On Mon, July 12, 2010 10:03, Tim Cook wrote: > Everyone's SNAPSHOTS are copy on write BESIDES ZFS and WAFL's. The > filesystem itself is copy-on-write for NetApp/Oracle, which is why there > is no performance degradation when you take them. > > Per Microsoft: > When a change to the original volume occurs, but before it is written to > disk, the block about to be modified is read and then written to a > differences area, which preserves a copy of the data block before it is > overwritten with the change. > > That is exactly how pretty much everyone else takes snapshots in the > industry, and exactly why nobody can keep more than a handful on disk at > any one time, and sometimes not even that for data that has heavy change > rates.
The nice thing about VSS is that they can be requested by applications. Though ZFS is ACID, and you can design an application to have ACID writes to disk, linking the two can be tricky. And not all applications are ACID (image editors, word processors, etc.). It'd be handy to have a mechanism where applications could register for snapshot notifications. When one is about to happen, they could be told about it and do what they need to do. Once all the applications have acknowledged the snapshot alert--and/or after a pre-set timeout--the file system would create the snapshot, and then notify the applications that it's done. Given that snapshots will probably be more popular in the future (WAFL NFS/LUNs, ZFS, Btrfs, VMware disk image snapshots, etc.), an agreed upon consensus would be handy (D-Bus? POSIX?). _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
