Compatibility and migration

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JFS file systems are fully compatible within AIX® 5.1 and AIX 5.2. Previous supported versions of the operating system are compatible with the current JFS, although file systems with a nondefault fragment size, NBPI value, or allocation group size might require special attention if migrated to a previous version.

Note: JFS file systems are not supported on disks that have a sector size of 4 KB. Therefore, when you create a file system or perform backup operations, ensure that the disks are not of 4 KB sector size.

JFS2 file systems, with the exception of snapshots, are compatible within AIX 5.1 and AIX 5.2, but not with earlier versions of the operating system. JFS2 file systems with snapshots are not supported in AIX 5.1. Always ensure a clean unmount of all JFS2 file systems before reverting to an earlier version of AIX because the logredo command does not necessarily run against a file system created for a later release.

Note: JFS2 file systems created or converted to v2 format cannot be accessed on prior releases of AIX.

The following list describes aspects that might cause problems with file systems created under earlier versions of the operating system:

JFS file system images: Any JFS file system image created with the default fragment size and NBPI value of 4096 bytes, and default allocation group size (agsize) of 8 can be interchanged with JFS file system images created under AIX 4.3 and later versions of this operating system without requiring any special migration activities.
Note: JFS2 Snapshots: JFS2 snapshots created or accessed in AIX 5L Version 5.2 with the 5200-01 Recommended Maintenance package are not accessible on earlier releases. These snapshots must be deleted before the filesystem can be mounted.
Backup and restore between JFS file systems: Backup and restore sequences can be performed between JFS file systems with different block sizes, however because of increased disk utilization, restore operations can fail due to a lack of free blocks if the block size of the source file system is smaller than the block size of the target file system. This is of particular interest for full file system backup and restore sequences and can even occur when the total file system size of the target file system is larger than that of the source file system.
While backup and restore sequences can be performed from compressed to uncompressed file systems or between compressed file systems with different fragment sizes, because of the enhanced disk utilization of compressed file systems, restore operations might fail due to a shortage of disk space. This is of particular interest for full file system backup and restore sequences and might even occur when the total file system size of the target file system is larger than that of the source file system.
JFS and JFS2 device driver limitations: A device driver must provide disk block addressability that is the same or smaller than the JFS file system fragment size or the JFS2 block size. For example, if a JFS file system was made on a user supplied RAM disk device driver, the driver must allow 512 byte blocks to contain a file system that had 512 byte fragments. If the driver only allowed page level addressability, a JFS with a fragment size of 4096 bytes could only be used.