[Sheepdog] [PATCH v6 7/8] collie: read the cached objects if any for collie operation

[MORITA Kazutaka]

At Sat, 31 Mar 2012 15:34:10 +0800,
Liu Yuan wrote:
> 
> On 03/31/2012 03:20 PM, MORITA Kazutaka wrote:
> 
> > - Read requests:
> >   read from a object cache always when the cache exists
> > 
> > - Write requests:
> >   flush and remove a object cache if SD_FLAG_CMD_CACHE is not set
> > 
> > Isn't this okay for collie, too?
> 
> 
> Who set SD_FLAG_CMD_CACHE for collie? user or let collie decide itself?

We don't need to set the same cache flag to the collie requests.

Collie doesn't send write requests to the opened VDIs, so collie
doesn't flush the existing caches with my idea.

> 
> For Guests, SD_FLAG_CMD_CACHE is set once we start up QEMU with object
> cache enabled. So we'd need to distinguish R&W requests. If we see
> SD_FLAG_CMD_CACHE, sheep can simply flush and delete the cache. So this
> looks extraneous for Guest request handling.

All requests (including collie's ones) must follow the following
rules:

 - If the target object is writable, only one client can send requests
   against it.  If other ones read data from the writable object, the
   read data may not be the latest.

 - If the target object is read-only, no client can send write
   requests against it, but any clients can send read ones.

If sheep receives a non-cache write request, it means no other clients
don't open the VDI with a writeback mode, so we can flush and remove
the object cache safely.  And, even if sheep receives a non-cache read
request, other clients may open the VDI with a writeback mode, so it
is not good to flush the cache in the such case.  This is the key idea
behind my suggestion.

However, if you think it is not good to distinguish read and write requests
for cache handling, I don't mind to drop my idea.

Thanks,

Kazutaka