Jeff Dean on Wed, 17 Feb 1999 12:12:01 -0500 (EST)

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Re: Stupid, Stupid RAID questions...

> Stupid, Stupid RAID questions...

They're just questions.  Stupid would be failing to ask!

> Can Linux see _hardware based_ RAID as a single disk?

Yes.  There are flavors of hardware RAID.  How well suited
to Linux they are depends on which type you're talking

The most flexible type would be a SCSI-SCSI
type hardware RAID controller.  These are made by a
number of manufacturers and used by many integrators
building generic storage boxes.  They require absolutely
no OS drivers and can be moved between and sometimes
shared between various systems, even differen OSs at the
same time (Imagine a big disk subsystem with two system
ports: a Linux/Intel and NT/Alpha on another - we do just
that).  You simply put a supported SCSI adapter in your
system and hang the RAID controller there with one or
more of its host ports.  The RAID controller just appears
as a big disk or disks with one or more partitions
(using multiple LUNs).  Very easy.  Downside: takes a
long time to format and is difficult to monitor via SNMP;
also more expensive due to extra SCSI controller in the
host.  Example: CMD CRD-5440 4-port UltraWide SCSI.

Embedded controllers sit inside your host system.
There are a number of vendors making these.  Because
they are a proprietary controller, they require an OS driver.
Often, as in the case of Adaptec, Linux may be a supported OS.
They're fast to format because they understand the host
OS' filesystem, and they're easy to monitor because
monitoring software usually comes with the adapters.
Downside: you can't multihost these adapters, and they're
rooted in the host so they're nearly impossible
to move from system to system.  I don't know of a
particular example with a Linux driver, but Dell claims
that their OEMed host-based RAID controllers are supported.

Finally, you may see a few boards with little embedded
RAID controllers meant to extend the functionality of
embedded SCSI.  This is a special case of the
embedded controller above, but since they're low-cost
you're not likely to see Linux drivers for these for a while
(Adaptec - prove me wrong on the ARO-1130!!!).

> Is the new kernel's RAID actually _software based_ RAID?

Yes.  I've not yet done it with Linux yet, but the idea is that
you're configuring multiple partitions, most likely on multiple
physical disks, into a single volume.  You get performance
and/or redundancy by doing this.

Hope this helps.

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