NonStop Systems Introduction

Transaction Management
NonStop Systems Introduction—527825-001
5-13
File Recovery
Audit dumps are tape or disk copies of the disk files containing the TMF audit trails.
The operations staff dumps the audit trail files to tape periodically to preserve space on
the audit trail disk. During the file recovery operation, the online dump and audit
dumps are automatically restored back to disk.
The file recovery procedure reapplies the after-images of all committed transactions in
the audit trails to the archive copy but does not reapply the record images of any
transactions that aborted or were incomplete at the time of the failure. The net result is
that the database is returned to the consistent state it was in just prior to the
catastrophic disk failure.
Here is a simple example of how file recovery applies after-images to the archive copy
of the database. Suppose that the audit trail for the warehouse database indicates that
three transactions updated the database after the last online dump and before the
failure:
1. The first transaction (a customer order) subtracted 50 units from an original
quantity on hand of 800 units for part number A6.
2. The second transaction (a delivery from a supplier) added 90 units to the original
quantity of hand on 240 units for part number D7.
3. The third transaction (an inventory adjustment resulting from a recount of stock)
subtracted 5 units from an original quantity on hand of 70 units for part number B9.
The audit trail reflects these three committed transactions in the before and after-
images of the records for the three parts. The before-image for part A6 shows a value
of 800 in the QOH field (quantity on hand), while the after-image shows a value of 750.
The before-image for part D7 shows a value of 240 in the QOH field, while the
Figure 5-7. TMF File Recovery
Sales Office
Headquarters
Warehouse
Audit Trail
Warehouse
Database Restored
Audit Trail
Databases
Audit Trail
Online
Dump
Audit
Trail
Dumped
Last
Online
Dump
Audit
Dump
Tape
Catastrophe!
Dual Media
Failure
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