In recent Linux releases, it’s available a tiny module called zram, that permits us to create RAM based block devices (named /dev/zramX), which will be kept in memory as compressed data. These ram-based block devices allow very fast I/O, and compression provides a reasonable amounts of memory saving.
We can use it as a drop-in replacement for the well-known tmpfs (used for speeding up compilation tasks or for /tmp), or better as a primary swap device, that will lead to virtually increase memory capacity, at the expense of a slightly increased CPU usage to compress/decompress the swapped data.
Nowadays RAM is very cheap, so why bother with compression? Because there are some situations where you can’t upgrade memory (netbooks) or you want to over-commit real resources (virtualization hosts).
For Ubuntu Precise and later:
Starting with Ubuntu Precise, there is an official upstart script for Ubuntu by Adam Conrad to configure zram in the main repository:
sudo apt-get install zram-config
For other distributions or older Ubuntu:
Googlin’ around to find a nice way to configure zram devices as swap, I found a very nice upstart script that will create a bunch of ramz devices depending on the number of CPU cores available, with a total size of the available memory:
Copy the script to the init.d folder, mark it as executable and enable autostart on boot:
Try it manually executing it for the first time with:
Depending on the kernel version you are running, you may need to adjust the module parameter name to num_devices on line 26 to:
modprobe zram num_devices=$num_cpus
or keep as is for newer kernels:
modprobe zram zram_num_devices=$num_cpus
Checking if it’s working
If everything went smooth, you will find a few notices on dmesg:
zram: module is from the staging directory, the quality is unknown, you have been warned.
zram: Creating 4 devices ...
Adding 1497864k swap on /dev/zram0. Priority:100 extents:1 across:1497864k SS
Adding 1497864k swap on /dev/zram1. Priority:100 extents:1 across:1497864k SS
Adding 1497864k swap on /dev/zram2. Priority:100 extents:1 across:1497864k SS
Adding 1497864k swap on /dev/zram3. Priority:100 extents:1 across:1497864k SS
meaning that the zram device have been created and enabled as swap devices with highest priority.
You can discover the increased swap space available with free -m:
Time is almost ripe for start using the native ZFS port on Linux (http://zfsonlinux.org/), and to increase the performances, reliability and space usage of our affordable distributed opensource storage solution.
Installing ZFS on Debian/Ubuntu is straightforward: you need first to build the SPL (Solaris Porting Layer) and after ZFS itself.
sudo apt-get install zlib1g-dev uuid-dev libblkid-dev libselinux-dev parted lsscsi
dpkg -i *.deb
ZFS is now ready to be used.
Let’s create our first pool with:
zpool create tank raidz [devices]
Devices can be partitions, UUIDs or entire disk. Is often a very good practice to use an entire disk using the disks id as found on /dev/disk/by-id/* (it should be advisable to not mix up existing drives with one of an existing volume).
sudo zpool status tank
Check pool status. :)
Now, the interesting features:
zfs set compression=on tank
zfs set dedup=on tank
Et voilà, your space usage is highly optimized compressing and deduplicating data.
Now it’s the GlusterFS turn, download the latest version and install it:
GlusterFS replica 2 on 2 servers, anything unexpected here.
root@debz-1:/gtank# ls -lh
-rw-r--r-- 1 scorp scorp 253K 1 gen 1980 Fattura Garanzia redcoon dns-323.pdf
-rw-r--r-- 1 scorp scorp 68K 1 gen 1980 fattura scontrino eeepc 900a.pdf
-rw-r--r-- 1 scorp scorp 55K 1 gen 1980 Fattura Scontrino WD HD Caviar Green Videofantasy.pdf
-rw-r--r-- 1 scorp scorp 681M 23 ago 19.29 lubuntu-11.04-desktop-i386.iso
-rw-r--r-- 1 scorp scorp 1021K 1 gen 1980 Scontrino xbox 360.pdf
-rw-r--r-- 1 root root 98M 27 ago 12.00 test2_random.dat
-rw-r--r-- 1 root root 98M 27 ago 11.59 test_random_copy.dat
-rw-r--r-- 1 root root 98M 27 ago 11.55 test_random.dat
-rw-r--r-- 1 root root 293M 27 ago 12.06 test_random-with-zero-hole.dat
-rw-r--r-- 1 root root 98M 27 ago 11.53 test_zero.dat
-rw-r--r-- 1 root root 674M 18 ago 16.43 ubuntu-11.04-server-amd64.iso
Here I’ve copied some PDFs, 2 ISO, and some dd generated file:
test_zero.dat is a dd if=/dev/zero
test_random.dat and test2_random.dat are 2 different iteration with dd if=/dev/urandom
test_random_copy.dat is a cp of test_random.dat
test_random-with-zero-hole.dat is the result of cat test_random.dat + test_random.dat + test2_random.dat
The following are the real disk usage:
root@debz-1:/gtank# du -sh *
259K Fattura Garanzia redcoon dns-323.pdf
69K fattura scontrino eeepc 900a.pdf
55K Fattura Scontrino WD HD Caviar Green Videofantasy.pdf
1,1M Scontrino xbox 360.pdf
As you can see, compression is doing its work with ISO and test_zero.dat, but isn’t effective with PDFs and random data (do you remember that if you zip an already zip file the total size will increase?).
And what about the dedup? You should check it with:
root@debz-1:/gtank# zpool list
NAME SIZE ALLOC FREE CAP DEDUP HEALTH ALTROOT
tank 9,94G 1,60G 8,34G 16% 1.20x ONLINE -
so there are ~ 200 MB of available deduped space.
Other “batteries included” functions with ZFS are:
Decrease disk I/O bottleneck using fast SSDs as caches with L2ARC
Copy-on-write transactions: no need for fsck after hard reboot, data is always consistent on disk.
Online Repair: ZFS store a checksum for every data block, and can notify data alternation on avery access or during a scheduled online scrub operation
Let me know if you have some good usage tips to submit!