Linux Software RAID a bit of a weakness?

["Colin Simpson" <csimpson@csl.co.uk>]

Hi, 

We had a small server here that was configured with a RAID 1 mirror,
using two IDE disks. 

Last week one of the drives failed in this. So we replaced the drive and
set the array to rebuild. The "good" disk then found a bad block and the
mirror failed.

Now I presume that the "good" disk must have had an underlying bad block
in either unallocated space or a file I never access. Now as RAID works
at the block level you only ever see this on an array rebuild when it's
often catastrophic. Is this a bit of a flaw? 

I know there is the definite probability of two drives failing within a
short period of time. But this is a bit different as it's the
probability of two drives failing but over a much larger time scale if
one of the flaws is hidden in unallocated space (maybe a dirt particle
finds it's way onto the surface or something). This would make RAID buy
you a lot less in reliability, I'd have thought. 

I seem to remember seeing in the log file for a Dell perc something
about scavenging for bad blocks. Do hardware RAID systems have a
mechanism that at times of low activity search the disks for bad blocks
to help guard against this sort of failure (so a disk error is reported
early)?

On Software RAID, I was thinking apart from a three way mirror, which I
don't think is at present supported. Is there any merit in say, cat'ing
the whole disk devices to /dev/null every so often to check that the
whole surface is readable (I presume just reading the raw device won't
upset thing, don't worry I don't plan on trying it on a production
system). 

Any thoughts? As I presume people have thought of this before and I must
be missing something.

Colin


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