KVM I/O question for VPS providers

[libro22]

I've read about KVM I/O and the first issue I'm concerned with is the best way to store virtual disks for the guest VMs? They say using LVM is better than disk file images. Assuming there is hardware RAID in place, what's the best way to go? Also, raw or qcow2 or other storage format?

Any pros and cons for LVM against other backend storing types (i.e. caching, sudden power failure)?

I hope to see some nice answers regarding this matter with the goal of improving I/O on guests nodes.

PS. I'm not going to sell VPSs lol

 

[splitice]

I dont do much with it myself but raw is apparently faster than qcow2.

 

[perennate]

There's no single optimal storage format that works on all hardware (or even all RAID setups). Also you have to balance reliability vs I/O speed, for example you can enable writeback disk caching which usually speeds up disk operations, but depending on your filesystem there is possibility of disk corruption in event of power failure (newest ext3 and also ext4 are said to be fully safe though; the writeback cache is exposed to guest OS so guest can determine how to use it). The "best way" would be to test each configuration and see what fits your needs; for qcow2/raw (you may also want to test with sparse raw file vs preallocated raw, also qcow2 offers metadata preallocation) and none/writeback/writethrough it is pretty easy to test the combinations, lvm may be slightly more difficult but still won't take you too long.