[TriLUG] Setting Up RAID-5

Josh Vickery josh at vickeryj.com
Fri Dec 8 09:39:14 EST 2006


CPU power may be cheap, but unless I'm mistaken, hard drives are even
cheaper.  Have you considered multiple raid 1 arrays instead of a
single raid 5?  The CPU overhead should be lower at the expense of
less disk space.  It would also probably present the easiest recovery
situation, since any single disk in the collection would be usable all
by itself.

Josh

On 12/8/06, Matt Bidwell <bidwell at dead-city.org> wrote:
> Jason Tower wrote:
> > i'm sure someone can find my old post(s), but there are three major
> > problems with software raid:
> >
> > 1. the bootloader is installed on the MBR of the first disk only.  if
> > that disk goes bye bye you're in trouble.  sure, you can install
> > lilo/grub on multiple MBRs but that's a pita and an inelegant solution.
> >
> > 2. some sata chipsets flat out don't work with software raid 5, they
> > will crash the system hard either during initialization or under heavy
> > i/o.  i've personally seen this happen on no less than three totally
> > different systems with multiple distros.
> >
> > 3. if a disk dies suddenly, the system is gonna crash regardless of raid
> > because the kernel can no longer communicate with /dev/sdx, it just
> > disappears.  go ahead, set up software raid with hot swap disks then
> > yank one out while the system is running, see what happens.  the data
> > itself is probably ok (you'll have a degraded array upon reboot) but
> > availability is shot.  plus your fstab may no longer be accurate once a
> > disk is removed.
> >
> > there are probably workaround for these issues, or if they don't bother
> > you then knock yourself out with software raid.  i use it myself if
> > circumstances justify it.  but i've built enough systems to know that
> > hardware raid exists for a reason.  if you want to do the job right get
> > a 3ware card and sleep peacefully.
> >
> > jason
> Jason,
> As per number one, here is how you write to /dev/md0, /dev/hda and
> /dev/hdc in lilo.  It's straight from the software raid docs.
>
> boot=/dev/md0
> root=/dev/md0
> #this writes the boot signatures to either disk.
> raid-extra-boot=/dev/hda,/dev/hdc
> image=/vmlinuz
> label=RAID
> read-only
>
> As far as whether to do raid5 in software, I haven't done it.
> I do trust mdadm to do raid1.
>
> Matt
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