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

Subject: Re: [kvm-devel] [Qemu-devel] [PATCH] QEMU: fsync AIO writes on flush request
From: Jamie Lokier
Date: Sat, 29 Mar 2008 02:11:26 +0000
Paul Brook wrote:
> > That'll depend on what kind of device is emulated.  Does the SCSI
> > emulation handle multiple in-flight commands with any guarantee on
> > order?
> 
> SCSI definitely allows (and we emulate) multiple in flight commands.
> I can't find any requirement that writes must complete before a
> subsequent SYNCHRONISE_CACHE. However I don't claim to know the spec
> that well,

Aren't there SCSI tagged barrier commands or something like that,
which allow a host to request ordering between commands?

> it's probably not a bad idea have them complete anyway. Preferably
> this would be a completely asynchronous operation. i.e. the sync
> command returns immediately, but only completes when all preceding
> writes have completed and been flushed to disk.

I agree, that seems the optimal implementation.

-- Jamie


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