On 12/09/2015 08:34 PM, Dallas Clement wrote: > Hi all. I'm trying to determine best and worst case expected > sequential write speeds for Linux software RAID with spinning disks. > > I have been assuming on the following: > > Best case RAID 6 sequential write speed is (N-2) * X, where is is > number of drives and X is write speed of a single drive. > > Worst case RAID 6 sequential write speed is (N-2) * X / 2. > > Best case RAID 5 sequential write speed is (N-1) * X. > > Worst case RAID 5 sequential write speed is (N-1) * X / 2. > > Could someone please confirm whether these formulas are accurate or not? Confirm these? No. In fact, I see no theoretical basis for stating a worst case speed as half the best case speed. Or any other fraction. It's dependent on numerous variables -- block size, processor load, I/O bandwidth at various choke points (Northbridge, southbridge, PCI/PCIe, SATA/SAS channels, port mux...), I/O latency vs. queue depth vs. drive buffers, sector positioning at block boundaries, drive firmware housekeeping, etc. Where'd you get the worst case formulas? -- 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
