Subject: Re: hot spares for RAID 1 sets?
To: Malcolm Herbert <firstname.lastname@example.org>
From: Chuck Swiger <email@example.com>
Date: 08/15/2005 21:21:37
Malcolm Herbert wrote:
> On Mon, Aug 15, 2005 at 08:22:28AM -0400, Chuck Swiger wrote:
> |>I have a host with three 120G internal disks and have a RAID 1 mirror
> |>set up which autoconfigures at boot to be the root device.
> |Set up a three-way mirror. Having a hot spare sitting around isn't as
> erm, I thought mirrors with more than two disks wasn't supported in
> RAIDFrame. Excerpt from raidctl manpage:
> In this case, /dev/sd20e and /dev/sd21e are the two components of the
> mirror set. While no hot spares have been specified in this configura-
> tion, they easily could be, just as they were specified in the RAID 5
> case above. Note as well that RAID 1 sets are currently limited to only
> 2 components. At present, n-way mirroring is not possible.
The documentation is likely to be right, I'm familiar enough with RAID to
recommend a 3-way mirror for your usage case, but I didn't know RAIDframe's
limitation on this specific area. :-)
3-way mirroring was one of the techniques that people used to use to make a
consistent dump of a live filesystem: you'd split one disk off the mirror,
leaving two disks mirrored for continued redundancy, then dump the single disk
(which would be frozen since nothing would be writing to it, thus not changing
underneath the dump process), and then reattach the mirror.
Later came the ability to perform a filesystem snapshot, originally found in
some higher-end NAS filesystems (think Auspex & NetApp), and more recently even
in FFSv2 aka UFS and the "-L" flag to dump (which may be a FreeBSD-ism).