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-rw-r--r--Documentation/blockdev/zram.txt74
1 files changed, 47 insertions, 27 deletions
diff --git a/Documentation/blockdev/zram.txt b/Documentation/blockdev/zram.txt
index 436c5e98e1b6..4df0ce271085 100644
--- 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