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Re: [Qemu-devel] [RFC] Disk integrity in QEMU
From: |
Jamie Lokier |
Subject: |
Re: [Qemu-devel] [RFC] Disk integrity in QEMU |
Date: |
Mon, 13 Oct 2008 19:22:31 +0100 |
User-agent: |
Mutt/1.5.13 (2006-08-11) |
Rik van Riel wrote:
> >When cache=on, read requests may not actually go to the disk. If a
> >previous read request (by some application on the system) has read the
> >same data, then it becomes a simple memcpy(). Also, the host IO
> >scheduler may do read ahead which means that the data may be available
> >from that.
>
> This can be as much of a data integrity problem as
> asynchronous writes, if various qemu/kvm guests are
> accessing the same disk image with a cluster filesystem
> like GFS.
If there are multiple qemu/kvm guests accessing the same disk image in
a cluster, provided the host cluster filesystem uses a fully coherent
protocol, ordinary cached reads should be fine. (E.g. not NFS).
The behaviour should be equivalent to a "virtual SAN".
(Btw, some other OSes have an O_RSYNC flag to force reads to hit the
media, much as O_DSYNC forces writes to. That might be relevant to
accessing a disk image file on non-coherent cluster filesystems, but I
wouldn't recommend that.)
-- Jamie
Re: [Qemu-devel] Re: [RFC] Disk integrity in QEMU, Mark Wagner, 2008/10/13
Re: [Qemu-devel] [RFC] Disk integrity in QEMU, Rik van Riel, 2008/10/13
- Re: [Qemu-devel] [RFC] Disk integrity in QEMU,
Jamie Lokier <=
Re: [Qemu-devel] [RFC] Disk integrity in QEMU, Ian Jackson, 2008/10/28