Re: Spare disk could not sleep / standby

[Molle Bestefich <molle.bestefich@gmail.com>]

Tobias wrote:
[...]
> I just found your mail on this list, where I have been lurking for
> some weeks now to get acquainted with RAID, but I fear my mail would
> be almost OT there:

Think so?  It's about RAID on Linux isn't it?
I'm gonna CC the list anyway, hope it's okay :-).

>> I was just curious about the workings of MD in 2.6, since it sounded
>> a bit like it wasn't possible to put a RAID array to sleep.  I'm about
>> to upgrade a server to 2.6, which "needs" to spin down when idle.

> Which is exactly what I am planning to do at my home - currently, I have

[...]

> Thus my question: Would you have a link to info on the net concerning
> safely powering down an unused/idle Raid?

No, but I can tell you what I did.

I stuffed a bunch of cheap SATA disks and crappy controllers in an old system.
(And replaced the power supply with one that has enough power on the 12V rail.)

It's running 2.4, and since it's IDE disks, I just call 'hdparm
-S<whatever>' in rc.local,
which instructs the disks to go on standby whenever they've been idle
for 10 minutes.

Works like a charm so far, been running for a couple of years.
There does not seem to be any issues with MD and timing because of the
disks using 5 seconds or so to spin up, MD happily waits for them, and
no corruption or wrong behaviour has stemmed from putting the disks in
sleep mode.

There have been a couple of annoyances, though.

One is that MD reads from the disks sequentially, thus spinning up the
disks one by one.
The more disks you have, the longer you will have to wait for the
entire array to come up :-/.
Would have been beautiful if MD issues the requests in parallel.

Another is that you need to have your root partition outside of the array.
The reason for this is that some fancy feature in your favorite distro
with guarantee periodically writes something to the disk, which will
make the array spin up constantly.
Incidentally, this also makes using Linux as a desktop system a PITA,
since the disks are noisy as hell if you leave it on.
I'm currently using two old disks in RAID1 for the root filesystem,
but I'm thinking that there's probably a better solution.
Perhaps the root filesystem can be shifted to a ramdisk during
startup.  Or you could boot from a custom made CD - that would also be
extremely handy as a rescue disk.