RDF/IMP and IMPX System Management Manual (RDF 1.3+)

Managing RDF
Compaq NonStop™ RDF/IMP and IMPX System Management Manual—522204-001
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Reading the Backup Database
8. Turn on updating.
9. When RDF has caught up, do a planned switchover from \B to \A (as described
earlier in this section).
Reading the Backup Database
Unlike databases protected by TMF, backup databases for RDF protection have no locks
on rows or records, even while these rows or records are being updated. Therefore,
applications can read the backup databases at any time; the data can, however, be
inconsistent because reading and updating can occur simultaneously.
Except immediately after a takeover operation, after the updaters have stopped as the
result of a STOP TMF command, or after the updaters have stopped as the result of a
STOP UPDATE, TIMESTAMP command (discussed in the next topic, below), you only
have the equivalent of BROWSE ACCESS to the backup database. BROWSE ACCESS,
a SQL/MP access option for transaction consistency, provides immediate access to the
data; however, the data can be inconsistent because a transaction might not be
completely applied to the backup database when the query is in progress. This access
provides the lowest consistency but the highest concurrency.
Immediately after a takeover operation or after the updaters have stopped as the result of
a STOP TMF or STOP UPDATE timestamp command, you have the equivalent of
STABLE ACCESS to the backup database; at those points, the backup database is
consistent with regard to all transactions whose outcomes (commit or abort) are known
at the backup system.
The following example shows the kind of data inconsistency that can occur if the backup
database is read while the database is being updated:
Suppose that a file named FILEA resides on $VOL1 on the primary system and that a
file named FILEB resides on $VOL2 on this primary system. Suppose transaction
number 50 causes changes to both FILEA and FILEB on the primary system.
Now suppose that the updater for $VOL1 has processed transaction 50, but the updater
for $VOL2 has not.
If the backup database is read at this point, the database reflects the incomplete updating
for transaction 50.
To read the backup database while RDF is running, you should open the backup files
with SHARED READ-ONLY access.
Access to Backup Databases in a Consistent State
Because the RDF updaters work asynchronously with respect to one another and to
transaction boundaries, when you use the backup database as a read-only resource you
are almost always accessing an inconsistent database.
One way to ensure that the backup database is in a logically consistent state with respect
to transaction boundaries is to stop TMF on the primary system (requiring that you first