[sheepdog] [PATCH v0, RFC] sheep: writeback cache semantics in backend store

[Hitoshi Mitake]

Thanks for your comments, Liu and Christoph.

On Sat, Aug 18, 2012 at 3:09 AM, Christoph Hellwig <hch at infradead.org> wrote:
> On Fri, Aug 17, 2012 at 11:01:41AM +0800, Liu Yuan wrote:
>> > +static int syncfs(int fd)
>> > +{
>> > +   return syscall(__NR_syncfs, fd);
>> > +}
>> > +
>>
>> syncfs only appeared in Linux 2.6.39, rather new, I think most of
>> systems won't support it.
>
> Yeah, we probably need to test that it works during startup, similar to
> what we do for xattrs.

Yes. I have to modify build process for checking the existence of syncfs().
It seems that check with ENOSYS is also required.

>
> Some systems use a plain sync for fallback, and we could also do that.
> The only issue is that it can lead to deadlocks if the client is running
> on the same systems as the actual backend storage.

I don't know well about this problem. How can be the deadlocks caused?

>
>> With current implementation, you only flush inode object, not all the
>> data objects belong to targeted VDI. I think this is hardest part to
>> implement. Probably you need to call exec_local_request() to take
>> advantage of retry mechanism.
>
> Yes, that also was the issue last time I looked into it.  One simple
> approach could be to simply forward the flush to all nodes.
>

Thanks for your advices. I'll try to solve this problem in the next version.


-- 
Hitoshi Mitake
h.mitake at gmail.com