On Wed, Sep 13, 2006 at 07:38:22AM -0700, Nicolas Dorfsman wrote: > There's something really bizarre in ZFS snaphot specs : "Uses no separate > backing store." .
It's not at all bizarre once you understand how ZFS works. I'd suggest reading through some of the documentation available at http://www.opensolaris.org/os/community/zfs/docs/ , in paricular the "Slides" available there. > Hum...if I want to mutualize one physical volume somewhere in my SAN as THE > snaphots backing-store...it becomes impossible to do ! Really bad. > > Is there any chance to have a "backing-store-file" option in a future > release ? Doing this would have a significant hit on performance if nothing else. Currently when you do a write to a volume which is snapshotted the system has to : 1) Write the new data (Yes, that's it - one step. OK, so I'm ignoring metadata, but...) If there was a dedicated backing store, this would change to : 1) Read the old data 2) Write the old data to the backing store 3) Write the new data 4) Free the old data (ok, so that's metadata only, but hey) ZFS isn't copy-on-write in the same way that things like ufssnap are. ufssnap is copy-on-write in that when you write something, it copies out the old data and writes it somewhere else (the backing store). ZFS doesn't need to do this - it simply writes the new data to a new location, and leaves the old data where it is. If that old data is needed for a snapshot then it's left unchanged, if it's not then it's freed. Scott _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
