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Re: [Qemu-devel] Re: [PATCH 2/2] Add flush=off parameter to -drive
From: |
Jamie Lokier |
Subject: |
Re: [Qemu-devel] Re: [PATCH 2/2] Add flush=off parameter to -drive |
Date: |
Tue, 11 May 2010 19:13:22 +0100 |
User-agent: |
Mutt/1.5.13 (2006-08-11) |
Anthony Liguori wrote:
> qemu-img create -f raw foo.img 10G
> mkfs.ext3 foo.img
> mount -oloop,rw,barrier=1 -t ext3 foo.img mnt
>
> Works perfectly fine.
Hmm, interesting. Didn't know loop propagated barriers.
So you're suggesting to use qemu with a loop device, and ext2 (bit
faster than ext3) and barrier=0 (well, that's implied if you use
ext2), and a raw image file on the ext2/3 filesystem, to provide the
effect of flush=off, becuase the loop device caches block writes on
the host, except for explicit barrier requests from the fs, which are
turned off?
That wasn't obvious the first time :-)
Does the loop device cache fs writes instead of propagating them
immediately to the underlying fs? I guess it probably does.
Does the loop device allow the backing file to grow sparsely, to get
behavious like qcow2?
That's ugly but it might just work.
> >2. barrier=0 does _not_ provide the cache=off behaviour. It only
> >disables barriers; it does not prevent writing to the disk hardware.
>
> The proposal has nothing to do with cache=off.
Sorry, I meant flush=off (the proposal). Mounting the host filesystem
(i.e. not using a loop device anywhere) with barrier=0 doesn't have
even close to the same effect.
> >>The problem with options added for developers is that those options are
> >>very often accidentally used for production.
> >>
> >We already have risky cache= options. Also, do we call fdatasync
> >(with barrier) on _every_ write for guests which disable the
> >emulated disk cache?
>
> None of our cache= options should result in data corruption on power
> loss. If they do, it's a bug.
(I might have the details below a bit off.)
If cache=none uses O_DIRECT without calling fdatasync for guest
barriers, then it will get data corruption on power loss.
If cache=none does call fdatasync for guest barriers, then it might
still get corruption on power loss; I am not sure if recent Linux host
behaviour of O_DIRECT+fdatasync (with no buffered writes to commit)
issues the necessary barriers. I am quite sure that older kernels did not.
cache=writethrough will get data corruption on power loss with older
Linux host kernels. O_DSYNC did not issue barriers. I'm not sure if
the behaviour of O_DSYNC that was recently changed is now issuing
barriers after every write.
Provided all the cache= options call fdatasync/fsync when the guest
issues a cache flush, and call fdatasync/fsync following _every_ write
when the guest has disabled the emulated write cache, that should be
as good as Qemu can reasonably do. It's up to the host from there.
-- Jamie
- Re: [Qemu-devel] Re: [PATCH 2/2] Add flush=off parameter to -drive, (continued)
- Re: [Qemu-devel] Re: [PATCH 2/2] Add flush=off parameter to -drive, Paul Brook, 2010/05/11
- Re: [Qemu-devel] Re: [PATCH 2/2] Add flush=off parameter to -drive, Anthony Liguori, 2010/05/11
- Re: [Qemu-devel] Re: [PATCH 2/2] Add flush=off parameter to -drive, Paul Brook, 2010/05/11
- Re: [Qemu-devel] Re: [PATCH 2/2] Add flush=off parameter to -drive, Anthony Liguori, 2010/05/11
- Re: [Qemu-devel] Re: [PATCH 2/2] Add flush=off parameter to -drive, Paul Brook, 2010/05/11
- Re: [Qemu-devel] Re: [PATCH 2/2] Add flush=off parameter to -drive, Anthony Liguori, 2010/05/11
- Re: [Qemu-devel] Re: [PATCH 2/2] Add flush=off parameter to -drive, Paul Brook, 2010/05/11
- Re: [Qemu-devel] Re: [PATCH 2/2] Add flush=off parameter to -drive, Avi Kivity, 2010/05/11
- Re: [Qemu-devel] Re: [PATCH 2/2] Add flush=off parameter to -drive, Jamie Lokier, 2010/05/11
- Re: [Qemu-devel] Re: [PATCH 2/2] Add flush=off parameter to -drive, Anthony Liguori, 2010/05/11
- Re: [Qemu-devel] Re: [PATCH 2/2] Add flush=off parameter to -drive,
Jamie Lokier <=
- Re: [Qemu-devel] Re: [PATCH 2/2] Add flush=off parameter to -drive, Alexander Graf, 2010/05/11
- Re: [Qemu-devel] Re: [PATCH 2/2] Add flush=off parameter to -drive, Jamie Lokier, 2010/05/11
- Re: [Qemu-devel] Re: [PATCH 2/2] Add flush=off parameter to -drive, Paul Brook, 2010/05/11
- Re: [Qemu-devel] Re: [PATCH 2/2] Add flush=off parameter to -drive, Paul Brook, 2010/05/11
- Re: [Qemu-devel] Re: [PATCH 2/2] Add flush=off parameter to -drive, Jamie Lokier, 2010/05/12
- Re: [Qemu-devel] Re: [PATCH 2/2] Add flush=off parameter to -drive, Christoph Hellwig, 2010/05/17
- Re: [Qemu-devel] Re: [PATCH 2/2] Add flush=off parameter to -drive, Markus Armbruster, 2010/05/14
- Re: [Qemu-devel] Re: [PATCH 2/2] Add flush=off parameter to -drive, Christoph Hellwig, 2010/05/17
- Re: [Qemu-devel] Re: [PATCH 2/2] Add flush=off parameter to -drive, Alexander Graf, 2010/05/17
- Re: [Qemu-devel] Re: [PATCH 2/2] Add flush=off parameter to -drive, Avi Kivity, 2010/05/11