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Subject[PATCHv7 7/7] zram: document max_comp_streams
Add max_comp_streams device attribute documentation.

Signed-off-by: Sergey Senozhatsky <>
Documentation/ABI/testing/sysfs-block-zram | 9 ++++++++-
Documentation/blockdev/zram.txt | 31 +++++++++++++++++++++++++-----
2 files changed, 34 insertions(+), 6 deletions(-)

diff --git a/Documentation/ABI/testing/sysfs-block-zram b/Documentation/ABI/testing/sysfs-block-zram
index 8aa0468..0da9ed6 100644
--- a/Documentation/ABI/testing/sysfs-block-zram
+++ b/Documentation/ABI/testing/sysfs-block-zram
@@ -50,7 +50,6 @@ Description:
The failed_reads file is read-only and specifies the number of
failed reads happened on this device.

What: /sys/block/zram<id>/failed_writes
Date: February 2014
Contact: Sergey Senozhatsky <>
@@ -58,6 +57,14 @@ Description:
The failed_writes file is read-only and specifies the number of
failed writes happened on this device.

+What: /sys/block/zram<id>/max_comp_streams
+Date: February 2014
+Contact: Sergey Senozhatsky <>
+ The max_comp_streams file is read-write and specifies the
+ number of backend's zcomp_strm compression streams (number of
+ concurrent compress operations).
What: /sys/block/zram<id>/notify_free
Date: August 2010
Contact: Nitin Gupta <>
diff --git a/Documentation/blockdev/zram.txt b/Documentation/blockdev/zram.txt
index b31ac5e..d7f2f8f 100644
--- a/Documentation/blockdev/zram.txt
+++ b/Documentation/blockdev/zram.txt
@@ -21,7 +21,28 @@ Following shows a typical sequence of steps for using zram.
This creates 4 devices: /dev/zram{0,1,2,3}
(num_devices parameter is optional. Default: 1)

-2) Set Disksize
+2) Set max number of compression streams
+ Compression backend may use up to max_comp_streams compression streams,
+ thus allowing up to max_comp_streams concurrent compression operations.
+ By default, compression backend uses single compression stream.
+ Examples:
+ #show max compression streams number
+ cat /sys/block/zram0/max_comp_streams
+ #set max compression streams number to 3
+ echo 3 > /sys/block/zram0/max_comp_streams
+In order to enable compression backend's multi stream support max_comp_streams must
+be initially set to desired concurrency level before ZRAM device initialisation.
+Once the device initialised as a single stream compression backend (max_comp_streams
+equals to 0) changing the value of max_comp_streams will not take any effect, because
+single stream compression backend implemented as a special case and does not support
+dynamic max_comp_streams. Multi stream backend supports dynamic max_comp_streams
+adjustment in range [2, N].
+3) 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.
@@ -38,14 +59,14 @@ 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.

-3) Activate:
+4) Activate:
mkswap /dev/zram0
swapon /dev/zram0

mkfs.ext4 /dev/zram1
mount /dev/zram1 /tmp

-4) Stats:
+5) Stats:
Per-device statistics are exported as various nodes under
@@ -60,11 +81,11 @@ size of the disk when not in use so a huge zram is wasteful.

-5) Deactivate:
+6) Deactivate:
swapoff /dev/zram0
umount /dev/zram1

-6) Reset:
+7) Reset:
Write any positive value to 'reset' sysfs node
echo 1 > /sys/block/zram0/reset
echo 1 > /sys/block/zram1/reset

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