[Gllug] Discs for a RAID1 pair

Nix nix at esperi.org.uk
Mon Jul 4 22:36:43 UTC 2011

On 4 Jul 2011, Chris Bell verbalised:
>    Thanks for all the replies, what I was most worried about was "disk not
> ready" problems with green discs,

I'm using four WD GreenPower disks in my array and have no trouble with
them at all. Sure, perhaps the Areca RAID controller is hiding any
errors, but I really doubt it.

Bear in mind that these disks can service I/O perfectly well even when
spun down: it's just slower until the disk is spinning at full speed
again (something it doesn't bother doing unless quite a bit of I/O is
done, so flickers of activity won't trigger spinups and the drives are
likely to spend nearly all their time spun down -- which should lengthen
their lives, as well.)

I cannot imagine how horrendous the code must be to service I/Os on a
drive which is being spun up. This is some impressive engineering.

>                                and data written to and then held in cache
> until actually written to disc, with changes possibly copied in the wrong
> order.

That happens anyway if you're using RAID: the only way to disguise it is
to ensure that you never, ever get power failures while doing I/O on the
multipath path of the RAID (if using md, that'd be the whole system: if
using a hardware RAID controller, only the controller needs to be
protected like that, so a battery-backed controller will suffice).

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