zram: Compressed RAM based block devices

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

• Introduction

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

• Compiling
  • 'make': This will compile zram module against your kernel
  • Optional (HIGHLY RECOMMENDED): Only for kernels older than 2.6.35
  • Apply the patch found in compcache/patches/ directory and just compile the kernel as usual. This patch enables 'swap free notify' feature which allows the kernel to send callback to zram as soon as a swap slot becomes free. So, we can immediately free memory allocated for the corresponding page, eliminating any stale data in (compressed) memory. Currently, patches are available for 2.6.29 and 2.6.33 kernels but it should apply to 'nearby' kernel versions too.
  • Uncomment '#define CONFIG_SWAP_FREE_NOTIFY' in compcache/compat.h before compiling compcache against this patched kernel. Otherwise, this swap notify callback will not be used.
  • Note that this patch is applicable only when zram devices are intended to be used as swap disks. They do not help when used for other purposes (see examples below).

Following binaries are created:

• zram.ko (kernel driver)

• 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' (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:

• Mailing list: linux-mm-cc at laptop dot org
• Issue tracker: http://code.google.com/p/compcache/issues/list

Nitin Gupta ngupta@vflare.org