From mboxrd@z Thu Jan 1 00:00:00 1970 From: John Robinson Subject: Re: mdadm RAID and drives spinning up/down Date: Sun, 21 Mar 2010 22:01:34 +0000 Message-ID: <4BA6973E.7080401@anonymous.org.uk> References: <5bdc1c8b1003211440n7e0b7848s7c069c3394d487a3@mail.gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <5bdc1c8b1003211440n7e0b7848s7c069c3394d487a3@mail.gmail.com> Sender: linux-raid-owner@vger.kernel.org To: Mark Knecht Cc: Linux-RAID List-Id: linux-raid.ids On 21/03/2010 21:40, Mark Knecht wrote: > I'm moving forward on my first machine using RAID. This will be a > fairly lightly used home server storing MythTV recordings and backups > coming from other Linux machine. Due to the light workload I expect > there will be times, sometimes lasting hours or maybe days, when the > drives are unused and might (should?) spin down. Additionally with > everyone trying to save power and be 'green' more and more drives > probably do this by design anyway. > > OK, so never having build a RAID array and knowing nothing about this > I'm curious about how mdadm handles this sort of thing. If the drives, > using something like hdparm to set parameters, have times that shut > them down for a while, when the system needs them spinning again are > there ways to buffer write data and delay read data until everything > is ready to roll again? I.e. - it's the middle of the night and Myth > wants to start a recording. Everything is shut down and not it needs > to start. > > Is it a problem if one drive spins up more slowly? Could that fool the > RAID software into thinking the drive has died when it's actually just > asleep? > > Sorry for such newbish questions. I think the answer to this is much the same as the other thread about using "RAID-class" drives: md doesn't set timeouts itself, so whether things work or not depends on whether the device driver underneath fails reads while a drive is waking up. I'd say if you can make single drives sleep and spin up again without problems, you can make md arrays do it - because the Linux md layer knows nothing about it. The ReadyNAS, formerly from Infrant but now from Netgear, does precisely this and works well, and uses Linux md, but I don't know whether they've tuned anything. It's open source of course, so it might be worth taking a look. Cheers, John.