HP ProLiant 4500 Disk Subsystem Performance and Scalability - Page 12

WHITE PAPER (cont.) SCSI Bus Idle Time can be calculated as follows: [sustained transfer rate] x [number of drives] ÷ [SCSI bus transfer rate]= amount of time the SCSI bus is busy. Subtracting this result from 1 provides the SCSI bus idle time. For example, [5 MB/s] x [1 drive] ÷ [40 MB/s] = 12.5%. Subtracting this value from 1 equals 87.5%. ECG025.0997 ... SCSI Channel Transfer Rates The disk controllers being used today can transfer data up to 40 MB/s to and from the hard disk to the disk controller by way of the SCSI bus. However, if your disk drive can sustain a transfer rate of only 5 MB/s, the SCSI bus is going to be idle 87.5% of the time. In this example, the disk drive is the bottleneck because it transfers data slower than the SCSI bus. The key to improving system performance is to maximize data throughput by minimizing the time the disk subsystem has to wait to send or receive data. If the cumulative sustained transfer rate of the drives is less than the transfer rate of the SCSI channel, there is a significant chance that the drives will limit the throughput. Alleviate the disk bottleneck by adding additional drives to the system. For maximum performance, the total disk transfer rate should be equal to or greater than the SCSI channel transfer rate. For example, if the SCSI channel transfer rate is 40 MB/s (Wide-Ultra SCSI), add six 9.1 GB drives (6 x 7 MB/s = 42 MB/s) to reach a sustained transfer rate equal or greater than the SCSI channel. Disk Controller Transfer Rates Disk controllers are continuously being upgraded to support wider data paths and faster transfer rates. Currently, Compaq supports three industry standard SCSI interfaces on their disk controllers, as shown in Table 5. Controller Name Compaq Fast-SCSI-2 Compaq Fast-Wide SCSI-2 Compaq Wide-Ultra SCSI Table 5: Compaq Disk Controllers Description SCSI interface that uses an 8-bit data path with transfer rates up to 10 MB/s SCSI interface that uses a 16-bit data path with transfer rates up to 20 MB/s SCSI interface that uses a 16-bit data path with transfer rates up to 40 MB/s Disk controllers can be a common cause of disk subsystem bottlenecks. For example, if a disk subsystem contains a Compaq Wide-Ultra SCSI Controller transferring data up to 40 MB/s, ideally it would take three controllers to saturate the PCI Bus, which transfers data at the rate of 133 MB/s. Again, similar to the disk transfer rate example discussed earlier, concurrency would increase once you begin to add more controllers to the disk subsystem. The additional controllers enable the system to process more I/O requests, thus improving overall system performance. 12