>>>>> "Dallas" == Dallas Clement <dallas.a.clement@xxxxxxxxx> writes: Dallas> Hi all. I'm trying to determine best and worst case expected Dallas> sequential write speeds for Linux software RAID with spinning disks. Dallas> I have been assuming on the following: Dallas> Best case RAID 6 sequential write speed is (N-2) * X, where is is Dallas> number of drives and X is write speed of a single drive. Dallas> Worst case RAID 6 sequential write speed is (N-2) * X / 2. Dallas> Best case RAID 5 sequential write speed is (N-1) * X. Dallas> Worst case RAID 5 sequential write speed is (N-1) * X / 2. Dallas> Could someone please confirm whether these formulas are accurate or not? Dallas> I am not even getting worst case write performance with an Dallas> array of 12 spinning 7200 RPM SATA disks. Thus I suspect Dallas> either the formulas I am using are wrong or I have alignment Dallas> issues or something. My chunk size is 128 KB at the moment. I think you're over-estimating the speed of your disks. Remember that disk speeds are faster on the outer tracks of the drive, and slower on the inner tracks. I'd setup two partitions, one at the start and one at the outside and do some simple: dd if=/dev/zero of=/dev/inner,outer bs=8192 count=100000 oflag=direct and look at those numbers. Then build up a table where you vary the bs= from 512 to N, which could be whatever you want. That will give you a better estimate of individual drive performance. Then when you do your fio tests, vary the queue depth, block size, inner/outer partition, etc, but all on a single disk at first to compare with the first set of results and to see how they correlate. THEN you can start looking at the RAID performance numbers. And of course, the controller you use matters, how it's configured, how it's setup for caching, etc. Lots and lots and lots of details to be tracked. Change one thing at a time, then re-run your tests. Automating them is key here. -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
