[Sheepdog] [PATCH] sheep: disable object cache by default

[Liu Yuan]

On 05/09/2012 04:25 PM, MORITA Kazutaka wrote:

> So 'collie vdi convert' doesn't use the write cache by default?  I
> disagree it.  It makes sense to convert image with a writeback mode by
> default for any users because we never use the image in the conversion
> process.
> 


I don't get it, what users get when he convert image as writeback, and
the sheepdog doesn't support it at all(assume no object cache)

Further more, converting use 'writeback' is nonsense, because you never
write any block for twice. So the whole process time = convert it in
local writeback cache (if any) + flush time. Why need this extra
writeback cache phase? I think users can only expect it write it to the
cluster storage for once.

> It's a bug that the sheepdog write cache doesn't allow both writeback
> and writethrough requests to the same image.  We should provide a way
> to disable it until it is fixed.
> 


I don't think any sane client would both issue interleaved writethrough
+ writeback requests.

> What's the problem to you to introduce another option to disable the
> object cache?  It will really helpful for users who don't like the
> current cache semantics; it doesn't replicate data at all until the
> data is flushed.


I don't get it again. Is there any cache that replicates itself to
backend storage and keep consistent with backing storage all the time
instead of flush to get consistent? If so, kernel's page cache and
flashcache(or bcache) are all fundamentally wrong.

I think if user want strong consistency, he should open VDI with default
QEMU setup command or use cache=writethrough option to disable object
cache instead of object cache to keep consistent all the time with the
backing storage.

Thanks,
Yuan