Hello Maurice, On Sat, Aug 9, 2008 at 1:25 AM, Maurice Volaski <[EMAIL PROTECTED]> wrote: > AVS has a mechanism for autosynchronization. This almost sounds like it > is an "intelligence" option. Isn't autosynchronization the point of AVS for > replication? > Why would anyone ever want to turn it off?
I'm not going to go through all your points, but this one in particular deserves an answer. Being able to disable auto-synchronisation may seem like a mistake, but once you understand how AVS replication performs a resynchronisation operation, and the common failure modes of networking you may appreciate why many customers of SNDR when it was a separate product used to run with auto-sync turned off. To start with resynchronisation: SNDR performs a re-sync by merging the bitmaps from primary and secondary and then iterating through the combined bitmap and where a bit is set it transfers the corresponding data from the primary to the secondary (or the other way for reverse synchronisation). This iteration is done in address order, not chronological order. Thus during the re-sync the target volume is *not logically consistent* as the IO operations are not being replaying in strict chronological order, so breaking the write ordering used by whatever application (or filesystem) originally wrote the data to the disk. The target volume will only become self-consistent again when the re-sync completes. (This is also why AVS can be configured such that it will automatically take a snapshot of the target volume before starting a re-sync.) Now consider that networks do not tend to hard break and then return cleanly; there is usually a degree of network bouncing such that to an application using the network (such as AVS) it appears that the network comes back, then goes, then comes back again. So if AVS is configured to auto-sync, the danger is that the network will come back up, and a re-sync will start. Then the network will bounce again and the re-sync will fail, leaving the target volume inconsistent. Then the network comes back and another re-sync starts and let's say it completes this time. So by enabling auto-sync the time period during which the target is inconsistent has been increased. If instead a sync had been started manually when the netadmin or sysadmin could be confident of the connectivity, the period would have been reduced to just the time taken for the re-sync itself. In many commercial environments this kind of detail matters far more than always having the secondary maintained in synchronisation. Regards, Simon _______________________________________________ storage-discuss mailing list [email protected] http://mail.opensolaris.org/mailman/listinfo/storage-discuss
