Re: [Qemu-devel] [RFC v2 PATCH 2/6] block: add live block commit functionality

[Jeff Cody <jcody@redhat.com>]

On 09/06/2012 05:16 PM, Eric Blake wrote:
> On 09/06/2012 02:37 PM, Jeff Cody wrote:
>> On 09/06/2012 10:00 AM, Kevin Wolf wrote:
>>> Am 30.08.2012 20:47, schrieb Jeff Cody:
>>>> This adds the live commit coroutine.  This iteration focuses on the
>>>> commit only below the active layer, and not the active layer itself.
>>>>
>>>> The behaviour is similar to block streaming; the sectors are walked
>>>> through, and anything that exists above 'base' is committed back down
>>>> into base.  At the end, intermediate images are deleted, and the
>>>> chain stitched together.  Images are restored to their original open
>>>> flags upon completion.
>>>>
> 
>>
>>> What should we do with backing files that are smaller than the image to
>>> commit? In this version, data is copied up to the size of the backing
>>> file, and then we get -EIO from bdrv_co_do_writev().
>>>
>>
>> We could leave it like that, and let it receive -EIO in that case.
>> Alternatively, we could try and determine before the commit if the data
>> will fit in the base, and return -ENOSPC if not.
> 
> Neither sounds appealing.  Why can't we first try to resize the base to
> the new data size being committed, and only fall back to -ENOSPC or -EIO
> if the resize fails?
> 

OK - we will attempt to resize the base, and return the appropriate
error on failure or if unsupported for the format.