[Sheepdog] Some setattr/getattr strangeness

[MORITA Kazutaka]

At Thu, 13 Oct 2011 22:00:05 +0900,
MORITA Kazutaka wrote:
> 
> At Thu, 13 Oct 2011 13:35:06 +0100,
> Chris Webb wrote:
> > 
> > MORITA Kazutaka <morita.kazutaka at lab.ntt.co.jp> writes:
> > 
> > > Sheepdog uses a corosync multicast for all global atomic operations,
> > > so I think the correct way is to implement a SD_OP_ATOMIC_WRITE_OBJ
> > > operation with the multicast.
> > >
> > > But this limits the size of a vdi attribute to the maximum multicast
> > > size (a few hundreds KB?).  Is it okay for you?
> > 
> > Hi. That definitely wouldn't cause me any problems, as I'm only using them
> > for locks (twenty bytes identifying uniquely what has claimed the vdi) and
> > very simple textual properties (like a drive name). To be honest, I had
> > assumed that they were intended for very small amounts of metadata like this
> > rather than for bulk data storage, for which we have vdis themselves, and
> > didn't realise they'd hold such large chunks of data successfully.
> 
> Okay, I'll create a patch to support atomic I/Os.

After a close look at the codes again, I found that setattr is already
an atomic operation.  Sheepdog uses a corosync multicast to allocate a
vdi attr object id, so setattr -x works correctly even if multiple
hosts send the requests at the same time.

Sorry for my confusing answer.


Thanks,

Kazutaka