From mboxrd@z Thu Jan 1 00:00:00 1970 From: Bill Davidsen Subject: Re: sw raid0 read bottleneck Date: Wed, 14 Mar 2007 13:41:14 -0500 Message-ID: <45F841CA.8040704@tmr.com> References: <17911.3814.749745.77134@notabene.brown> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: Sender: linux-raid-owner@vger.kernel.org To: Mark Hahn Cc: Neil Brown , Tomka Gergely , linux-raid@vger.kernel.org List-Id: linux-raid.ids Mark Hahn wrote: >> 15 drives and 64K chunks gives 960K per stripe. >> The raid0 code should set the read-ahead to twice that: 1920K >> which I would have thought would be enough, but apparently not. > > choosing the RA size should depend in some way on speed, shouldn't it? > after all, the goal is to have enough reads queued to avoid a "stall" > while un-read-ahead-ed sectors pass under the head. for a typical > 60 MB/s, 7200 rpm disk, a track is .5 MB. the heuristic above only reads > ahead .12 MB... While I'm sure you realize it, some reader won't, so I'll point out that a cylinder is all the tracks which can be read without a seek, and cylinder size is track size times number of data heads. If you want max speed you can read that much off a drive at a time... -- bill davidsen CTO TMR Associates, Inc Doing interesting things with small computers since 1979