JFS online backups and JFS2 snapshots

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You can make a point-in-time image of a JFS file system or of a JFS2 file system, which you can then use for backup purposes. There are differences, however, in the requirements and behavior of this image for each file system type.

For a JFS file system, you can split off a read-only static copy of a mirrored copy of the file system. Typically, a mirrored copy is updated whenever the original file system is updated, but this point-in-time copy does not change. It remains a stable image of the point in time at which the copy was made. When this image is used for backing up, any modifications that begin after you begin the procedure to create the image might not be present in the backup copy. Therefore, it is recommended that file system activity be minimal while the split is taking place. Any changes that occur after the split is made will not be present in the backup copy.

For a JFS2 file system, the point-in-time image is called a snapshot. The snapshot remains static and it retains the same security permissions as the original file system (called the snappedFS) had when the snapshot was made. Also, you can create a JFS2 snapshot without unmounting or quiescing the file system. You can use a JFS2 snapshot to use as an online backup of the file system, to access the files or directories as they existed when the snapshot was taken, or to back up to removable media. Note the following about JFS2 snapshots:
  • A snapshot image of the root (/) or /usr file system is overwritten when the system is rebooted. Snapshots of other file systems can be preserved by unmounting the file system before rebooting. Snapshots created in AIX® 5.2 with 5200-01 are recoverable. When fsck or logredo runs on a JFS2 filesystem with a snapshot created on AIX 5.2 with 5200-01, the snapshot will be preserved. A cleanly unmounted filesystem with an AIX 5.2-created snapshot will also be recoverable once it is mounted on an AIX 5.2 with 5200-01 system.
  • Running the defragfs command against a file system with snapshots is not recommended. Every block that is moved during defragmentation must also be copied to the snapshot, which is both time consuming and a waste of space in the snapshot logical volume.
  • If a snapshot runs out of space, all snapshots for that snappedFS are deleted. This failure writes an entry to the error log.
  • If a write to a snapshot fails, all snapshots for that snappedFS are deleted. This failure writes an entry to the error log.
  • A snapshot that is created or accessed on a AIX 5.2 with 5200-01 system cannot be accessed on an AIX 5.2 system. These snapshots must be deleted before the filesystem can be mounted.
  • A JFS2 file system that has a snapshot on AIX 5.3 cannot be accessed on any releases prior to AIX 5.2 with 5200-01. If the system is to be moved back, the snapshots must be deleted first to allow file system access.