Merge branch 'async-scsi-resume' of git://git.kernel.org/pub/scm/linux/kernel/git...

[linux.git] / Documentation / blockdev / zram.txt

diff --git a/Documentation/blockdev/zram.txt b/Documentation/blockdev/zram.txt
index 2eccddffa6c8fde5cdd4d5cc49cecab0b82b8206..0595c3f56ccfaadaa8f36d76021d8c6ce1a66140 100644 (file)
--- a/Documentation/blockdev/zram.txt
+++ b/Documentation/blockdev/zram.txt
@@ -21,7 +21,43 @@ 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
+
+Note:
+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 1), you will see error if you try to change
+the value of max_comp_streams because single stream compression backend
+implemented as a special case by lock overhead issue and does not support
+dynamic max_comp_streams. Only multi stream backend supports dynamic
+max_comp_streams adjustment.
+
+3) Select compression algorithm
+       Using comp_algorithm device attribute one can see available and
+       currently selected (shown in square brackets) compression algortithms,
+       change 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
+
+4) 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:
@@ -33,32 +69,38 @@ Following shows a typical sequence of steps for using zram.
             echo 512M > /sys/block/zram0/disksize
             echo 1G > /sys/block/zram0/disksize
 
-3) Activate:
+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.
+
+5) Activate:
        mkswap /dev/zram0
        swapon /dev/zram0
 
        mkfs.ext4 /dev/zram1
        mount /dev/zram1 /tmp
 
-4) Stats:
+6) Stats:
        Per-device statistics are exported as various nodes under
        /sys/block/zram<id>/
                disksize
                num_reads
                num_writes
+               failed_reads
+               failed_writes
                invalid_io
                notify_free
-               discard
                zero_pages
                orig_data_size
                compr_data_size
                mem_used_total
 
-5) Deactivate:
+7) Deactivate:
        swapoff /dev/zram0
        umount /dev/zram1
 
-6) Reset:
+8) Reset:
        Write any positive value to 'reset' sysfs node
        echo 1 > /sys/block/zram0/reset
        echo 1 > /sys/block/zram1/reset