RE: OT: RAID Help

["Paul Smith" <wheelbarrowhandle@xxxxxxx>]

Bruno,


      Thanks for a very useful insight into raid.
I can now understand
that I can get no more useful space from the current setup with out
compromising the mirror.

      I can therefore do 1 of two things, get a
raid 5 card or decide
that the contents of the array are not important enough and simply
remove the mirroring and live with the risks.

Since the space is used for video editing as long as I back up the
project files I can re create the set up should a disk fail.


H'mm i'm now not sure which way to go. 


Once again thanks for you help with this.


Any one care to recommend a Raid 5 pci card ?


Regards,

Paul


-----Original Message-----
From: Bruno Prior [mailto:bruno@xxxxxxx] 
Sent: 23 September 2002 23:55
To: ukha_d@xxxxxxx
Subject: Re: [ukha_d] OT: RAID Help

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

>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.


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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