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.
Disk space can sometimes become an issue. Storing overlapping versions has the potential to use a lot of disk space, depending on the number and size of versions in use simultaneously. Metadata and database page versions are retained until they are dropped, either at a RELEASE SAVEPOINT or when the last transaction that can see a given version commits or rolls back. The space is then reclaimed.
Delays due to locking are minimal. Individual commits, rollbacks, and checkpoints can block other read or write transactions only very briefly.
Remember that all of these performance and disk use factors only affect your system in the degree to which you take advantage of IQ's concurrent read and write capabilities. Disk space requirements in particular can vary widely, depending on how long write transactions take before they commit, how many read transactions take place during write transactions, the number of rows these transactions affect, and whether you allow the release of data pages at interim savepoints.
For an explanation of how Sybase IQ uses the resources discussed in this section, see Chapter 4, “Managing System Resources” in Performance and Tuning Guide.
