Re: [Qemu-devel] [PATCH] qemu-img: let qemu-img behave as the manual advertise

[Liu Yuan]

On 04/11/2012 08:27 PM, Kevin Wolf wrote:

> Am 11.04.2012 12:33, schrieb Kevin Wolf:
>> Am 11.04.2012 12:05, schrieb Liu Yuan:
>>> On 04/11/2012 03:49 PM, Kevin Wolf wrote:
>>>
>>>> I don't agree with this patch. If the documentation says that qemu-img
>>>> always uses writeback, then the documentation must be fixed.
>>>>
>>>> We really don't care about flushes during an image conversion. It should
>>>> just go as fast as it can. If any error happens in the middle, you'll
>>>> have to throw the image away anyway, because it contains only half of
>>>> what you need. So not having flushed doesn't really make any difference.
>>>>
>>>> As soon as a guest begins using the image with data that we really care
>>>> about, it's probably running in a VM with non-unsafe mode, and then it
>>>> gets flushed.
>>>
>>>
>>> But we can set cache mode for qemu-img convert. So at least if we set
>>> explicitly with other modes(for e.g, writeback) that ask to flush the
>>> dirty bits, I think we should call brdv_flush().
>>>
>>> If you agree, I'll update the patch with this modification.
>>
>> Yes, adding the flush is fine, I just wouldn't change the default cache
>> mode.
> 
> On second thought, wouldn't make it more sense to add the flush in
> bdrv_close(), so that all callers get it?
> 

Hmm, yes, if high level bdrv_close() calls bdrv_flush() internally, we
can safely call bdrv_close() from users (qemu-img) to ensure data
writeback with proper cache mode.

Thanks,
Yuan