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

From: Ian Jackson
Subject: Re: [Qemu-devel] [PATCH] QEMU: fsync AIO writes on flush request
Date: Fri, 28 Mar 2008 17:25:41 +0000

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

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 ...


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