TMF Introduction (G06.26+)

Transaction Protection and Database Recovery
HP NonStop TMF Introduction—522414-002
3-10
Volume Recovery
Volume Recovery
Volume recovery provides relatively quick database recovery after a volume, disk, or
node failure occurs in a NonStop system. The precise recovery time varies depending
on the number of files to be recovered, the system resources (particularly processors)
available, and transaction lengths. Nevertheless, it usually takes only a few minutes to
recover a database.
Volume recovery is initiated automatically whenever an audited volume must be
recovered. For example, when TMF is started after a failure or an audited volume
comes up after a system failure, the volume recovery process runs.
Volume recovery uses audit-trail records to return the database to a consistent state.
With files protected by TMF, database updates are first written to the audit trail, then to
cache for the database volumes, and finally to disk. If a failure causes data to be lost,
TMF recovers the volume by applying before-images and after-images from the audit
trail to the database.
Volume recovery performs the following operations:
•
Backs out transactions that had not committed all their changes at the time of the
failure. To back out transactions, TMF applies before-images from the audit trail to
the appropriate database.
•
Applies committed transactions that were in cache at the time of the failure and not
yet applied to the database. To apply committed transactions, TMF applies after-
images from the audit trail to the appropriate database.
The volume recovery operation is illustrated in Figure 3-5. Volume recovery applies
the after-images for transactions 3 and 4 to the database and backs out the partial
effects of transaction 2, which was not committed.