Re: kvm-83 write performance raw

[Nikola Ciprich <extmaillist@linuxbox.cz>]

Hi,
I think DRBD *MIGHT* be Your problem anyways...
Can You try repeating Your measurments with
no-disk-barrier, no-disk-flushes, no-disk-drain
options for Your drbd devices and report the results?
nik

On Mon, Mar 02, 2009 at 04:00:57PM -0500, Malinka Rellikwodahs wrote:
> On Mon, Mar 2, 2009 at 15:53, Mark van Walraven <markv@netvalue.net.nz> wrote:
> > 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,
> 
> It does, however unless I'm missing something the performance is being
> lost not in the lvm/raid/drbd config, because I'm using the same setup
> for other partitions which are used for data on the host and write
> performance to those drives is just fine.
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