RAID0 on gytha

[[!tag linux mdadm]]

I've just more than doubled my desktop's disk I/O speed. Neat, eh?

My desktop machine has four identical hard disks. I keep backups of anything I care about, and I run some disk-intensive things from time to time. Thus, I want the disk subsystem to be as fast as possible, and I don't care about using RAID to recover from disk problems.

When I initially installed the system, however, I couldn't coax the Ubuntu installer to create a RAID0 for me, so it used plain LVM only. Yesterday I spent a day to re-install the system, and got it done. Not entirely straightforward, though.

The alternate installer lets you create RAID systems. However, it fails to install a boot loader if you do that on the entire disks. The final partitioning scheme I ended up with uses LVM for the root and swap partitions, and then RAID0 for a big scratch partition. I also have /boot on a separate partition, to get the boot loader (GRUB or LILO) to work.

In other words, every disk has three partitions:

a 10 GiB LVM physical volume
a 500 MiB volume
the rest as a RAID0 physical volume

The 500 MiB volume on the first disk is used for /boot, and is unused on the three other disks, wasting 1.5 GiB, but that's small enough that I don't care.

Was this worth it? I think so. Russell Coker's bonnie++ gives me the following results for the non-striped LVM volume, i.e., essentially for one disk:

     ------Sequential Output------ --Sequential Input- --Random-
     -Per Chr- --Block-- -Rewrite- -Per Chr- --Block-- --Seeks--
Size K/sec %CP K/sec %CP K/sec %CP K/sec %CP K/sec %CP  /sec %CP
 16G 54932  78 58243  14 28037   6 58412  67 61662   6 261.2   0

In comparison, the RAID0 gets the following results:

     ------Sequential Output------ --Sequential Input- --Random-
     -Per Chr- --Block-- -Rewrite- -Per Chr- --Block-- --Seeks--
Size K/sec %CP K/sec %CP K/sec %CP K/sec %CP K/sec %CP  /sec %CP
 16G 68732  95 143030  29 65331  14 76887  97 152345  17 394.6   1

That's a speedup of about 2.5 times. I'm sure it could be better: in theory, a speedup of 4 times would be possible, but that's the theory. I'm going to be satisified with this, for now.