupta@vflare.org |