On Sun, Nov 1, 2009 at 12:45 AM, Theodore Tso <tytso@xxxxxxx> wrote: > On Fri, Oct 30, 2009 at 10:20:35AM -0400, Greg Freemyer wrote: >> Ignoring computer forensics, with LVM snapshots, hardware raid array >> snapshots, etc. even in the presence of a dirty log, we need to be >> able to mount a drive in true read-only fashion fro many backup >> operations to function correctly. > > Can you go into more detail about "many backup operations"? One example is a hardware raid array that creates readonly snapshots or clones. (Lots of those exist in the real world). So the typical backup procedure is: ==== Queisce application (databases, etc. have utils to do this.) Queisce filesystem (xfs_freeze -f can be done from userspace. is there a ext4 util?) issue raid array command to create snapshot. release filesystem (xfs_freeze -u) release the app (util provided by app). Mount the snapshot readonly (true readonly with zero writes to the block device). Backup the readonly snapshot (to tape, etc.). === I believe XFS had 2 issues related to this process when first implemented in linux. 1) It required the UUID to be unique. Obviously in the above scenario it is not, so "mount -o nouuid" was added for xfs. 2) Journal replay was originally aways attempted in the above process, so the "mount -o norecovery" option was added to force a true readonly mount. ext4 may already support mounting of readonly clones, but if not it needs to before it will qualify as a data center ready filesystem. Greg -- Greg Freemyer Head of EDD Tape Extraction and Processing team Litigation Triage Solutions Specialist http://www.linkedin.com/in/gregfreemyer Preservation and Forensic processing of Exchange Repositories White Paper - <http://www.norcrossgroup.com/forms/whitepapers/tng_whitepaper_fpe.html> The Norcross Group The Intersection of Evidence & Technology http://www.norcrossgroup.com -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
