On 22/07/2011 21:59, C Anthony Risinger wrote:
> is the disk's cache mode set to none [or maybe writeback]
> (virtmanager)?
I tested various options: the most important is the cache mode.
cache=none is a little bit better than cache=default (writethrough), but
there is an huge difference using cache=unsafe. This option is not
supported by virt-manager, but by kvm. Using cache=unsafe, the
installation of FreeBSD is ~100x faster:
- cache=default: write at 1 to 8 KB/sec
- cache=none: write at 40 KB/sec
- cache=unsafe: write at 1200 KB/sec
According to agraf__ on IRC (#kvm on FreeNode), the cache mode has the
following effect:
- cache=writethrough calls fsync() after every write()
- cache=none uses O_DIRECT
- cache=writeback calls fsync() when the guest issues a barrier()
(don't use O_DIRECT)
- cache=unsafe doesn't do any fsync() (but don't use O_DIRECT)
I still have to test writeback ;-)
I found another mail thread, of last summer (July 2010), which is
exactly the same problem that I had: "BTRFS: Unbelievably slow with
kvm/qemu" on this mailing list. It contains another advice:
"Make sure you build your file system with "mkfs.btrfs -m single -d
single /dev/whatever". You may well be writing duplicate copies of
everything."
http://lkml.org/lkml/2010/7/12/5
--
FreeBSD installation in VirtualBox is as fast (or maybe a little bit
slower) than the installation in kvm using cache=unsafe. I suppose that
VirtualBox uses something like cache=unsafe or cache=writeback.
Victor
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