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Date: Wed, 14 Dec 2011 13:05:19 +0100
From: Kevin Wolf <kwolf@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH 00/17] Support mismatched host and guest
 logical block sizes
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>, qemu-devel@nongnu.org, Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>

Am 14.12.2011 12:47, schrieb Paolo Bonzini:
> On 12/14/2011 12:13 PM, Kevin Wolf wrote:
>> As we discussed before, the really interesting point here is defaults,
>> and whatever you choose to do is wrong in some respect.
>>
>> So it looks like you chose to make the virtual device default to the
>> host block size.
> 
> ... wait wait, I default to 512. :)
> 
> Here is the rationale.  512-over-4k may be slow, but is safe (but it is 
> not slow if you align partitions properly).  4k-over-512 is unsafe.  So, 
> defaulting to 512 seemed the right thing after all.

Which means bounce buffers by default on 4k hosts. Is this going to
become our next cache=writethrough? At some point 4k disks will be in
wide use, but we'll still be stuck with a slow default of 512.

No matter what we decide here, I think it might really be a good idea to
save the block size in the image and use that as the default if nothing
else is specified on the command line.

Kevin