On Thu, Dec 10, 2015 at 2:06 PM, Phil Turmel <philip@xxxxxxxxxx> wrote: > 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? > > Where'd you get the worst case formulas? Google search I'm afraid. I think the assumption for RAID 5,6 worst case is having to read and write the parity + data every cycle. -- 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
