TMF Introduction (G06.26+)

Application Performance
HP NonStop TMF Introduction—522414-002
4-3
Writes to Database Files
I/O for these transactions on a database protected by TMF includes writes to the
database and writes to audit-trail files. Both types of writes include features that
enhance performance for database files under TMF protection.
Writes to Database Files
Whereas the I/O for reading data is the same whether or not an application is using
TMF, the I/O for writing the updated records is different:
•
Writes for files under TMF protection can be buffered and held collectively in
cache.
•
Writes for nonaudited files, however, should not be buffered and held collectively in
cache because these writes are not completed until they actually arrive on
electronic media.
Buffering writes is similar to keeping a list of instructions that you have to give
someone, and then giving them all in one telephone conversation instead of making a
separate call for each instruction. Many buffered writes are stored in cache, and then
written to disk in one physical access. Because more time is used by the physical
access than by any of the other operations involved in writing records to disk, using
buffered writes greatly improves performance. For example, an application can end a
transaction while the transaction is held in cache; the application does not have to wait
for disk access.
With files under TMF protection, 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 in cache, TMF protects the database with the before-images and after-images
in the audit trail.
I/O on files without TMF protection should be unbuffered so that the updates are
written immediately to disk. Data records written only to cache can be lost if a double
processor failure occurs. Unbuffered and buffered writes are illustrated in Figure 4-3.