[sclug] hot drives

[Alex Butcher]

On Mon, 19 Jun 2006, Dickon Hood wrote:

> On Tue, Jun 20, 2006 at 00:12:41 +0200, Pieter Claassen wrote:
> : On Monday 19 June 2006 01:40, Dickon Hood wrote:
>
> : > : I have two 120GB Maxtors in a soft raid config (actually they are
> : > : plugged into a cheap PCI PATA raid card but after config, the linux
> : > : kernel still sees them as two disks so I just went with soft raid).
>
> : > It works.
>
> : Simple question. Why when I configure the SATA raid card and add the two
> : drives to an array do I still get presented with a two scsi devices in the
> : kernel? What does this mean? I was under the impression that a successful hw
> : config would present only a single raid device to the kernel.
>
> I can imagine scenarios with either one or three SCSI devices being
> presented: one HW RAID device only, or one HW RAID device, plus one each
> of the raw devices (so you can query them for status (temperature, that
> sort of thing)).

As far as I'm aware, all the fakeraid (Promise, Highpoint, et al) drivers
remove the raw devices and present a single SCSI device for each configured
array. IME, this means you lose the ability to use SMART query tools to
gather the information you mention above.

> : Even more intriguing, what happens if I configure HW raid as well as SW raid
> : and then have a drive fail?
>
> I should imagine you're not seeing the HW RAID device at all, so the
> chances are, the software RAID will sort it.

My guess is that the next time you reboot, the BIOS will complain that
there's no operating system because you'll have corrupted the fakeraid
array(s) by writing to the raw devices. YMMV, though.

>  You might want to check that you've got the right RAID driver compiled in
> / module loaded.
>
> I've played with hardware RAID under Linux once.  I hate it.  Use
> software.  If you need the performance, buy a faster (or second...) CPU...

HW RAID doesn't, IME, gain you performance, either with the various striping
and mirroring options, or even with parity-based arrays (e.g. RAID5/6). In
fact, in my experience, Linux software RAID5 is actually significantly
faster (like 3-4x) when running on a modern CPU (i.e. Xeon 2.8GHz vs. LSI
MegaRAID 300-8x).

HW RAID is a win where you want ease of configuration, ease of maintenance
and maximum availability (i.e. replacing failed drives in a true HW RAID
array is dead easy, whilst you need to know a little OS-specific stuff when
doing the same with SW RAID, and this can consequently reduce availability
if you don't have the appropriate expertise readily available).

> This particular thing had a standardish chipset which could be used in
> JBOD mode (which I did), and some RAID thing.  I gave up fighting with the
> module (binary blob) eventually.

After some bad experiences with nVidia's binary blob driver, I've resolved
to avoid using such hardware whenever there's an acceptable FOSS-only driver
alternative.

Best Regards,
Alex.
-- 
Alex Butcher, Bristol UK.                           PGP/GnuPG ID:0x5010dbff

"[T]he whole point about the reason why I think it is important we go for
identity cards and an identity database today is that identity fraud and
abuse is a major, major problem. Now the civil liberties aspect of it, look
it is a view, I don't personally think it matters very much."
  - Tony Blair, 6 June 2006 <http://www.number-10.gov.uk/output/Page9566.asp>