From mboxrd@z Thu Jan 1 00:00:00 1970 From: Bill Davidsen Subject: Re: Distributed spares Date: Tue, 14 Oct 2008 19:20:04 -0400 Message-ID: <48F52924.7030607@tmr.com> References: <48F3C2A1.3080607@tmr.com> 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: Justin Piszcz Cc: Neil Brown , Linux RAID List-Id: linux-raid.ids Justin Piszcz wrote: > > > On Mon, 13 Oct 2008, Bill Davidsen wrote: > >> Over a year ago I mentioned RAID-5e, a RAID-5 with the spare(s) >> distributed over multiple drives. This has come up again, so I >> thought I'd just mention why, and what advantages it offers. >> >> By spreading the spare over multiple drives the head motion of normal >> access is spread over one (or several) more drives. This reduces >> seeks, improves performance, etc. The benefit reduces as the number >> of drives in the array gets larger, obviously with four drives using >> only three for normal operation is slower than four, etc. And by >> using all the drives all the time, the chance of a spare being >> undetected after going bad is reduced. >> >> This becomes important as array drive counts shrink. Lower cost for >> drives ($100/TB!), and attempts to drop power use by using fewer >> drives, result in an overall drop in drive count, important in >> serious applications. >> >> All that said, I would really like to bring this up one more time, >> even if the answer is "no interest." >> >> -- >> Bill Davidsen >> "Woe unto the statesman who makes war without a reason that will still >> be valid when the war is over..." Otto von Bismark >> -- >> To unsubscribe from this list: send the line "unsubscribe linux-raid" in >> the body of a message to majordomo@vger.kernel.org >> More majordomo info at http://vger.kernel.org/majordomo-info.html >> > > Bill, > > Not a bad idea; however, can the same not be acheived (somewhat) by > performing daily/smart, weekly/long tests on the drive to validate its > health? I find this to work fairly well on a large scale. Not really, the performance benefit comes from spreading head motion to (at least) one more drive. You can get a check on basic functionality with SMART, but it doesn't beat the drive the way real load does. Add to that the unfortunate problem that more realistic testing also takes up i/o bandwidth for non-productive transfers. Better to be doing actual live data transfers to those drives if you can. -- Bill Davidsen "Woe unto the statesman who makes war without a reason that will still be valid when the war is over..." Otto von Bismark