From: Nix on
On 7 Dec 2009, jasee(a) verbalised:

> Theo Markettos wrote:
>>> Really? dd uses sector access, which should be about as quick as
>>> you can get. Or do you mean it slows down a lot when it needs to
>>> retry?
> This is what I've heard (100 times) and from my experience this time
> it certainly is, even without the errors I'm getting this time.

Bear in mind that consumer hard drives will retry failed reads on their
own when they hit a bad sector. They retry a lot so may take many
seconds to fail to read each duff sector, and while they do that they're
dead to the world.

Some (more expensive) hard drives are meant for use in RAID arrays, and
retry less (on the basis that the RAID array will be able to supply the
data and write it (which will automatically use a spare sector); such
drives also allegedly have higher vibration resistance on the assumption
that they'll have to cope with the presence of lots of other disks in
the same chassis. It's somewha ironic that, despite the I in RAID
standing for 'Inexpensive', the drives used in RAID arrays often cost
more than ordinary ones. (But you can use cheap disks if you want, and
normally I do. If we're hitting retries often, we're going to want to
chuck the whole disk anyway, in which case it's a good thing that the
disk was cheap!)