========================================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 storedin memory itself. These disks allow very fast I/O and compression providesgood 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 attributesb) 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-linuxdocumentation, zramctl man-page or `zramctl --help`. Please be informedthat zram maintainers do not develop/maintain util-linux or zramctl, shouldyou 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 theexamples 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.-EAGAIN re-try operation later (e.g. when attempting to run recompress and writeback simultaneously).======== ============================================================= If you use 'echo', the returned value is set by the 'echo' utility,and, in general case, something like:: echo foo > /sys/block/zram0/comp_algorithm if [ $? -ne 0 ]; then handle_error fi should suffice. 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 bepre-created. Default: 1. Select compression algorithm============================ Using comp_algorithm device attribute one can see available andcurrently selected (shown in square brackets) compression algorithms,or change the selected compression algorithm (once the device is initialisedthere 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 shows only compressionalgorithms that are supported by zram. Set compression algorithm parameters: Optional============================================== Compression algorithms may support specific parameters which can betweaked for particular dataset. ZRAM has an `algorithm_params` deviceattribute which provides a per-algorithm params configuration. For example, several compression algorithms support `level` parameter.In addition, certain compression algorithms support pre-trained dictionaries,which significantly change algorithms' characteristics. In order to configurecompression algorithm to use external pre-trained dictionary, pass fullpath to the `dict` along with other parameters:: #pass path to pre-trained zstd dictionary echo "algo=zstd dict=/etc/dictionary" > /sys/block/zram0/algorithm_params #pass path to pre-trained zstd dictionary and compression level echo "algo=zstd level=8 dict=/etc/dictionary" > \ /sys/block/zram0/algorithm_params #same, but using algorithm priority echo "algo=zstd priority=1" > /sys/block/zram0/recomp_algorithm echo "priority=1 dict=/etc/dictionary" > \ /sys/block/zram0/algorithm_params Each write to `algorithm_params` replaces the entire set of parameters ofthe corresponding algorithm, parameters that are not listed in the writeare reset to their default values. Configure all of the parameters of analgorithm in one write:: #WRONG: the second write resets level back to its default value echo "algo=zstd level=8" > /sys/block/zram0/algorithm_params echo "algo=zstd dict=/etc/dictionary" > /sys/block/zram0/algorithm_params #RIGHT echo "algo=zstd level=8 dict=/etc/dictionary" > \ /sys/block/zram0/algorithm_params Select the compression algorithm before configuring its parameters. Theparameters of one algorithm are not necessarily valid for another one, sochanging the algorithm of a particular priority resets that priority'sparameters:: #WRONG: comp_algorithm write resets the previously configured level echo "level=8" > /sys/block/zram0/algorithm_params echo zstd > /sys/block/zram0/comp_algorithm #RIGHT echo zstd > /sys/block/zram0/comp_algorithm echo "algo=zstd level=8" > /sys/block/zram0/algorithm_params Parameters are algorithm specific: not all algorithms support pre-traineddictionaries, not all algorithms support `level`. Furthermore, for certainalgorithms `level` controls the compression level (the higher the value thebetter the compression ratio, it even can take negatives values for somealgorithms), for other algorithms `level` is acceleration level (the higherthe value the lower the compression ratio). Parameters are handed over to the compression algorithm when the device isinitialised, hence invalid parameters (or parameters that the selectedalgorithm does not support) are reported by the `disksize` write, and notby the `algorithm_params` write that has configured them. 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 memorysince we expect a 2:1 compression ratio. Note that zram uses about 0.1% of thesize of the disk when not in use so a huge zram is wasteful. 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 Activate======== :: mkswap /dev/zram0 swapon /dev/zram0 mkfs.ext4 /dev/zram1 mount /dev/zram1 /tmp Add/remove zram devices======================= zram provides a control interface, which enables dynamic (on-demand) deviceaddition and removal. In order to add a new /dev/zramX device, perform a read operation on the hot_addattribute. This will return either the new device's device id (meaning that youcan 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 Stats===== Per-device statistics are exported as various nodes under /sys/block/zram<id>/ A brief description of exported device attributes follows. For more detailsplease read Documentation/ABI/testing/sysfs-block-zram. ====================== ====== ===============================================Name access description====================== ====== ===============================================disksize RW show and set the device's disk sizeinitstate RO shows the initialization state of the devicereset WO trigger device resetmem_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 datawriteback_limit WO specifies the maximum amount of write IO zram can write out to backing device as 4KB unitwriteback_limit_enable RW show and set writeback_limit featurewriteback_batch_size RW show and set maximum number of in-flight writeback operationscompressed_writeback RW show and set compressed writeback featurecomp_algorithm RW show and change the compression algorithmalgorithm_params WO setup compression algorithm parameterscompact WO trigger memory compactiondebug_stat RO this file is used for zram debugging purposesbacking_dev RW set up backend storage for zram to write outidle 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 fordetails. File /sys/block/zram<id>/io_stat The stat file represents device's I/O statistics not accounted by blocklayer and, thus, not available in zram<id>/stat file. It consists of asingle line of text and contains the following stats separated bywhitespace: ============= ============================================================= 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 singleline 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 ofa 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 ============== ============================================================= Deactivate========== :: swapoff /dev/zram0 umount /dev/zram1 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================ IDLE pages tracking------------------- zram has built-in support for idle pages tracking (that is, allocated butnot used pages). This feature is useful for e.g. zram writeback andrecompression. In order to mark pages as idle, execute the following command:: echo all > /sys/block/zramX/idle This will mark all allocated zram pages as idle. The idle mark will beremoved only when the page (block) is accessed (e.g. overwritten or freed).Additionally, when CONFIG_ZRAM_TRACK_ENTRY_ACTIME is enabled, pages can bemarked as idle based on how many seconds have passed since the last access toa particular zram page:: echo 86400 > /sys/block/zramX/idle In this example, all pages which haven't been accessed in more than 86400seconds (one day) will be marked idle. writeback--------- With CONFIG_ZRAM_WRITEBACK, zram can write idle/incompressible pageto 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 Admin can request writeback of 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 pagesthis can be accomplished with:: echo huge_idle > /sys/block/zramX/writeback If a user chooses to writeback only incompressible pages (pages that none ofalgorithms can compress) this can be accomplished with:: echo incompressible > /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 In Linux 6.16 this interface underwent some rework. First, the interfacenow supports `key=value` format for all of its parameters (`type=huge_idle`,etc.) Second, the support for `page_indexes` was introduced, which specify`LOW-HIGH` range (or ranges) of pages to be written-back. This reduces thenumber of syscalls, but more importantly this enables optimal post-processingtarget selection strategy. Usage example:: echo "type=idle" > /sys/block/zramX/writeback echo "page_indexes=1-100 page_indexes=200-300" > \ /sys/block/zramX/writeback We also now permit multiple page_index params per call and a mix ofsingle pages and page ranges:: echo page_index=42 page_index=99 page_indexes=100-200 \ page_indexes=500-700 > /sys/block/zramX/writeback If there are lots of write IO with flash device, potentially, it hasflash wearout problem so that admin needs to design write limitationto 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 limitany writeback. IOW, if admin wants to apply writeback budget, they shouldenable writeback_limit_enable via:: $ echo 1 > /sys/block/zramX/writeback_limit_enable Once writeback_limit_enable is set, zram doesn't allow any writebackuntil admin sets the budget via /sys/block/zramX/writeback_limit. (If admin doesn't enable writeback_limit_enable, writeback_limit's valueassigned via /sys/block/zramX/writeback_limit is meaningless.) If admin wants to limit writeback as per-day 400M, they could do itlike 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 ofwriteback happened until you reset the zram to allocate extra writebackbudget in next setting is user's job. By default zram stores written back pages in decompressed (raw) form, whichmeans that writeback operation involves decompression of the page beforewriting it to the backing device. This behavior can be changed by enabling`compressed_writeback` feature, which causes zram to write compressed pagesto the backing device, thus avoiding decompression overhead. To enablethis feature, execute:: $ echo yes > /sys/block/zramX/compressed_writeback Note that this feature should be configured before the `zramX` device isinitialized. Depending on backing device storage type, writeback operation may benefitfrom a higher number of in-flight write requests (batched writes). Thenumber of maximum in-flight writeback operations can be configured via`writeback_batch_size` attribute. To change the default value (which is 32),execute:: $ echo 64 > /sys/block/zramX/writeback_batch_size If admin wants to measure writeback count in a certain period, they couldknow it via /sys/block/zram0/bd_stat's 3rd column. recompression------------- With `CONFIG_ZRAM_MULTI_COMP`, zram can recompress pages using alternative(secondary) compression algorithms. The basic idea is that alternativecompression algorithm can provide better compression ratio at a price of(potentially) slower compression/decompression speeds. Alternative compressionLines 1–500 of 610. Load more
======================================== 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. -EAGAIN re-try operation later (e.g. when attempting to run recompress and writeback simultaneously). ======== ============================================================= If you use 'echo', the returned value is set by the 'echo' utility, and, in general case, something like:: echo foo > /sys/block/zram0/comp_algorithm if [ $? -ne 0 ]; then handle_error fi should suffice. 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. 