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Re: [Qemu-devel] [PATCH for-1.2] qed: refuse unaligned zero writes with
From: |
Stefan Hajnoczi |
Subject: |
Re: [Qemu-devel] [PATCH for-1.2] qed: refuse unaligned zero writes with a backing file |
Date: |
Tue, 28 Aug 2012 14:37:03 +0100 |
On Tue, Aug 28, 2012 at 2:25 PM, Paolo Bonzini <address@hidden> wrote:
> Il 28/08/2012 15:04, Stefan Hajnoczi ha scritto:
>> Zero writes have cluster granularity in QED. Therefore they can only be
>> used to zero entire clusters.
>>
>> If the zero write request leaves sectors untouched, zeroing the entire
>> cluster would obscure the backing file. Instead return -ENOTSUP, which
>> is handled by block.c:bdrv_co_do_write_zeroes() and falls back to a
>> regular write.
>>
>> The qemu-iotests 034 test cases covers this scenario.
>
> Reviewed-by: Paolo Bonzini <address@hidden>
>
> Makes sense since both streaming and copy-on-read will do cluster-aligned
> writes.
>
> The "right fix" would not be much more complex though, something like this,
> right?
> (untested).
Yes but it's more complicated. To do a really good job we should
slice off the first/last clusters if they are unaligned, handle them
like regular allocating writes, and handle the middle of the request
as a zero write.
I decided to do the simplest implementation since this scenario only
occurs in test cases, not real guests.
Stefan