Windows supports only fixed-length I/O devices. This means that each read or write to tape must be the same size as the one that preceded it and the one that follows. If any read/write operation exceeds the capacity of the hardware device, the operation fails. For backup and restore operations, this means that your backup (or restore) fails unless all of your writes (or reads) are the size the hardware is configured for.
The Sybase IQ defaults are designed to make your read and write operations as efficient as possible on each platform. However, if you override the default block size when you create an IQ database, you need to adjust the block factor when you back up that database.
For any backup or restore:
block size x block factor ≈ I/O size
To adjust the block factor on a Windows system, you must know the maximum physical block size that can be handled by your tape device. This information usually is not documented by the drive manufacturer. To determine the value, typically 64KB, you need to write a small applet using WIN32 API calls. You must then use the block size of the database and the BLOCK FACTOR option of the BACKUP command to optimize backup performance. For complete syntax and usage, see the Sybase IQ Reference Manual.
The closer to the maximum block size you can make each I/O operation, the better your backup performance will be. Use an integral BLOCK FACTOR that when multiplied by the block size yields as close to the drive's block size as possible.
Keep in mind that Sybase IQ adds some extra data to each block as it is written, for data integrity. So, if your database block size is 8192, and the maximum block size handled by the tape device is 128KB, you cannot use a block factor of 16, even though 8192 * 16 = 128KB. You have to account for the extra data added on each I/O operation by Sybase IQ and use a BLOCK FACTOR of 15. Note that 15 is the default block factor on Windows for the default database block size and the default IQ page size of 128KB.
