From mboxrd@z Thu Jan 1 00:00:00 1970 From: ross@lug.udel.edu (Ross Vandegrift) Subject: Re: Linux RAID Enterprise-Level Capabilities and If It Supports Raid Level Migration and Online Capacity Expansion Date: Thu, 22 Dec 2005 10:06:31 -0500 Message-ID: <20051222150631.GC31400@lug.udel.edu> References: <20051222100703.23157.qmail@web52909.mail.yahoo.com> <62b0912f0512220431l4d1ec130q137b737c1956ddd9@mail.gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <62b0912f0512220431l4d1ec130q137b737c1956ddd9@mail.gmail.com> Sender: linux-raid-owner@vger.kernel.org To: Molle Bestefich Cc: Rik Herrin , linux-raid@vger.kernel.org List-Id: linux-raid.ids On Thu, Dec 22, 2005 at 12:31:25PM +0000, Molle Bestefich wrote: > Since you say "we", I assume you're part of a very large corporation > and thus intend to RAID a whole bunch of disks. Go with RAID6 + a > couple of spares for that. If you intend to use really many disks, > make multiple arrays. (Not sure whether you can share spares across > arrays, but I think you can.) A recent foray through mdadm's code verifies this. If it noticies a failure and there is a spare, it attempts to migrate the spare to the array that needs it. Very cool feature! > I've seen lots of MD tests, but none that covered profiling MD's > random access performance. So I suppose that most hardware solutions > will do a lot better than MD here since they have been profiled with > this in mind. Well, it depends on the RAID level, disk, configuration, and how you're using it. In general, RAID 0+1 has better seek properties because reads can be done independantly from many disks. RAID5 is always going to be slow because n-1 disks need to all simultaneously read their stripe, and this can cause spindle contention. Of course, you lose more space to overhead as RAID 0+1 arrays grow... -- Ross Vandegrift ross@lug.udel.edu "The good Christian should beware of mathematicians, and all those who make empty prophecies. The danger already exists that the mathematicians have made a covenant with the devil to darken the spirit and to confine man in the bonds of Hell." --St. Augustine, De Genesi ad Litteram, Book II, xviii, 37