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xen-devel
Re: [Xen-devel] Shouldn't backend devices for VMX domain disks be opened
Philip R. Auld wrote:
Hi,
Rumor has it that on Thu, Feb 02, 2006 at 04:28:37PM -0600 Steve Dobbelstein
said:
aliguori@xxxxxxxxxxxxxxxxxxxxxxx wrote on 02/02/2006 03:46:11 PM:
I would doubt it. Since it's usually opening a file, and qemu-dm is
emulating a contigous disk, you probably want the buffer cache to
reorder events.
I guess we're not usual since our backend is an LVM volume. :)
I can appreciate how writing to the buffer cache can speed up the response
to the I/O and make it more efficient in its writing to the backend device
by reordering events. However, I'm still wondering if we have a data
corruption issue should dom0 crash before it writes the data in the buffer
cache to disk, data that the domain expects to be on the disk but won't be
there when the domain is restarted.
I agree. It sounds like a correctness problem. It's just like disks
with write caching enabled.
Referring to the original question, which has been quoted away,
journaling doesn't require that data be written to disk per-say but that
writes occur in a particular order. A journal is always recoverable
given that writes occur in the expected order. A buffer cache will have
no effect on that order so you're no more likely to have corruption than
if you disabled the buffer cache.
You especially want the buffer cache if you have LVM partitions.
Sectors on an LVM disk are not necessarily contiguous and can even span
multiple disks. You definitely want the IO scheduler involved there.
If anything, what you really want (from a performance perspective) is to
disable the buffer cache in the domU and leave it enabled in the dom0
(this is what the paravirtual drivers should be doing IIRC).
Does this address your corruption concerns?
Regards,
Anthony Liguori
Are you seeing a performance improvement? Should be easy to check.
It's more about correctness and data integrity than performance.
Cheers,
Phil
We just started doing the first runs of disk performance tests when we
noticed this behavior and thought we should bring it up on the list. We
don't have enough data points to compare yet. We'll post problems/issues
if/when we find them.
Steve D.
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