diff options
Diffstat (limited to 'Documentation/blockdev/zram.txt')
| -rw-r--r-- | Documentation/blockdev/zram.txt | 80 |
1 files changed, 72 insertions, 8 deletions
diff --git a/Documentation/blockdev/zram.txt b/Documentation/blockdev/zram.txt index 3c1b5ab54bc0..436c5e98e1b6 100644 --- a/Documentation/blockdev/zram.txt +++ b/Documentation/blockdev/zram.txt @@ -164,11 +164,14 @@ 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 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. @@ -220,6 +223,17 @@ line of text and contains the following stats separated by whitespace: pages_compacted the number of pages freed during compaction huge_pages the number of incompressible pages +File /sys/block/zram<id>/bd_stat + +The stat file represents 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 @@ -237,11 +251,60 @@ line of text and contains the following stats separated by whitespace: = writeback -With incompressible pages, there is no memory saving with zram. -Instead, with CONFIG_ZRAM_WRITEBACK, zram can write incompressible page +With CONFIG_ZRAM_WRITEBACK, zram can write idle/incompressible page to backing storage rather than keeping it in memory. -User should set up backing device via /sys/block/zramX/backing_dev -before disksize setting. +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 partition at this moment. +If admin want to use incompressible page writeback, they could do via + + "echo huge > /sys/block/zramX/write" + +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 request access of the block. +IOW, unless there is access request, those pages are still idle pages. + +Admin can request writeback of those idle pages at right timing via + + "echo idle > /sys/block/zramX/writeback" + +With the command, zram writeback idle pages from memory to the storage. + +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". +The "writeback_limit"'s default value is 0 so that it doesn't limit +any writeback. If admin want to measure writeback count in a certain +period, he could know it via /sys/block/zram0/bd_stat's 3rd column. + +If admin want to limit writeback as per-day 400M, he could do it +like below. + + MB_SHIFT=20 + 4K_SHIFT=12 + echo $((400<<MB_SHIFT>>4K_SHIFT)) > \ + /sys/block/zram0/writeback_limit. + +If admin want to allow further write again, he could do it like below + + echo 0 > /sys/block/zram0/writeback_limit + +If admin want to see remaining writeback budget since he set, + + cat /sys/block/zram0/writeback_limit + +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. = memory tracking @@ -251,16 +314,17 @@ 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 ..h + 300 75.033841 .wh. + 301 63.806904 s... + 302 63.806919 ..hi First column is zram's block index. Second column is access time since the system was booted Third column is state of the block. (s: same page w: written page to backing store -h: huge page) +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 |