At 1/24/2002 03:22 PM +0000, you wrote:
>I'm (hopefully) going to end up with 6 9GB Hot-Plug Untra2 SCSI disks on an
>Integrated Dual Channel Wide-Ultra SCSI-3 Controller. This will not provide
>H/w raid, so I'm looking at doing it in s/w.
Software RAID is limited to 0 (striping) and 1 (mirroring). RAID-0 will
give you more speed in read/write over n > 1 disks, RAID-1 will give you
more speed in read but not write with pairs of disks, since its primary
purpose is to make sure that one disk is good if the other fails.
>1) I intended to use striping (raid 5?) over the six disks. Am I right in
>thinking that this improves performance by spreading the workload more evenly
>over the disks? If one of the drives fails, I understand the system will
>carry on but generate warnings. Is the S/w raid in Linux good enough to let
>me swap out and rebuild the disk without loss of service?
You're *thinking* of RAID-5, which is striping with parity check
information distributed among the disks. RAID-5 does provide many of the
benefits of RAID-0 in terms of speed, *and* it can survive the hardware
loss of one drive.
However, the parity calculations for RAID-5 are CPU-intensive-enough that
RAID-5 is *only* supported in hardware; with s/w RAID you're limited to 0
and 1. Your RAID controller (and the bays in which you put your drives) are
also what determine if your drives are hot-swappable.
If you do RAID-0 over the whole array, then it's one big disk. If any part
of it fails, it's all gone. Lots of speed but the same fault-tolerance as a
single disk (i.e. none).
>4) Can anyone suggest better alternatives?
I'll modify Ashley's suggestion:
1. Mirror two disks (RAID-1) for the entire system except /home;
no server I've ever set up has needed more than 9GB of system space.
2. Use Mondo (http://www.microwerks.net/~hugo/index.html) to make
a backup image of your freshly-installed perfect system onto CD so you can
restore the system *quickly* if you need to.
3. Stripe the other four disks (RAID-0) so you get a fast 36GB for
/home. Put your swap partition on this array as well, to separate it from
the rest of the system and to make it faster.
4. Make good backups of your /home. With four drives, your chances
of a drive failure are now four times greater than the chances of a single
drive failing. Still minute and tiny, mind you... I'm not trying to scare
you here. But yes, four times greater.
--
Rodolfo J. Paiz
[EMAIL PROTECTED]
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