131 строка
4.1 KiB
Plaintext
131 строка
4.1 KiB
Plaintext
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) Set memory limit: Optional
|
|
Set memory limit by writing the value to sysfs node 'mem_limit'.
|
|
The value can be either in bytes or you can use mem suffixes.
|
|
In addition, you could change the value in runtime.
|
|
Examples:
|
|
# limit /dev/zram0 with 50MB memory
|
|
echo $((50*1024*1024)) > /sys/block/zram0/mem_limit
|
|
|
|
# Using mem suffixes
|
|
echo 256K > /sys/block/zram0/mem_limit
|
|
echo 512M > /sys/block/zram0/mem_limit
|
|
echo 1G > /sys/block/zram0/mem_limit
|
|
|
|
# To disable memory limit
|
|
echo 0 > /sys/block/zram0/mem_limit
|
|
|
|
6) Activate:
|
|
mkswap /dev/zram0
|
|
swapon /dev/zram0
|
|
|
|
mkfs.ext4 /dev/zram1
|
|
mount /dev/zram1 /tmp
|
|
|
|
7) 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
|
|
mem_used_max
|
|
|
|
8) Deactivate:
|
|
swapoff /dev/zram0
|
|
umount /dev/zram1
|
|
|
|
9) 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
|