Compaq ProLiant 1000 I/O Performance Tuning of Compaq Servers - Page 11

I/O Performance Tuning of Compaq Servers 11 interface and because its backward compatibility provides investment protection for Compaq customers. P In general, use devices with the highest SCSI specification that your controller can support. In the case of Fibre Channel Arrays, use Wide-Ultra SCSI-3 compatible drives for maximum performance. In all SCSI configurations, note that: • Wide-SCSI will outperform narrow SCSI. • Higher clock-rate interfaces (Fast and Ultra) will usually give performance gains. Compaq does not recommend mixing SCSI revisions or protocols within RAID arrays. If you were to put a SCSI-1 device as part of a RAID array of Wide-Ultra SCSI-3 devices, the performance of the entire array would suffer. However, if drives are configured as independent drives (not as a RAID array), protocols may be mixed to some extent. Wide and narrow devices may be used on the same bus without affecting the performance of the individual devices. That is, Wide devices will transfer 16 bits per clock; and Narrow devices will use 8-bit transfers. However, when mixing non-Ultra and Ultra drives, you must be aware of potential performance implications. When the SCSI bus is initialized, the host adapter negotiates the highest transfer protocol that each drive is capable. Ideally, the controller will use this maximum speed to do all transfers to that device. In practice, however, operating a SCSI bus at Ultra speeds places strict requirements on the configuration in order to maintain signal integrity. For any component of your Compaq SCSI chain to use Ultra speeds you must adhere to the following: • Both the device and your controller must be capable of Ultra SCSI. • The SCSI controller must not be in the middle of a SCSI chain. • For every 5 Narrow devices, you must have at least one Wide device. • Any device that communicates at Ultra speeds must be Wide. If any one of the above rules is broken, no device on the SCSI bus will be able to communicate at Ultra speeds. In this case, the bus will fall back to Fast, 10 MHz transfers. SCSI Bus Balancing Just as with PCI, balancing the load across multiple SCSI busses can increase sustained throughput. Here again, balancing SCSI controller loads is not as simple as evenly distributing the number of disks between the busses. Proper bus balancing evenly distributes the loads generated by access to each disk. All Compaq SMART-2 SCSI controllers, Compaq SMART-2 SCSI Array Controllers, and most ProLiant Storage Systems are available with multiple SCSI busses. ECG044.0399