On 2009-11-02, at 14:59, Greg Freemyer wrote:
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 thought Takashi Sato was working on allowing a filesystem freeze ioctl from userspace? This would hook into the filesystem-specific freeze code so that when the ioctl() returns the on-disk filesystem is fully consistent and does not even require journal replay.
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.
Cheers, Andreas -- Andreas Dilger Sr. Staff Engineer, Lustre Group Sun Microsystems of Canada, Inc. -- 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
