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

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.