On Mon, Mar 02, 2009 at 03:11:59PM -0500, Malinka Rellikwodahs wrote: > when running with a raw disk image as a file or a raw disk image on an > lvm vg, I'm getting very low performance on write (5-10 MB/s) however > when using qcow2 format disk image the write speed is much better > (~30MB/s), which is consistant with a very similar setup running > kvm-68. Unfortunately when running the test with qcow2 the system > becomes unresponsive for a brief time during the test. > The host is running raid5 and drbd (drive replication software), > however performance on the host is performaning well and avoiding the > drbd layer in the guest does not improve performance, but running on > qcow2 does. > > Any thoughts/suggestions of what could be wrong or what to do to fix this? RAID1 has *much* better write performance. With striping RAIDs, alignment is important. RAID controllers sometimes introduce hidden alignment offsets. Excessive read-ahead is a waste of time with a lot of small random I/O, which is what I see mostly with guests on flat disk images. With LVM, it pays to make sure the LVs are aligned to the disk. I prefer boundaries with multiples of at least 64-sectors, which makes the LVM overhead virtually disappear. I align the guest filesystems too, when I can. I don't think DRBD has an effect on alignment, but you might look at keeping the metadata on another drive. Block - rather than file - images are much faster. Hope this helps, Mark. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
