From mboxrd@z Thu Jan 1 00:00:00 1970 From: Stan Hoeppner Subject: Re: Suboptimal raid6 linear read speed Date: Tue, 15 Jan 2013 20:48:23 -0600 Message-ID: <50F614F7.20104@hardwarefreak.com> References: <20130115123301.GA11948@rabbit.us> <50F55046.7050605@turmel.org> <20130115125507.GA12184@rabbit.us> Reply-To: stan@hardwarefreak.com Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20130115125507.GA12184@rabbit.us> Sender: linux-raid-owner@vger.kernel.org To: Peter Rabbitson Cc: Phil Turmel , linux-raid@vger.kernel.org List-Id: linux-raid.ids On 1/15/2013 6:55 AM, Peter Rabbitson wrote: > On Tue, Jan 15, 2013 at 07:49:10AM -0500, Phil Turmel wrote: >> You are neglecting each drive's need to skip over parity blocks. If the >> array's chunk size is small, the drives won't have to seek, just wait >> for the platter spin. Larger chunks might need a seek. > >> Either way, you >> won't get better than (single drive rate) * (n-2) where "n" is the >> number of drives in your array. (Large sequential reads.) > > This can't be right. As far as I know the md layer is smarter than that, and > includes various anticipatory codepaths specifically to leverage multiple > drives in this fashion. Fwiw raid5 does give me the near-expected speed > (n * single drive). It is right. You're likely confusing the "smarts" of RAID1/10 optimizations. In that case you have more than one copy of each block on more than one drive allowing for additional parallelism. With a 4 drive RAID6 you only have one copy of each block on one drive. Thus as Phil states the best performance you can get here is 2 spindles of throughput, which is why you're seeing a max of ~250MB/s for the array. Unless you plan to expand this array in the future by adding more drives and doing a reshape, I'd suggest you switch to RAID10. It will give you 3x or more write throughput with greatly reduced latency, substantially faster rebuild times, and possibly a little extra read throughput. With only 4 drives RAID6 doesn't make sense as RAID10 is superior in every way. -- Stan