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zram: Compressed RAM based block devices
----------------------------------------
* Introduction
The zram module creates RAM based block devices named /dev/zram<id>
(<id> = 0, 1, ...). Pages written to these disks are compressed and stored
in memory itself. These disks allow very fast I/O and compression provides
good amounts of memory savings. Some of the usecases include /tmp storage,
use as swap disks, various caches under /var and maybe many more :)
Statistics for individual zram devices are exported through sysfs nodes at
/sys/block/zram<id>/
* Usage
Following shows a typical sequence of steps for using zram.
1) Load Module:
modprobe zram num_devices=4
This creates 4 devices: /dev/zram{0,1,2,3}
(num_devices parameter is optional. Default: 1)
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:
# Initialize /dev/zram0 with 50MB disksize
echo $((50*1024*1024)) > /sys/block/zram0/disksize
# Using mem suffixes
echo 256K > /sys/block/zram0/disksize
echo 512M > /sys/block/zram0/disksize
echo 1G > /sys/block/zram0/disksize
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
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
zero_pages
orig_data_size
compr_data_size
mem_used_total
7) Deactivate:
swapoff /dev/zram0
umount /dev/zram1
8) Reset:
Write any positive value to 'reset' sysfs node
echo 1 > /sys/block/zram0/reset
echo 1 > /sys/block/zram1/reset
This frees all the memory allocated for the given device and
resets the disksize to zero. You must set the disksize again
before reusing the device.
Nitin Gupta
ngupta@vflare.org
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