4.14 We are considering adding additional disk drives to our 9x9. Presently, we have 3 HASS enclosures containing 10 F/W SCSI 2GB disks on one channel mirrored with 10 F/W SCSI 2GB disks on another channel. I'm considering adding 2 x 4GB disks, but that will result in 11 devices on each mirrored F/W SCSI channel. The F/W SCSI maximum is 15 devices, but only 10 are recommended. Will this be OK?
From Bill Lancaster:
Chances are you will probably be OK with 11 but, as with any performance question, the answer is "It depends". Before you can decide whether or not you can put the 11th drive on the channel, you have to see how much demand the other devices put on the channel. If you look at the sustained disk I/O rate on any given disk drive, you are probably sustaining only 1-5 I/O's per second. Adding it all up would indicate a maximum sustained disk I/O rate on the channel of only 50 I/O's per second. However, this is only the high average sustained I/O rate. You need to look at burst activity. If your burst activity is significantly higher, you may run into periodic disk I/O problems. This will most likely occur during a transaction manager checkpoint post to disk.
You may very will be able to live with the results. Remember that HP's performance recommendations are generally conservative. The channel can handle approximately 150 I/O's per second.