Zram

Few months ago, i tried very cool feature called ‘zram’. It is linux kernel module that allow to create compressed block devices into memory, it can be used for creating compressed fs in ram  (/tmp for example) or for swap.
May be you think, that in context of swap, it is dumb to keeping  memory pages in memory when OS need that memory. =) But compress and store pages in memory is faster than write it onto disk, in most cases memory pages can be heavily compressed, that will help OS to free RAM, if you have SSD it will save life of your disk, also you can continue using swap on disk. If you want to keep you swap partition on-line, you must give higher priority for swap in zram, when zram will full, OS will started to using swap on disk.

I used that init.d script for debian, but i changed it to use not  a whole RAM for zram devices, but half of all memory (in worst case, when pages can not be compressed, zram will use only half of my memory). If you want to do same modification, just change echo $((mem_total / num_cpus )) to echo $((mem_total / num_cpus / 2)) in that script.
Without modifications this script will slice you memory by number of CPU core in your system, create swaps on that slices and attach it to your system with priority 100 (usualy swap partitions have priority -1).
I made simple test of compression ratio for zram:
Detached one of my swaps:

$ sudo swapoff /dev/zram3

Created core file of iceweasel process and wrote it into zram:

$ pgrep -lf icewea
3375 sh -c /usr/bin/iceweasel
3376 /usr/bin/iceweasel
$ gcore 3376
[Thread debugging using libthread_db enabled]
[New Thread 0x7f7c0b9fd700 (LWP 8455)]
...blablabla...
0x00007f7c62a57c13 in poll () from /lib/libc.so.6
Saved corefile core.3376
$ sudo dd if=./core.3376 of=/dev/zram3
dd: writing to `/dev/zram3': No space left on device
2027297+0 records in
2027296+0 records out
1037975552 bytes (1,0 GB) copied, 6,79003 s, 153 MB/s

Core file does not fit completely into zram device, but it is dose not mater, let’s look at compression ratio:

$ cd /sys/block
$ echo `cat ./zram3/orig_data_size`/`cat ./zram3/compr_data_size`|bc
2.68475926964795164955

So, in most cases zram has compress ratio more than 2.5.
Huh, i think it is pretty cool.

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