Messages in this thread | | | Date | Fri, 8 Oct 1999 10:38:18 -0600 (MDT) | From | Scott Marlowe <> | Subject | Re: Potential SCSI system bottleneck. |
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On Fri, 8 Oct 1999, ishikawa wrote:
> Here is a potential performance problem uncovered > by a set of benchmark tests. > Namely two SCSI adaptors don't speed up raid 0/1 perceptibly > in comparison to the performance of a single SCSI adaptor system. > (and under dual CPU system for that matter .)
Hi, do you know how many disks he was spinning? It looks like just two drives, in which case, there will NO real advantage on having dual SCSI controllers, since the SCSI bus is NOT the bottleneck. The primary reason for dual scsi controllers in this instance is to insure that any failure that completely kills one scsi buss doesn't kill the other. I.e. cabling, fried SCSI card, a drive that holds a line at ground forever and ever amen.)
Assuming the SCSI controllers are using UW or U2W busses, the max throughput on the buss should be 40 or 80 MBytes/second. A single server class hard drive will likely have a throughput of between 5 and 15 Megs a second. Judging by the numbers listed in the RAID-0 tests would appear to be from a pair of hard drives each pushing out about 6-7 Megs a second.
Further, since Ultra SCSI is usually limited to a maximum of 1.5m cable run length with about 0.25m to 0.30m spacing for stability reasons, the maximum number of devices, counting the controller tops out at about 7 devices (6 hard drives), your hard drives would have to pump out 11+ Megs/sec to saturate a U2W card or half that for the UW cards.
So, on a U2W, until you fill one card with 6 drives, there's probably no performance advantage til you add the second card. Then you should see a difference. So you probably need to spin 10 or 12 fast hard drives on multiple controllers to really see a gain from the second card. The gain will likely NOT be linear, but each drive should speed things up a large percentage of it's raw speed in a RAID 0.
My testing on Linux RAID-1 is that it is no faster reading right now than the single drive, just more reliable.
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