[sheepdog] [PATCH 1/2] dog: add a new option for reducing identical snapshots

[Liu Yuan]

On Thu, Feb 12, 2015 at 03:59:51PM +0900, Hitoshi Mitake wrote:
> At Thu, 12 Feb 2015 14:38:37 +0800,
> Liu Yuan wrote:
> > 
> > On Mon, Feb 09, 2015 at 05:25:48PM +0900, Hitoshi Mitake wrote:
> > > Current "dog vdi snapshot" command creates a new snapshot
> > > unconditionally, even if a working VDI doesn't have its own
> > > objects. In such a case, the created snapshot is redundant because
> > > same VDI is already existing.
> > 
> > What kind of use case will create two identical snapshots? This logic is simple
> > and code is clean, but I doubt if there is real users of this option.
> 
> Generally speaking, taking snapshot periodically is an ordinal usecase
> of enterprise SAN. Of course sheepdog can support this use case. In a
> case of sheepdog, making cron job (e.g. daily) which invokes "dog vdi
> snapshot" simply enables it.
> 
> But if a VDI doesn't have COWed objects, the snapshot will be
> redundant. So I want to add this option.

Okay, your patch makes sense for periodic snapshot. But if dog have found
identical snapshots, it won't create a new one and return success to the caller.

I assume the caller is some middleware, if there is no new vdi returned, will
this cause trouble for it? This means it will need to call 'vdi list' to check
if new vdi created or not? 

I'm not agasint this patch, but I have some questions. For identical snapshots,
the overhead is just an inode object created, no? Looks to me the overhead is
quite small and no need a special option to remove it.

Thanks
Yuan