From mboxrd@z Thu Jan 1 00:00:00 1970 From: Phil Turmel Subject: Re: best base / worst case RAID 5,6 write speeds Date: Thu, 10 Dec 2015 15:06:19 -0500 Message-ID: <5669DB3B.30101@turmel.org> References: Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: Sender: linux-raid-owner@vger.kernel.org To: Dallas Clement , linux-raid@vger.kernel.org List-Id: linux-raid.ids 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?