[TriLUG] Setting Up RAID-5

[Matt Bidwell]

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