[Qemu-devel] Re: [RFC][STABLE 0.13] Revert "qcow2: Use bdrv_(p)write_sync for metadata writes"

From: Anthony Liguori <anthony@codemonkey.ws>
To: Avi Kivity <avi@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>,
	stefanha@gmail.com, mjt@tls.msk.ru, qemu-devel@nongnu.org,
	hch@lst.de
Subject: [Qemu-devel] Re: [RFC][STABLE 0.13] Revert "qcow2: Use bdrv_(p)write_sync for metadata writes"
Date: Wed, 25 Aug 2010 08:37:45 -0500	[thread overview]
Message-ID: <4C751CA9.4090200@codemonkey.ws> (raw)
In-Reply-To: <4C7515AE.9020808@redhat.com>

On 08/25/2010 08:07 AM, Avi Kivity wrote:
>>
>>>   The next allocation can then be filled from memory, so the 
>>> allocation sync is amortized over many blocks.  A power fail will 
>>> leak the preallocated blocks, losing some megabytes of address 
>>> space, but not real disk space.
>>
>> It's a clever idea, but it would lose real disk space which is 
>> probably not a huge issue.
>
> Not real disk space since no pwrite() would ever touch the disk.  If 
> the image were copied, _then_ we'd lose the disk space, if the copy 
> command and filesystem don't optimize zeros away. \

Ok.

Regards,

Anthony Liguori

>>>>
>>>> And that's it.  There is no scenario where the disk is corrupted.
>>>
>>> _if_ that's the only failure mode.
>>
>> If we had another disk format that only supported growth and metadata 
>> for a backing file, can you think of another failure scenario?
>>
>
> I can't think of one, but that's not saying much.
>