On Mon, May 05, 2008 at 01:00:19PM -0500, Les Mikesell wrote:
> > (NB: Les and I have gone back and forth on this issue before, and I don't
> > sense any hostility involved; we just have different experiences and
> > requirements, so we disagree firmly but politely).
BTW: This statement made me laugh. :-) That's what I called a civilised
discussion!
> Yes, I have nothing in particular against hardware raid, but like the
> simplicity of software mirroring and the advantage of being able to
> recover the data from any single drive.
I've yet come to a conclusion regardin HW or SW RAID. I've used both.
I've had both crash on me and recover from crashes.
My last issue was with some Dell PERC3 with an external HW RAID attached
(yes, that's HW RAID over HW RAID and the setup sucks badly). It managed
to forget it's configuration somehow and hung at boot. I had to unplug
everything, then start adding disks back in so it recovered
configuration. After this, everything worked as expected, no data lost.
I also had a SW RAID5 (actually 3 disks with 3 partitions each: 1 swap,
1 for RAID1 and 1 for RAID5). I suppose the disks got damaged during
shipping - we replaced all of them until today. There was no hotplug, so
I had rebooting included. It wasn't easy, but no data was lost.
(BTW: I had to figure out the correct mdadm commands every time. At
least, it didn't do anything unreasonable while I was trying to tell it
what to do.)
But I also had an SW RAID with some mysterious missing member.
Supposedly, I did something wrong, but after all, I couldn't use mdadm
for monitoring any more because it always complained although there was
no problem.
> >>> That said; my experince with software RAID is that if one of the disks
> >>> dies,
> >>> the whole machine might fall over anyway. In most cases you have to down
> >>> the
> >>> box and reboot in order to swap in a new drive.
> >> That's typically related to IDE (PATA) hardware, not anything to do with
> >> RAID. SCA (swappable scsi) or SATA in a swappable tray wouldn't have
> >> those issues.
> >
> > I can't say I have firsthand experience that way; can you enlighten me how
> > it is that PATA failing can cause the OS to crash, but not SCA/SATA?
>
> It isn't the OS. The drives typically fail in a way that locks the
> controller/motherboard especially when booting. It is hard to simulate
> or test because removing the drive doesn't cause this.
That's because of the master-slave scheme with IDE. If one of them dies
in certain ways, it will block the whole bus. SCSI might be vulnerable
to that as well, SAS and SATA are probably not since they've got point
to point connections.
> > (Firewall is the machine you're trying to fix, so you can't
> > access the Net to look up the syntax, you can't find the documentation
> > on the machine itself because it's a minimal install, the customer is
> > hovering over your shoulder asking when it will be done and what's going
> > on...
>
> A wireless card in your laptop can take care of that sort of thing.
A local mirror of relevant documentation as well. :-) And, of course,
exercising that before the machine goes into production.
> >> That's a problem with consultants... The plus side of software raid is
> >> that there are no other dependencies. If you have nothing left but one
> >> drive (assuming raid1) you can plug it into just about any
> >> hardware-compatible controller or even a cheap USB adapter and access
> >> the data. With hardware controllers you'd probably need to keep
> >> matching spares on hand.
> >
> > This is a good point; but I've never found it to be a problem in the real
> > world.
>
> I start with the premise that I want to be able to restore from a
> mirrored copy of my backuppc archive. It hasn't actually been necessary
> but I still consider it a big plus that in a disaster recovery scenario
> I can plug this copy into a laptop and restore files instantly.
That's one of the issues still bugging me - I'd like to have a live copy
of the whole pool. RAID doesn't protect from filesystem corruption and a
lot of related accidents.
Bye,
Tino.
--
„What we resist, persists.” (Zen saying)
www.craniosacralzentrum.de
www.forteego.de
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