Re: [Hampshire] Gentoo adaptec RAD controller hangs

2008/11/9 Keith Edmunds <kae@???>

> If the appropriate solution seem seems to be to use more than five or so
> drives an any one RAID array then I'd venture to suggest the solution is
> incorrect. Anyone contemplating using 20 (or 11) drives in one array is
> either doing it deliberately for some specific reason or doesn't
> understand the implications of what they are doing.


Having large numbers of drives provides greater I/O throughput by increasing
the number of spindles on your array. However, there are caveats. It is
advisable to use RAID-6 or some other technology which uses multiple parity
drives to reduce the risk of data loss due to multiple drive failures and to
also keep hot spares online.

I've used this on Netapp filers, which use a proprietary double-parity
version of RAID-4. A 14 disk enclosure can give you 11 data disks, 2 parity
and 1 hot spare and this is a pretty standard configuration. Some sites do
apparently use multiple enclosure arrays. I'd think long and hard before
doing this on a Linux system without the level of engineering support that
Netapp provides though.

To return to the original point: RAID-1 in the Linux kernel is cheap,
> reliable and effective. If RAID-5 is required, a dedicated h/w controller
> that provides true RAID is the best solution. Of course there are other
> ways of doing things.
>

In the ideal world where your Linux boxes all use exactly the same RAID
controller I would agree with you. Linux software RAID does have some
advantages over hardware controllers in my view, in that it is always
possible to:

- Configure software RAID from a running Linux OS
- Detect disk failures from a running Linux OS
- Remove the drives from one Linux system and re-assemble the array in
another Linux system

More importantly, all these tasks can be automated easily - since the
commands are the same no matter what hardware is used - within automated
build system (Kickstart followed by puppet in my case) and no manual
intervention is required. This is a definite benefit in a large server farm
managed by a relatively small team.

For these reasons, I am considering simply ignoring the hardware RAID
controllers in the servers at work and using software RAID throughout.

As you say, there are other ways of doing things, but those are my thoughts
based on managing ~100 physical Linux hosts.

Cheers,
Graham