SAND CDBMS Administration Guide
Persistent Mode Operations (Time Travel)

Removing Invalid Snapshots from the Version Tree

When a merge operation fails due to lack of disk space, the database version tree will still register the creation of new Snapshots which, however, correspond to non-existent Update (.Uxx) Files. For example, consider the information displayed for Snapshots 1 and 5 below:

nmerge -v database-name

Snap  Parent Last modification time   Size(Mb)   Location
===== ====== ======================== ========== ============
0     (none) Tue Oct 15 22:14:21 2002 4096.000   ./ADD1.n00
3     0      Mon Nov 4 11:27:29 2002  198604.758 ./ADD1.U03
4     3      Wed Nov 6 09:59:55 2002  81368.310  ./ADD1.U04
1     0      not available            ?          ./ADD1.U01
2     4      Fri Nov 8 10:15:01 2002  19699.075  ./ADD1.U02
5     0      not available            ?          ./ADD1.U05

This represents a situation where a merge operation has failed twice, leaving two Snapshots 1 and 5 with a ? in the size field and a last modification time reported as "not available" (since the Update Files do not actually exist).

These invalid Snapshots can be safely removed using the nmerge -d <snapshot-number> <database-name> command:

nmerge -d 1 database-name
nmerge -d 5 database-name

nmerge does not perform such a cleanup operation automatically when a Snapshot is found to be associated with a non-existing Update File. This is so that, in cases where the Update (.Uxx) File has been deleted for some reason, the file may be restored and use of the Snapshot continued.