zram: Compressed RAM based block devices
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* 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