Re: kvm-83 write performance raw

[Mark van Walraven <markv@netvalue.net.nz>]

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.