: I've frequently heart the terms "hardware RAID" and "software RAID"
: bantered about this and other newsgroups. I'm confused.
: Any RAID solution requires both hardware (a SCSI controller -- either
: embedded in the system board or an adapter board) and software (either as
: part of the OS or using some utility), don't they?
Well, yes--at some level there is always "Array Management Software"
that makes the operating system think that multiple physical disks
are to be treated as one virtual logical disk. This is the software
which handles algorithms for things like maintaining data integrity
if a disk goes away, for handling caches, etc. This is not to be
confused with the software which allows users to monitor and
configure their RAID units, like through a GUI which notifies
the user of faults on disks, etc. The software that's being
discussed here is the software which handles shuffling around
checksums, and that sort of activity.
However, the question is where that software lives. In "hardware RAID"
the software lives on a controller card, usually inside the disk
array cabinet itself, though placing that controller card inside a
host is also a possibility. In "software RAID" the software runs
on the host computer.
The "RAID Advisory Board" calls "hardware RAID" a "subsystem-based
disk array," meaning that the software executes in the array's
controller card. The benefit of this type of array is that since it
generally uses SCSI buses to connect to the host, it can be connected
to a number of different types of hosts without needing to port
the RAID software. It is also the case that since the software runs
on the controller card, it doesn't have any impact on computing speed
on the host.
The RAB calls "software RAID" "host-based disk arrays." This type of
array also has advantages--one of which is that by distributing the
software and disks over multiple hosts you can give the system higher
availability. They are cheaper, and often more easily configurable.
However, they are usually less powerful and slower, since the software
usually has less low-level commands available to it with which to control
I hope I wasn't just repeating a bunch of stuff you already knew...
most of this information came more or less verbatim from the
RAID Advisory Board's publication "The RAIDBook," published in
June of 1993.
-David Spencer, Software Engineer, CLARiiON Advanced Storage Solutions