[sheepdog] [PATCH v1, RFC] sheep: writeback cache semantics in backend store

[Liu Yuan]

On 08/29/2012 12:10 PM, MORITA Kazutaka wrote:
> At Wed, 29 Aug 2012 11:36:35 +0800,
> Liu Yuan wrote:
>>>
>>> Adding a 'writeback' option is a bit confusing.  Should we extend a
>>> '-w' option so that we can specify types of caches we want to enable?
>>>
>>
>> Basically this is a sync operation, as we don't want to confuse users by
>> yet another writeback semantic, I'd suggest term it as sync operation.
>> Then how about -w {wrtiethrough,writeback},{nosync}? First two used for
>> object cache, and nosync used for backend. nosync means don't use D_SYNC
>> flag for backend write. For e.g
>>  -w writeback,nosync means use object cache and no D_SYNC flag for backend
>>  -w nosync means no D_SYNC flag for backend
>>  -w writethough means use object cache but still use D_SYNC flag for
>> backend write
> 
> I guess what sheep should do is only to specify what kinds of cache to
> be turned on.  Whether using writeback or writethrough should be
> specified by a qemu command line, no?
>

I think QEMU flag to control cache is really human-unfriendly, I guess most users will
get confused at 'cache=' options.
 
> 'nosync' sounds dangerous though this patch provides right block
> device semantics.  I vote for adding this patch as another write cache
> because we can say that this patch uses a local disk cache on each
> storage node as a Sheepdog disk cache.  In addition, we can consider
> yet another write cache implementation in future (e.g. use page cache
> on storage nodes (not gateway) as a Sheepdog disk cache).
> 

If you find 'nosync' kind of dangerous, how about 'buffered' which means we use buffered write
for backend. Then the new look will be:

-w writeback,buffered means use object cache and buffered write for backend
-w buffered means buffered write only for backend
-w writethrough means use object cache writethrough mode
-w writethrough,buffered, means use object cache writethrough mode and buffered write for backend.

The benefit:
 1) we don't need to modify QEMU at all. 2) we can control cache mode at our own side and write it more
human friendly and easy to use.

Thanks,
Yuan