Sybase 12.4.2 manual Performance implications, 309

CHAPTER 8 Transactions and Versioning

•Additional information about checkpoints that occurred during a transaction.

When you need to recover your database, instead of repeating all of the lengthy transactions that have occurred, Adaptive Server IQ restores quickly from the information in the transaction log and the checkpoint information. It uses the information about versions and free space to roll back transactions, and to release the disk space occupied by obsolete versions.

The transaction log requires very little space, only about 128 bytes for each committed transaction. The information about checkpoints and disk space availability are also very small.

The transaction log is deleted:

•Always after a full backup.

•Optionally after incremental backup.

•Always after backup files are restored following media failure, and a new log is started.

The checkpoint information is deleted at the next checkpoint. Information related to particular savepoints is deleted when the savepoint is released or rolled back.

For other concurrency issues relating to backing up and restoring databases, see “Concurrency and backups”.

Performance implications

Snapshot versioning should have a minimal impact on performance. The flexibility you gain by being able to update the database while other users read from it far outweigh any negative effects. There are certain resource issues you should be aware of, however:

•Buffer consumption may increase slightly, if multiple users are using different versions of the same database page simultaneously.

•Version management requires some overhead, but the effect on performance is minimal. See also the bullet on disk space.

•The thread control, which determines how many processing resources a user gets, and the sweeper controls, which use a small number of threads to sweep dirty data pages out to disk, have a minor impact on performance.

309