Software RAID issues (was Re: Suggestions solicited, server bring up)

Tom Buskey tom at buskey.name
Sat Nov 21 17:03:29 EST 2009


On Fri, Nov 20, 2009 at 2:42 PM, Bill McGonigle <bill at bfccomputing.com>wrote:

> On 11/18/2009 03:33 PM, Ben Scott wrote:
> >   I've found hardware RAID to be more reliable when booting with a
> > degraded disk set.  A smart controller will just fail the bad member
> > disk and ignore it.  Software-based solutions -- which don't kick in
> > until the OS is running -- sometimes get caught up trying to boot from
> > a failed disk.
>
> To steal a line from one of my LUG members, "Please don't use RAID-5".
> :)  A healthy, properly configured (and tested) RAID-1 will boot nicely.
>
>
I'm a big fan of an OS based software RAID-1 for the OS disks.  Put your
data on any other kind of RAID you want.

The OS should be able to boot the installation disks and understand and fix
the OS drives without any drivers, special hardware.  I put the data
elsewhere to isolate OS issues/performance/etc from the data drives.

I once saw a Jr Sysadmin turn a 15 minute job into an all nighter with
hardware.  It was a SunOS system with a RAID controller.  The OS was put on
the RAID to improve reliability.

After moving data partitions around, the admin changed fstab and rebooted.
When it didn't reboot because of a typo (we've all done that) he put the OS
CD in & it didn't have a driver to recognize the controller.  In order to
install the driver, he needed an OS.  It took awhile to get enough disk
space to install a minimal OS with the driver that could mount the RAID so
he could fix his typo in /etc/fstab.

This pretty much negated any reliabilty benefit from having the OS on RAID.

Thing are much better today with controller support.  I'm not sure software
RAID was built into SunOS back then either.

Nowadays most Linux, BSD and Solaris can do software RAID for the OS.  It's
save me a few times from doing a reinstall.  I was able to replace a failed
drive during a planned downtime instead.
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