Very interesting thread!

I just wondering if more disks are really better….

a) according reliability each disk increase failing probability.
(as each platter seem to increase failing probability)

b) how about the latency to send (split) signals to multiple disks
and to bundle them again?

Is there a break-even in raid-0 adding disks….
I see a step from single disk to 2disk raid 0, from 2 to 4…..
but what about from 4 to 8 or even 8 to 16?

my personal experience is that newer disks with less
platter and higher density give more performance increase
than adding disks to existing raid-0
(”subjective waiting time” on long seq IO)

…maybe i’m wrong?

Regarding the 12-drive RAID-10 compared to the mirrored 6-drive RAID-10s, the read performance should be identical. In either case, every drive should participate equally in the IO traffic - assuming the RAID stacks properly balance the workload. So if each drive can provide 100 IOPS, then both array configurations should deliver 1200 IOPS total. If you see differences in performance, then it’s due to issues in the RAID stacks and has nothing to do with the actual array configuration.

The downside of the mirrored 6-drive RAID-10 config is that (a) you get half the capacity, and (b) writes will have half the IOPS of the 12-drive RAID-10 because each data block is written four times. The upside is that you can withstand a minimum of two drive failures as opposed to just one.

Regarding splitting the array across busses. this will absolutely not hurt performance. And it could help performance if you’re bottlenecked on the SCSI bus. But typically the bottleneck is in the drives or the OS, so splitting rarely helps. This should especially be true if you’re doing mostly random IO because the SCSI bus is hardly being utilized. Most of the time is spent just waiting for the drives to seek.

The main reason to use two busses if (a) the IO load is mostly long sequential and exceeds the single bus speed, (b) you simply run out of device IDs (15) on a single bus, or (c) you want to spread your mirrors across two busses in order to protect against a SCSI cable failure.

I hope that helps. Thanks for reading.

TT

Here’s a tricky one for you. Which is the quickest (mainly for reads on large SQL server databases)?

One 12 disc RAID-10 array (hardware RAID).

Or two 6 disc RAID-10 arrays, this time mirrored as one logical disc by Windows 2000 server software raid? Ideally I’d like maximum redundancy by making the RAID-10 array redundant itself, so if a pair of mirror discs go on one array we should be okay.

… And a quickie.

I’ve currently got 6 discs in RAID-10, I wasn’t sure if I was better leaving all the discs on the same SCSI channel or splitting them. I’ve got a Adaptec SCSI RAID 2230SLP card which provides 640mb/s across two channels. Does splitting the channels in RAID-10 slow down the array - do you have to have the discs on the same channel by design?

Much appreciated,

Andy.

Also, the comment about RAID-5 parity being about 1/3 of the overhead is only true for a 3-drive RAID-5. For example a 10-drive RAID-5 has a 10% overhead. And this overhead really only affects performance in long sequential IO, which is usually not measured in IOPS. Long IO is measured in throughput, or MB/s. So to your point, which I agree with, long sequential writes to a RAID-5 are actually faster than to a RAID-10.

But I don’t think I agree with your comment about async writes. (Or maybe I just didn’t understand it.) In both RAID-10 and RAID-5, each short random host write will cause two drives to be touched. In the case of RAID-10, each drive is written once. In the case of RAID-5, each drive is read once and written once. Meanwhile, all of the other drives in the array are available for other host commands. So multiple simultaneous host commands are supported in both RAID levels.

TT

I am not convinced that measuring the quantity of IOPS produces an accurate representation of performance. Given the speed of modern scsi devices and controllers, the IOPS throughput is almost negligible in a Raid-5, as compared to having to write the data 2 times in a Raid-1(0). For example, the parity calculation offloaded to the controller card only causes a small hiccup in the throughput. What’s important to me is start to finish time. For a reasonably large amount of data being written, having to write it 2x in a Raid-1(0) environment pales to only having to write 1.35 the amount of data, (assuming the parity set is 1/3 the original data). I will certainly concede for small data blocks, there would be some disadvantage in the raid-5 scenario. Another area of discussion would be the async nature of scsi writes in a multi spindle raid-5. The main advantage here is that other devices in the raid set could be used simultaneously while that one is busy, which is not true of the raid-1(0).

So what am I missing?