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.
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