[Techtalk] Software vs. Hardware RAID

[Dave North]

I am the person who expressed a preference for software RAID. Since this
issue has been debated at great length elsewhere, I'd prefer to not do so
here. But for the information of those who wish to make their own
decision, here are some links and quotes:

First, from Linuxplanet's nice tutorial:
> Software RAID can be organized with the help of the OS and it doesn't
> need anything except additional CPU time for its support. But, CPU time
> is the cheapest resource among all that we have. There is myth that states
> hardware RAID is always better than software RAID. Hardware sellers like
> this myth very much, for reasons which you can well understand. We can
> also hear the same line of thinking from system administrators, too,
> though nobody knows why.

The rest of the remarks can be reviewed at:
	http://www.linuxplanet.com/linuxplanet/tutorials/4349/2/

However, from the NT era (1999 or earlier), we have this point of view:

> Hardware vs. Software RAID
> Just like any other application, software-based arrays occupy host
> system memory, consume CPU cycles and are operating system dependent. By
> contending with other applications that are running concurrently for
> host CPU cycles and memory, software-based arrays degrade overall server
> performance. Also, unlike hardware-based arrays, the performance of a
> software-based array is directly dependent on server CPU performance and
> load.

The rest of this work can be viewed at:
	http://www.uni-mainz.de/~neuffer/scsi/what_is_raid.html

Fishing around you'll find quite a few debates -- and some benchmarks --
to support both views, but the software crowd seems to be gaining ground
lately. I do not think the popularity of an idea is a good metric, though.
	The flexibility of the linux software approach is very appealing
to some. The OS-independence of hardware is appealing to others.

	This is presented not to try to convince anyone, but to let
everyone know there are rational people who think, even if one has a
hardware subsystem, there may be reasons to consider using software RAID
instead. This idea was first presented to me by someone doing just that,
having tested both systems.

	More to the point, if you have a hardware setup and would like to
compare its capabilities and idiosyncracies with software on linux, review
the excellent faq at:
	http://www.tldp.org/FAQ/Linux-RAID-FAQ/

My own experience setting it up with quad Reiser partitions took, if I
recall correctly, a couple of hours (mostly being nervous). But it was
very easy, and works fine. The partitions do not have to match (but should
be close if possible) and the disks do not have to match (but the slowest
will be the limiting factor in some setups).

I _do not_ think there is one _always right_ solution. I have, however,
become familiar with reports of several people having trouble with
hardware setups that disappeared when they switched to the linux
implementation. I would actually be interested to hear if anyone has seen
the reverse effect, as I have not heard of any such.
	However, that should not be surprising when one considers that the
software implementation costs less, and is easy to experiment with, so I
suspect quite a lot of new linux RAID users head straight for the software
setup without trying hardware at all (that would be, for example, me).
	And if hardware starts goofing up on you, the cheapest fix is, of
course, to just move to software. The converse is not true.


d