On Mon, 14 Jul 2008, [UTF-8] Søren Ragsdale wrote:It seems to me that the blanket 8% improvement statement by 'Black Jacque' is clearly the most technically correct even though it is a not a serious answer.
[i]"You tend to get better write performance when the number of disks in the raid is (a power of 2) + 1, as the parity calculations are more efficient. ... 5 drives is better than 4 because you are more likely to write an entire stripe if it is the same size as your IO block, which is a power of 2. There is a thread around here somewhere that talks about that. RAID-Z always writes a full stripe, so that may not apply as much."[/i]
This seems like a bunch of hog-wash to me. Any time you see even a single statement which is incorrect, it is best to ignore that forum poster entirely and if no one corrects him, then ignore the entire forum. For example, from what I have read, RAID-Z does not always write a full stripe. While RAID-Z uses parity similar to RAID-5 and writes a stripe across disks, it does not need to write across all the disks like RAID-5 does. If you had 10 disks, it might decide to stripe across 6 disks per block write.
It sounds like they're talking more about traditional hardware RAID but is this also true for ZFS? Right now I've got four 750GB drives that I'm planning to use in a raid-z 3+1 array. Will I get markedly better performance with 5 drives (2^2+1) or 6 drives 2*(2^1+1) because the parity calculations are more efficient across N^2 drives?
With ZFS and modern CPUs, the parity calculation is surely in the noise to the point of being unmeasurable. Even the ZFS fletcher checksum algorithm is an order or two of magnitude more costly than the parity calculation.
If you can afford the extra drives, then you can use them to obtain more performance. The solution to more performance (depending on the type of performance you are looking for) may be to create more small VDEVs using mirrors, RAID-Z, or RAID-Z2 and load share across them. With ZFS, the mirrored configuration is usually fastest since it uses the least device IOPS and does not split up the blocks.
ZFS is really easy to try out various incantations with so if you have the drives available there is no excuse to not test it for yourself with your own workload.
Bob
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