Compatibility Between AIX 6.1 and AIX 5L JFS2 Filesystems

Answer

The answer depends on the type of AIX 6 enhancement that has been added or created on the JFS2 filesystem.

$ lsvg -l efsvg
efsvg:
LV NAME         TYPE       LPs     PPs     PVs  LV STATE      MOUNT POINT
efslv01         jfs2       20      20      1    closed/syncd  /efstest
jfs2lv          jfs2       20      20      1    closed/syncd  /inlinesnap
fslv07          jfs2       20      20      1    closed/syncd  /jfstest
loglv01         jfs2log    1       1       1    closed/syncd  N/A

In this example, the filesystem /efstest is a JFS2 filesystem with EFS (Encrypted Filesystem) enabled on it.

# lsfs -q /efstest
Name            Nodename   Mount Pt               VFS   Size    Options    Auto Accounting
/dev/efslv01    --         /efstest               jfs2  655360  rw         yes  no
  (lv size: 655360, fs size: 655360, block size: 4096, sparse files: yes, inline log: no, inline log size: 0, EAformat: v2, Quota: no, DMAPI: no, VIX: yes, EFS: yes, ISNAPSHOT: no)

/inlinesnap is a filesystem created with Inline Snapshots

# lsfs -q /inlinesnap
Name            Nodename   Mount Pt               VFS   Size    Options    Auto Accounting
/dev/jfs2lv     --         /inlinesnap            jfs2  655360  rw         no   no
  (lv size: 655360, fs size: 655360, block size: 4096, sparse files: yes, inline log: no, inline log size: 0, EAformat: v2, Quota: no, DMAPI: no, VIX: yes, EFS: no, ISNAPSHOT: yes)

Attempt to import into an AIX 5.3 system, which does not contain these enhancements.

# importvg -y aix61vg hdisk19
aix61vg

# lsvg -l aix61vg
aix61vg:
LV NAME             TYPE       LPs     PPs     PVs  LV STATE      MOUNT POINT
efslv01             jfs2       20      20      1    closed/syncd  N/A
jfs2lv              jfs2       20      20      1    closed/syncd  /inlinesnap
fslv07              jfs2       20      20      1    closed/syncd  /jfstest
loglv01             jfs2log    1       1       1    closed/syncd  N/A


1. Volume group containing EFS-enabled JFS2 filesystem

The volume group will import, but the filesystem information will not be read correctly from the superblock. Note above after the import the mount point lists "N/A" for efslv01.

There will also not be any information for the filesystem added to /etc/filesystems.

# mkdir /efstest

# mount /dev/efslv01 /efstest
mount: 0506-322 Cannot determine log device to use for /dev/efslv01 (/efstest).

# mount -o log=/dev/loglv01 /dev/efslv01 /efstest
Replaying log for /dev/efslv01.
mount: 0506-324 Cannot mount /dev/efslv01 on /efstest: The media is not formatted or the format is not correct.

Running fsck on the filesystem produces these errors:

# fsck /dev/efslv01
fsck: 0506-040 Cannot find the Vfs value for file system /dev/efslv01.

# fsck -V jfs2 /dev/efslv011
The current volume is: /dev/efslv01
File system version contains unsupported features in primary superblock.

The reason you cannot use the EFS filesystem is because when changing or creating the filesystem with EFS enabled the change made to the filesysetm superblock is not backwards-compatible. Additionally if you had encrypted files in the filesystem they would be inaccessible to earlier versions of AIX since they did not have EFS.


2. Volume group containing JFS2 filesystem with inline snapshots

The volume group will import, and the filesystem will show the correct mount point information.
However the filesystem will only mount read-only, and the snapshot subdirectories will not be available.

On the AIX 6.1 system:

# snapshot -q /inlinesnap
Snapshots for /inlinesnap
Current  Name         Time
*        friday       Fri Jan 25 13:49:22 PST 2008

# cd /inlinesnap/.snapshot/friday
# pwd
/inlinesnap/.snapshot/friday

After importing on an AIX 5L system:

# mount /inlinesnap
mount: 0506-324 Cannot mount /dev/jfs2lv on /inlinesnap: JFS2 file system has
        read permission only.  The device is read-only or the file system
        contains unsupported features. Use "-o ro" flag to mount as read-only.

# mount -o ro /inlinesnap

# df
Filesystem    512-blocks      Free %Used    Iused %Iused Mounted on

/dev/jfs2lv       655360    648840    1%      138     1% /inlinesnap

# cd /inlinesnap/.snapshot/friday
sh: /inlinesnap/.snapshot/friday:  not found.

Since External Snapshots are a feature found in AIX 5.2 and 5.3 there is support for the filesystem in those earlier versions of AIX. However the Internal Snapshot feature cannot be used.