Re: Multiple disks per IO channel?

For SCSI, look at *around* 4-6 disks/adapter for a heavily used system. Like you say, there are no hard and fast rules - do your own monitoring and load testing on your own system with your own applications. That's what I do for every system I set-up. I also use IBM SSA adapters (as well as SCSI) and SSA drives, and these are fast but have a completely different architecture, and set-up right you could run 8-12 disks/adapter, even on a heavily used system.

As regards flooding the SCSI bus, it depends how your drives and configured and accessed. I find that most good drives will peak at around 10 M/s under heavy reading from Oracle, and less than that under heavy writing (both being sequential, BTW). Striping can help if you do lots of sequential I/O, but is less useful for random I/O. I've never been able to get a striped cluster of fast drives (3 or more) to operate at anything like their peak total bandwidth - I guess Oracle, CPU and memory and system bus bandwidth get in the way, as well as the overhead of keeping multiple drive heads in sync. It varies from system to system - I'd be interested to read of actual performance figures others have seen.

You also need to think about the hotspots of your database and access patterns of your database, obviously with an aim to getting the drives and adapters striped/grouped around highly used tables/indexes to give you more balanced I/O. You might also want to look at raw devices.

In summary, use to good Oracle and OS level monitoring tools and test, test, test. Every system is different, and only by getting 'under the metal' will you be able to find out what's best for your system.

MotoX

Aaron Buhr wrote in message <35A29F11.25CABD33_at_mediaone.net>...
> All Oracle documentation that I have seen indicates that every
>drive
>should be on its own IO channel, which for PC servers involves a great
>deal of expense. I understand that having only one drive per channel
>would
>be best, but that seems to me to be severe overkill.
>
> At what point do multiple IO channels make a difference? How many
>
>drives, how fast drives, what kind of transactions, etc? In our case we
>
>will have four 4GB 10K rpm drives configured as two RAID 1 pairs. Given
>
>that the RAID bus connected to the four drives is an Ultra SCSI bus, as
>I understand it the RAID bus throughput then is 80 megabytes/sec. Since
>
>each of the drives maxes out around 15MB/sec it would seem that the bus
>has sufficient headroom even with all four drives operating at full
>speed. However there may well be complications, for example can or will
>
>one drive tie up the bus for an extended period of time, thereby locking
>
>out the other drives? If a lengthy transaction, such as 0.1-1.0
>seconds, is initiated do the other drives have to wait until it
>completes?
>
> I realize there may not be clear-cut answers to the above but I
>would appreciate any feedback or guidelines that are available.
>
> Thanks in advance.
>
>Aaron Buhr
>ambuhr_at_southeast.net
>
Received on Wed Jul 08 1998 - 03:28:59 CDT