Re: KVM on top of BTRFS

[Roman Mamedov <rm@romanrm.ru>]

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On Mon, 11 Jun 2012 23:53:34 +0000 (UTC)
Alex <alex@bpmit.com> wrote:

> Running KVM image files on btrfs as of yesterday.
> Used mkfs.btrfs -l 32k -n 32k to create and default options only (no mount
> options bar defaults, so no compression (just asking for trouble!), or
> autodefrag). 3.4.1 official Debian AMD64 kernel. Multi-subvol including root
> with set-default enacted. 32k's adopted per Chris's post.
> 
> Install:
> virt-install --connect qemu:///system -n china -r 256 --disk
> path=/var/lib/libvirt/images/china.img,size=4 -c
> /home/alex/debian-testing-amd64-CD-1.iso --vnc --noautoconsole --os-type linux
> --os-variant debianwheezy --accelerate --network=bridge:br0 --hvm 
> 
> This runs much slower than expected - have done many debian bare minimum
> installs like this before. Can't hear any disk thrashing. Doesn't appear to be
> CPU or memory bound - will double check.


From man kvm:

           cache=cache
               cache is "none", "writeback", "unsafe", "directsync" or
               "writethrough" and controls how the host cache is used to
               access block data.

The default setting is writethrough, can you try writeback, then unsafe, to
see if they help increase performance in your case? AFAIK the default setting
may cause KVM to call sync a bit too often, and sync can be slow on btrfs.

-- 
With respect,
Roman

~~~~~~~~~~~~~~~~~~~~~~~~~~~
"Stallman had a printer,
with code he could not see.
So he began to tinker,
and set the software free."

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