From mboxrd@z Thu Jan 1 00:00:00 1970 From: Justin Piszcz Subject: Re: Time to deprecate old RAID formats? Date: Fri, 19 Oct 2007 13:05:54 -0400 (EDT) Message-ID: References: <18200.49267.763509.924873@stoffel.org> <18200.53593.687483.120827@stoffel.org> <1192810534.1666.68.camel@firewall.xsintricity.com> <18200.56684.14194.630264@stoffel.org> <18200.57042.989520.666476@stoffel.org> <1192813477.1666.74.camel@firewall.xsintricity.com> Mime-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII; format=flowed Return-path: In-Reply-To: <1192813477.1666.74.camel@firewall.xsintricity.com> Sender: linux-raid-owner@vger.kernel.org To: Doug Ledford Cc: John Stoffel , linux-raid@vger.kernel.org List-Id: linux-raid.ids On Fri, 19 Oct 2007, Doug Ledford wrote: > On Fri, 2007-10-19 at 12:45 -0400, Justin Piszcz wrote: >> >> On Fri, 19 Oct 2007, John Stoffel wrote: >> >>>>>>>> "Justin" == Justin Piszcz writes: >>> >>> Justin> Is a bitmap created by default with 1.x? I remember seeing >>> Justin> reports of 15-30% performance degradation using a bitmap on a >>> Justin> RAID5 with 1.x. >>> >>> Not according to the mdadm man page. I'd probably give up that >>> performance if it meant that re-syncing an array went much faster >>> after a crash. >>> >>> I certainly use it on my RAID1 setup on my home machine. >>> >>> John >>> >> >> The performance AFTER a crash yes, but in general usage I remember seeing >> someone here doing benchmarks it had a negative affect on performance. > > I'm sure an internal bitmap would. On RAID1 arrays, reads/writes are > never split up by a chunk size for stripes. A 2mb read is a single > read, where as on a raid4/5/6 array, a 2mb read will end up hitting a > series of stripes across all disks. That means that on raid1 arrays, > total disk seeks < total reads/writes, where as on a raid4/5/6, total > disk seeks is usually > total reads/writes. That in turn implies that > in a raid1 setup, disk seek time is important to performance, but not > necessarily paramount. For raid456, disk seek time is paramount because > of how many more seeks that format uses. When you then use an internal > bitmap, you are adding writes to every member of the raid456 array, > which adds more seeks. The same is true for raid1, but since raid1 > doesn't have the same level of dependency on seek rates that raid456 > has, it doesn't show the same performance hit that raid456 does. > >> >> Justin. > -- > Doug Ledford > GPG KeyID: CFBFF194 > http://people.redhat.com/dledford > > Infiniband specific RPMs available at > http://people.redhat.com/dledford/Infiniband > Got it, so for RAID1 it would make sense if LILO supported it (the later versions of the md superblock) (for those who use LILO) but for RAID4/5/6, keep the bitmaps away :) Justin.