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Date: Mon, 31 Aug 2009 23:55:16 +0100
From: Jamie Lokier <jamie@shareable.org>
Subject: Re: [Qemu-devel] [PATCH 1/4] block: add enable_write_cache flag
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To: Anthony Liguori <anthony@codemonkey.ws>
Cc: Christoph Hellwig <hch@lst.de>, qemu-devel@nongnu.org

Anthony Liguori wrote:
> Christoph Hellwig wrote:
> >>If an unsafe mode is desired (I think it is, for those throwaway
> >>testing VMs, or during OS installs), I suggest adding cache=volatile:
> >>
> >>    cache=none
> >>        O_DIRECT, fdatasync, advertise volatile write cache
> >>
> >>    cache=writethrough
> >>        O_SYNC, do not advertise
> >>
> >>    cache=writeback
> >>        fdatasync, advertise volatile write cache
> >>
> >>    cache=volatile
> >>        nothing (perhaps fdatasync on QEMU blockdev close)
> >>    
> >
> >Fine withe me, let the flame war begin :)
> >  
> 
> I think we should pity our poor users and avoid adding yet another 
> obscure option that is likely to be misunderstood.
> 
> Can someone do some benchmarking with cache=writeback and fdatasync 
> first and quantify what the real performance impact is?
> 
> I think the two reasonable options are 1) make cache=writeback safe, 
> avoid a massive perf decrease in the process 2) keep cache=writeback as 
> a no-guarantees option.

Right now, cache=writeback does set the bit for SCSI emulation, which makes it
safe for guests which understand that.

Removing that is a regression in safety, not merely a lack of change.

-- Jamie