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 shows only compression algorithms that are supported by zram. Set compression algorithm parameters: Optional ============================================== Compression algorithms may support specific parameters which can be tweaked for particular dataset. ZRAM has an `algorithm_params` device attribute which provides a per-algorithm params configuration. For example, several compression algorithms support `level` parameter. In addition, certain compression algorithms support pre-trained dictionaries, which significantly change algorithms' characteristics. In order to configure compression algorithm to use external pre-trained dictionary, pass full path to the `dict` along with other parameters:: #pass path to pre-trained zstd dictionary echo "algo=zstd dict=/etc/dictionary" > /sys/block/zram0/algorithm_params #pass path to pre-trained zstd dictionary and compression level echo "algo=zstd level=8 dict=/etc/dictionary" > \ /sys/block/zram0/algorithm_params #same, but using algorithm priority echo "algo=zstd priority=1" > /sys/block/zram0/recomp_algorithm echo "priority=1 dict=/etc/dictionary" > \ /sys/block/zram0/algorithm_params Each write to `algorithm_params` replaces the entire set of parameters of the corresponding algorithm, parameters that are not listed in the write are reset to their default values. Configure all of the parameters of an algorithm in one write:: #WRONG: the second write resets level back to its default value echo "algo=zstd level=8" > /sys/block/zram0/algorithm_params echo "algo=zstd dict=/etc/dictionary" > /sys/block/zram0/algorithm_params #RIGHT echo "algo=zstd level=8 dict=/etc/dictionary" > \ /sys/block/zram0/algorithm_params Select the compression algorithm before configuring its parameters. The parameters of one algorithm are not necessarily valid for another one, so changing the algorithm of a particular priority resets that priority's parameters:: #WRONG: comp_algorithm write resets the previously configured level echo "level=8" > /sys/block/zram0/algorithm_params echo zstd > /sys/block/zram0/comp_algorithm #RIGHT echo zstd > /sys/block/zram0/comp_algorithm echo "algo=zstd level=8" > /sys/block/zram0/algorithm_params Parameters are algorithm specific: not all algorithms support pre-trained dictionaries, not all algorithms support `level`. Furthermore, for certain algorithms `level` controls the compression level (the higher the value the better the compression ratio, it even can take negatives values for some algorithms), for other algorithms `level` is acceleration level (the higher the value the lower the compression ratio). Parameters are handed over to the compression algorithm when the device is initialised, hence invalid parameters (or parameters that the selected algorithm does not support) are reported by the `disksize` write, and not by the `algorithm_params` write that has configured them. 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. 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 Activate ======== :: mkswap /dev/zram0 swapon /dev/zram0 mkfs.ext4 /dev/zram1 mount /dev/zram1 /tmp 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 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 writeback_batch_size RW show and set maximum number of in-flight writeback operations compressed_writeback RW show and set compressed writeback feature comp_algorithm RW show and change the compression algorithm algorithm_params WO setup compression algorithm parameters 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 ============== ============================================================= Deactivate ========== :: swapoff /dev/zram0 umount /dev/zram1 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 ================ IDLE pages tracking ------------------- zram has built-in support for idle pages tracking (that is, allocated but not used pages). This feature is useful for e.g. zram writeback and recompression. In order to mark pages as idle, execute the following command:: echo all > /sys/block/zramX/idle This will mark all allocated zram pages as idle. The idle mark will be removed only when the page (block) is accessed (e.g. overwritten or freed). Additionally, when CONFIG_ZRAM_TRACK_ENTRY_ACTIME is enabled, pages can be marked as idle based on how many seconds have passed since the last access to a particular zram page:: 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. 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 Admin can request writeback of 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 a user chooses to writeback only incompressible pages (pages that none of algorithms can compress) this can be accomplished with:: echo incompressible > /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 In Linux 6.16 this interface underwent some rework. First, the interface now supports `key=value` format for all of its parameters (`type=huge_idle`, etc.) Second, the support for `page_indexes` was introduced, which specify `LOW-HIGH` range (or ranges) of pages to be written-back. This reduces the number of syscalls, but more importantly this enables optimal post-processing target selection strategy. Usage example:: echo "type=idle" > /sys/block/zramX/writeback echo "page_indexes=1-100 page_indexes=200-300" > \ /sys/block/zramX/writeback We also now permit multiple page_index params per call and a mix of single pages and page ranges:: echo page_index=42 page_index=99 page_indexes=100-200 \ page_indexes=500-700 > /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. By default zram stores written back pages in decompressed (raw) form, which means that writeback operation involves decompression of the page before writing it to the backing device. This behavior can be changed by enabling `compressed_writeback` feature, which causes zram to write compressed pages to the backing device, thus avoiding decompression overhead. To enable this feature, execute:: $ echo yes > /sys/block/zramX/compressed_writeback Note that this feature should be configured before the `zramX` device is initialized. Depending on backing device storage type, writeback operation may benefit from a higher number of in-flight write requests (batched writes). The number of maximum in-flight writeback operations can be configured via `writeback_batch_size` attribute. To change the default value (which is 32), execute:: $ echo 64 > /sys/block/zramX/writeback_batch_size If admin wants to measure writeback count in a certain period, they could know it via /sys/block/zram0/bd_stat's 3rd column. recompression ------------- With `CONFIG_ZRAM_MULTI_COMP`, zram can recompress pages using alternative (secondary) compression algorithms. The basic idea is that alternative compression algorithm can provide better compression ratio at a price of (potentially) slower compression/decompression speeds. Alternative compression