LinuxSA Mailing list archives

  From: Alan Kennington <akenning@topology.org>
  To  : LinuxSA <linuxsa@linuxsa.org.au>
  Date: Tue, 5 Jun 2001 07:11:14 +0930

Re: software RAID versus hardware RAID

Thanks zillions for all the hints.

I had been hoping for a simple judgement one way or the
other, more on reliability and manageability rather
than performance.

But the conclusion I come to from all of the comments is
that the hardware solution is not that much of an improvement,
reliability- and functionality-wise.

Concerning the earlier RAID thread, finishing 20 April 2001,
I had followed some of that when it occurred, but it was
mostly about speed performance, which doesn't really 
interest me at all.
I might only write a megabyte to the disk each day
or something. I'm only interested in the RAID 1 mode,
which I gather is called mirroring. 
I want to be able to deliver 1000 of these PCs to clients,
and for none of them to fail in the first year.
That's pretty crazy, I know. But more realistically, I'd
like no more than 1 per day to fail out of 1000.
That gives an MTBF of about 3 years (assuming that failures
are distributed Poisson, of course).

The thing that worried me was how much bother it was going to
be to set up the boot sequence with LILO, for which I've
found a good howto.

It sounds like I'm in for a lot of bother anyway if I use
the hardware RAID. SOmeone had told me it would just look like
a single drive etc. etc.
I hadn't realised that hardware RAID requires driver support
etc. to make it work. So it's still messy doing hardware RAID,
and it's yet another pieve of hardware that can fail.

Hot swapping is no big issue for me.
If a 1RU or 2RU rack-mount system fails, you still have
to get at the bad disk and replace it.
The rack-mount PC I'm working on right now won't let me get into it
without getting the PC out of the rack.
So with this sort of system, I'll be hot-replacing the whole
system using IP path bypass, and the dud system will go
back to the factory or something.

By the way, I found this note
http://www.linuxdoc.org/FAQ/Linux-RAID-FAQ/index.html#AEN127
on how to know if a disk has failed.
Apparently the /proc/mdstat file goes subtly different.
I wonder what it does if both disks fail.

-------------------------------------------------------
So my conclusion is that I should go for the linux software RAID.
I'll just have to take a deep breath and dive in.
If I don't surface again for a few weeks, I leave all my
PCs to my sister.

Cheers,
Alan Kennington.

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