[Engine-devel] [Libstoragemgmt-devel] [vdsm] RFC: Writeup on VDSM-libstoragemgmt integration

[Deepak C Shetty]

On 06/25/2012 08:38 PM, Tony Asleson wrote:
> On 06/25/2012 09:14 AM, Deepak C Shetty wrote:
>> On 06/25/2012 07:47 AM, Shu Ming wrote:
>>> I think VDSM-libstoragemgmt will let the storage array itself to make
>>> the snapshot and handle the coordination of the various atomic
>>> functions. VDSM should be blocked on the following access to the
>>> specific luns which are under snapshotting.
>> I kind of agree. If snapshot is being done at the array level, then the
>> array takes care of quiesing the I/O, taking the snapshot and allowing
>> the I/O, why does VDSM have to worry about anything here, it should all
>> happen transparently for VDSM, isnt it ?
> The array can take a snapshot in flight, but the data may be in an
> inconsistent state.  Only the end application/user of the storage knows
> when a point in time is consistent.  Typically the application(s) are
> quiesced, the OS buffers flushed (outstanding tagged IO is allowed to
> complete) and then the storage is told to make a point in time copy.
> This is the only way to be sure of what you have on disk is coherent.
>
> A transactional database (two-phase commit) and logging file systems
> (meta data) are specifically written to handle these inconsistencies,
> but many applications are not.
>

Thanks for clarifying Tony. So that means we need to do whatever from 
VDSM to quiese the I/O
and then VDSM should instruct the array to take the snapshot.