Messages in this thread | | | Date | Sat, 12 Dec 1998 17:08:17 -0500 (EST) | From | Eric Lee Green <> | Subject | Re: Dumb question: Which is "better" SCSI or IDE disks? |
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On Wed, 9 Dec 1998, Gregory Maxwell wrote: > What? Join the real world but. Raid-1 doubles read throughput and the > slowdown of writes is negligable.
Sorry, but I don't find that in actual benchmarks. Perhaps it is true with some RAID controllers, but definitely not with the ICP-Vortex GDT 6517. I get the same throughput from RAID1 on that setup that I get from the bare Seagate Cheetah on that setup (no RAID). The only way I can figure that they could make it faster would be to operate dual elevators (i.e. on one drive reading from the bottom half, on the other drive reading from the top half), which would reduce random-access seek times but which would not improve streaming performance one whit. They can't do striping because the drives have to be identical (mirrored), meaning meaning that the head would have to move to access the next track of data no matter which drive it is coming from. There could be some improvement from read-ahead (i.e., starting the other head moving while we're reading from the first head), but in real life that's a minimal improvement.
On the other hand, you are correct that the slowdown of writes is neglible. Even with the write cache turned off the GDT seems capable of issuing two writes in quick succession without slowing down the main processor. (The GDT has an i960 on-board, so the main processor isn't involved once it gets the request to the GDT). With the write-cache turned on there's no slowdown at all. I suspect that this would be different if the SCSI bus was heavily loaded, but it's not -- a 40mb/sec SCSI bus will feed two Cheetahs at full speed without flinching.
-- Eric Lee Green eric@linux-hw.com http://www.linux-hw.com/~eric "Linux represents a best-of-breed UNIX, that is trusted in mission critical applications..." -- internal Microsoft memo
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