mdadm RAID and drives spinning up/down

mdadm RAID and drives spinning up/down

[Mark Knecht]

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.

Thanks,
Mark

Re: mdadm RAID and drives spinning up/down

[John Robinson]

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.