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

[MORITA Kazutaka]

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?

'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).

How about the following?

  -w object,disk    turn on both object cache and disk cache
  -w object         turn on only object cache
  -w disk           turn on only disk cache

Thanks,

Kazutaka