Re: [PLUG] Raid Basics (was: Linux Raid)

["Forge XP" <forge@dca.net>]

> In case anyone is interested in reading, the RAID manual for mo'bo'
> AD70-SR is here: http://sj.dfi.com.tw/download/MANUAL/RAID.PDF

It's a Promise chip, that much I was 90%+ sure of, now 100%, I just can't
tell *which* Promise chip, thanks to that stupid sticker. Almost certainly a
PDC20265R.

> > > If the built-in controller is RAID enabled, how is that software-based
> > RAID?
> >
> > The controller is nothing but a standard ATA controller, the 'RAID'
portion
> > is entirely located in the firmware. The firmware is executed by the
CPU. In
> > Windows 9X, where accessing a device through the BIOS information was
> > acceptable, you could insall onto one of these RAIDs, format, move
files,
> > etc, without ever loading a driver. In NT and Linux, among others, a
device
> > driver is needed. Basically, the Windows and Linux drivers re-create
what
> > the firmware was doing, along with adding a few other tweaks.
>
> I may be off because I haven't used one of these controllers, but from
> what I read in the manual, the driver for the controller is like any
> other driver and the striped drives are presented to the OS as a
> single disk.  The additional software is mostly for monitoring.  If
> software is needed to create a RAID, why even bother with the special
> controller?  (That's what I need to know before bothering with a
> RAID'ed mo'bo'.)

The manual is written by marketing. Of course they will never use offensive
words like low-end or software-based while describing their low-end
software-based product. :)

The striped drives *are* presented to the OS as a single disk. The
additional software *IS* only for monitoring and can be ignored completely.
It is possible to create RAID arrays in the BIOS. You can access these RAID
arrays in DOS or Win9X without any driver by way of the firmware support.
HOWEVER.... In order to use the Fasttrak's RAID array in any HAL-enabled OS
(any of them worth a damn, IMHO), You are going to need a driver to perform
the  functions that were previously provided by the BIOS.

> > > >'RAID' users are actually using a form of bootable software RAID,
> > courtesy
> > > >of their Promise Fasttrak and HPT onboard controllers. There's
nothing
> > wrong
> > > >with these, I'm using a Fasttrak TX4 myself, but they should not be
> > confused
> > > >with true hardware RAID.
>
> What about caching?

What about it? The Promise driver for Windows will aggressively cache reads
and writes to the system memory, though I'm not sure of the Linux driver. I
haven't *seen* it gobbling up godawful amounts of RAM, anyway. This,
however, is nothing you can't also accomplish by tweaking the host OS a
little.


Basically, I'm just trying to clarify the categories here, a little.

There is true full-hardware RAID, where all calculations are performed
on-card, and the OS sees usually a SCSI controller and a large disk (or more
than one, etc).

There is hardware-assist/firmware-based/partial-software RAID, where the
BIOS (which is executed on the CPU), or a driver (also on the host CPU)
provides the RAID mechanisms, but the array is bootable by the system BIOS.

Lastly, there is full software RAID, where the OS abstracts the partitions
involved and provides RAID functions at the OS level only... Non-bootable,
obviously.



If there's anything I'm still not explaining well, please rephrase the
question again, as I'm probably not getting what you mean.

Also, one of the chief (only) real benefits of firmware RAID is that it's
multi-OS ready. I use my Fasttrak because I find 8+ drives unwieldy when I
have to convert Windows drive letters to Linux mount points in my head all
day. One or two big RAID arrays are easier to remember.

To loosely quote: 'why even bother with the special controller?  (That's
what I need to know before bothering with a RAID'ed mo'bo'.)' - Don't
bother, unless you are using Windows and a Linux. I believe FreeBSD also
supports the Fasttrak. If I were only running Windows *or* Linux, I would
never have bought this TX4, I would have just bought the (slightly better
supported) Ultra100 plain IDE controller, and done OS-based software RAID.
In either OS (2K logical volumes or Linux MD) it would have been *FASTER*.




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