Re: OT: RAID Help

[Bruno Prior <bruno@xxxxxxx>]

On Mon, 2002-09-23 at 16:52, john.benfield@xxxxxxx wrote:

> >I currently have an Abit AT7 MD with 4 channel IDE raid built
in.
> (Supports 8 Ide Drives)
> >I have at the moment 4 120gb disks. One on each ide channel.
> >These have been configure for Raid 0+1. Such that I have 230gb of
space.
> >I have now used this up and require to make it bigger.
> >Firstly I guess I can not just add disks to the array and hope
that my
> >data is safe.

>From bitter personal experience, I can say this is not safe. You could
probably add disks to a RAID-1 without much risk (although I know
nothing about what utilities you could use to achieve this with your
RAID controller). But increasing the size of striped arrays is
difficult, and not to be undertaken lightly, and especially not without
a backup. I doubt the drivers for your RAID controller support this,
anyway.

> >I plan to add 4 more disks to give me around 460gb of space.
However how
> >should I get the  raid laid out.
> >
> >Do I use all primary disks and then mirror on the secondarys or
should I
> >group 2 primaries and 2 secondary and then mirror then on the
others.

Depends why you are using RAID.

If it's for HA (High Availability, the other HA), then the simple
answer
is, don't put more than one disk on an IDE channel. If your master goes
down, it will take the child with it. If both master and child are
striped on the same mirror, then the other mirror will carry on in
degraded mode. _But_, if both master and child were striped on the same
mirror, you were paying dearly in performance terms, as the whole point
of RAID-0 is to enable disk access to occur in parallel, which cannot
happen on the same IDE channel. Basically, the RAID-0 part of 0+1 is a
waste of time in this configuration. On the other hand, if master and
child are on different mirrors, then they will take both mirrors down,
and you have gained nothing from having the RAID-1 option.

Why not stick with your 4 disks and go for a RAID-5 configuration? This
will give you decent read speed, but slower writes, single disk
redundancy, and an increased capacity of around 350Gb. Alternatively,
if
you must have more disks, have a look at the Promise cards, which will
give you extra IDE channels (as long as you have enough IRQs). Either
run two separate mirrors, one on the onboard controller and one on the
Promise, or use software-RAID to combine the disks on both controllers.
I don't know your motherboard, but most onboard IDE RAID is not really
hardware RAID anyway, so it shouldn't make much difference to
performance if you don't use the onboard RAID drivers.

Cheers,

Bruno Prior

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