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Re: [kvm-devel] [Qemu-devel] [PATCH] QEMU: fsync AIO writes on flush req
From: |
Marcelo Tosatti |
Subject: |
Re: [kvm-devel] [Qemu-devel] [PATCH] QEMU: fsync AIO writes on flush request |
Date: |
Fri, 28 Mar 2008 16:11:08 -0300 |
User-agent: |
Mutt/1.4.2.1i |
On Fri, Mar 28, 2008 at 05:25:41PM +0000, Ian Jackson wrote:
> Marcelo Tosatti writes ("[Qemu-devel] [PATCH] QEMU: fsync AIO writes on flush
> request "):
> > Its necessary to guarantee that pending AIO writes have reached stable
> > storage when the flush request returns.
>
> Surely it isn't necessary to call qemu_aio_flush ? Because those
> pending AIO writes have not yet been returned to the guest as
> complete, the guest is not entitled to assume that a FLUSH CACHE
> command (issued before those writes have completed) completing
> successfully means that those interleaved writes have reached stable
> storage.
By the time qemu_aio_flush() finishes, all pending requests submitted
before aio_fsync() will have reached stable storage (aio_fsync will be
queued in the last position of the priority 0 AIO queue).
Unless the scheduling priority of the QEMU process is changed between
AIO write's and the fsync().
> Also, this patch does a synchronous flush (which is bad because it
> stalls the guest while the flush takes place) and it ignores any error
> return (which is quite bad - see my other messages about bdrv_flush,
> caches, etc.)
>
> So I think it would be better to apply
> - my bdrv_flush patch from February which I've reposted today
> and then
> - the asynchronous FLUSH CACHE patch which I've posted today
>
> I think we concluded last time that the change of fsync to fdatasync
> is correct but I think we should wait for the dust to settle before
> introducing another change on top of all this ...
Alright, please also fix the SCSI driver to perform asynchronous flush
with bdrv_aio_flush() and complete the SYNC_CACHE command on the callback.
- Re: [kvm-devel] [Qemu-devel] [PATCH] QEMU: fsync AIO writes on flush request, (continued)
- Re: [kvm-devel] [Qemu-devel] [PATCH] QEMU: fsync AIO writes on flush request, Marcelo Tosatti, 2008/03/28
- Re: [kvm-devel] [Qemu-devel] [PATCH] QEMU: fsync AIO writes on flush request, Jamie Lokier, 2008/03/28
- Re: [kvm-devel] [Qemu-devel] [PATCH] QEMU: fsync AIO writes on flush request, Paul Brook, 2008/03/28
- Re: [kvm-devel] [Qemu-devel] [PATCH] QEMU: fsync AIO writes on flush request, Jamie Lokier, 2008/03/28
- Re: [kvm-devel] [Qemu-devel] [PATCH] QEMU: fsync AIO writes on flush request, Paul Brook, 2008/03/28
- Re: [kvm-devel] [Qemu-devel] [PATCH] QEMU: fsync AIO writes on flush request, Jamie Lokier, 2008/03/28
- Re: [kvm-devel] [Qemu-devel] [PATCH] QEMU: fsync AIO writes on flush request, Marcelo Tosatti, 2008/03/28
- Re: [kvm-devel] [Qemu-devel] [PATCH] QEMU: fsync AIO writes on flush request, Jamie Lokier, 2008/03/28
- Re: [kvm-devel] [Qemu-devel] [PATCH] QEMU: fsync AIO writes on flush request, Marcelo Tosatti, 2008/03/29
Re: [Qemu-devel] [PATCH] QEMU: fsync AIO writes on flush request, Ian Jackson, 2008/03/28
- Re: [kvm-devel] [Qemu-devel] [PATCH] QEMU: fsync AIO writes on flush request,
Marcelo Tosatti <=