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Re: [Qemu-devel] [PATCH 0/2] Enable qemu block layer to not flush
From: |
Jamie Lokier |
Subject: |
Re: [Qemu-devel] [PATCH 0/2] Enable qemu block layer to not flush |
Date: |
Wed, 12 May 2010 13:50:08 +0100 |
User-agent: |
Mutt/1.5.13 (2006-08-11) |
Stefan Hajnoczi wrote:
> On Wed, May 12, 2010 at 10:42 AM, Jamie Lokier <address@hidden> wrote:
> > Stefan Hajnoczi wrote:
> >> Why add a nop AIO operation instead of setting
> >> BlockDriverState->enable_write_cache to zero? In that case no write
> >> cache would be reported to the guest (just like cache=writethrough).
> >
> > Hmm. If the guest sees write cache absent, that prevents changing the
> > cache policy on the host later (from not flushing to flushing), which
> > you might want to do after an OS install has finished and booted up.
>
> Right. There are 3 cases from the guest perspective:
>
> 1. Disable write cache or no write cache. Flushing not needed.
> 2. Disable flushing but leave write cache enabled.
> 3. Enable write cache and use flushing.
>
> When we don't report a write cache at all, the guest is always stuck at 1.
>
> If you're going to do this for installs and other temporary workloads,
> then enabling the write cache again isn't an issue. After installing
> successfully, restart the guest with a sane cache= mode.
That only works if you're happy to reboot the guest after the process
finishes. I guess that is usually fine, but it is a restriction.
Is it possible via QMP to request that the guest is paused when it
next reboots, so that QMP operations to change the cache= mode can be
done (as it's not safe to change the guest-visible disk write cache
availability when it's running, and probably a request to do so should
be denied).
-- Jamie
- [Qemu-devel] Re: [PATCH 2/2] Add flush=off parameter to -drive, (continued)
- Re: [Qemu-devel] [PATCH 0/2] Enable qemu block layer to not flush, Anthony Liguori, 2010/05/10
- Re: [Qemu-devel] [PATCH 0/2] Enable qemu block layer to not flush, Alexander Graf, 2010/05/10
- Re: [Qemu-devel] [PATCH 0/2] Enable qemu block layer to not flush, Anthony Liguori, 2010/05/10
- Re: [Qemu-devel] [PATCH 0/2] Enable qemu block layer to not flush, Jamie Lokier, 2010/05/11
- Re: [Qemu-devel] [PATCH 0/2] Enable qemu block layer to not flush, Stefan Hajnoczi, 2010/05/12
- Re: [Qemu-devel] [PATCH 0/2] Enable qemu block layer to not flush, Jamie Lokier, 2010/05/12
- Re: [Qemu-devel] [PATCH 0/2] Enable qemu block layer to not flush, Stefan Hajnoczi, 2010/05/12
- Re: [Qemu-devel] [PATCH 0/2] Enable qemu block layer to not flush,
Jamie Lokier <=