zram: idle writeback fixes and cleanup

[linux] / Documentation / blockdev / zram.txt

diff --git a/Documentation/blockdev/zram.txt b/Documentation/blockdev/zram.txt
index 436c5e9..4df0ce2 100644 (file)
--- a/Documentation/blockdev/zram.txt
+++ b/Documentation/blockdev/zram.txt
@@ -156,22 +156,23 @@ Per-device statistics are exported as various nodes under /sys/block/zram<id>/
 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
-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
+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.
@@ -280,32 +281,51 @@ 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.
+
+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 want to apply writeback budget, he 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 set 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 meaninless.)
 
 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.
+       $ 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 admin want to allow further write again, he could do it like below
+If admin want to allow further write again once the bugdet is exausted,
+he could do it like below
 
-    echo 0 > /sys/block/zram0/writeback_limit
+       $ echo $((400<<MB_SHIFT>>4K_SHIFT)) > \
+               /sys/block/zram0/writeback_limit
 
 If admin want to see remaining writeback budget since he set,
 
-    cat /sys/block/zram0/writeback_limit
+       $ cat /sys/block/zramX/writeback_limit
+
+If admin want to disable writeback limit, he 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 want to measure writeback count in a certain period, he 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