Re: [kvm-devel] [Qemu-devel] [PATCH] QEMU: fsync AIO writes on flush request

[Marcelo Tosatti <marcelo@kvack.org>]

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.