JFS2 snapshots and J2_SNAP_EIO error

Answer

This relates to the JFS2 feature called "snapshots", which are a point-in-time image of a filesystem's data. The JFS2 snapshot command creates a new logical volume that contains the data of the original one, at the time the snapshot was created.

For example, say we have an original filesystem, and we create a snapshot logical volume for it:


# lsfs -q /origfs
Name Nodename Mount Pt VFS Size Options Auto
Accounting
/dev/fslv02 -- /origfs jfs2 4194304 rw no
no
(lv size: 4194304, fs size: 4194304, block size: 4096, sparse files: yes, inline log: no, inline log size: 0, reserved: 0, reserved: 0, DMAPI: no, VIX: yes)

In the lsfs -q output the size is reported in 512-byte blocks. So in the above example the filesystem and logical volume are 2Gb in size. We'll make the snapshot filesystem 204Mb (10% of the original).

# snapshot -o snapfrom=/origfs -o size=419430
Snapshot for file system /origfs created on /dev/fslv05

Use of rmlv to remove a snapshot logical volume will result in the generation of J2_SNAP_EIO errors when the original filesystem is mounted again.

# mount /origfs

# errpt -a

--------------------------------------------------------------------------
LABEL: J2_SNAP_EIO
IDENTIFIER: 9C110A13

(...)

Description
Unable to write into a JFS2 snapshot

Probable Causes
Write error on JFS2 snapshot

Recommended Actions
The snapshots for the JFS2 file system are now invalid.
Correct the I/O problem and then create a new snapshot.

(...)

Detail Data
Snapshot Major/Minor device number
0024 0007
Snapshot generation number
0000 0000
FILE SYSTEM DEVICE AND MOUNT POINT
/dev/fslv02, /origfs

In order to recover from the accidental deletion of a snapshot in this manner, use fsck on the original filesystem. You will see a message similar to this:

# umount /origfs
# fsck -y /origfs

The current volume is: /dev/fslv02
Primary superblock is valid.
J2_LOGREDO:log redo processing for /dev/fslv02
Primary superblock is valid.
One or more snapshots in the filesystem have failed.
Snapshots must be removed to continue fsck processing (RELEASED)
*** Phase 1 - Initial inode scan
*** Phase 2 - Process remaining directories
*** Phase 3 - Process remaining files
*** Phase 4 - Check and repair inode allocation map
*** Phase 5 - Check and repair block allocation map
File system is clean.

At this point you can mount the original filesystem and create new snapshots against it.