Re: [vdsm] [Libstoragemgmt-devel] [Engine-devel] 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. Regards, Tony ___ vdsm-devel mailing list vdsm-devel@lists.fedorahosted.org https://fedorahosted.org/mailman/listinfo/vdsm-devel
Re: [vdsm] [Libstoragemgmt-devel] [Engine-devel] RFC: Writeup on VDSM-libstoragemgmt integration
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. ___ vdsm-devel mailing list vdsm-devel@lists.fedorahosted.org https://fedorahosted.org/mailman/listinfo/vdsm-devel
