kowr / compcache

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zram: Compressed RAM based block devices

Project home: http://compcache.googlecode.com/

The zram module creates RAM based block devices named /dev/zram ( = 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/

Help page: http://compcache.googlecode.com/CompilingAndUsingNew

Following binaries are created:

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' (in bytes). If disksize is not given, default value of 25% of RAM is used.

# Set disksize of 50MB for /dev/zram0
echo $((50*1024*1024)) > /sys/block/zram0/disksize

NOTE: disksize cannot be changed if the disk contains any
data. So, for such a disk, you need to issue 'reset' (see below)
before you can change its disksize.

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

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

4) Statistics: Per-device statistics are exported as various nodes under /sys/block/zram/ disksize num_reads num_writes invalid_io notify_free discard zero_pages orig_data_size compr_data_size mem_used_total

A helper script is included (sub-projects/scripts/zram_stats)
which shows these stats for devices containing any data. It also
shows (derived) values for average compression ratio and memory
overhead.

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).

Please report any problems at:

Nitin Gupta ngupta@vflare.org