zram: factor out single stream compression

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

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 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.

3) Activate:
	mkswap /dev/zram0
	swapon /dev/zram0

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

4) 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

5) Deactivate:
	swapoff /dev/zram0
	umount /dev/zram1

6) 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