Re: [Qemu-devel] [RFC] block-queue: Delay and batch metadata writes

[Kevin Wolf <kwolf@redhat.com>]

Am 20.09.2010 17:51, schrieb Anthony Liguori:
> On 09/20/2010 10:08 AM, Kevin Wolf wrote:
>>
>>> If you're comfortable with a writeback cache for metadata, then you
>>> should also be comfortable with a writeback cache for data in which
>>> case, cache=writeback is the answer.
>>>      
>> Well, there is a difference: We don't pollute the host page cache with
>> guest data and we don't get a virtual "disk cache" as big as the host
>> RAM, but only a very limited queue of metadata.
> 
> Would it be a mortal sin to open the file twice and have a cache=none 
> version for data and cache=writeback for metadata?

Is the behaviour for this well-defined and portable?

> The two definitely aren't consistent with each other but I think the 
> whole point here is that we don't care.

What we do care about is ordering between data and metadata writes, for
example when doing COW, the copy of the data should have completed
before we update the L2 table.

Also, what happens (in qcow2) when we free a data cluster and reuse it
as metadata (or the other way round). Does this work or is there a
chance that the old content is resurrected?

Kevin