[Wolves] Software RAID

[David Goodwin]

Adam Sweet wrote:
> Hi all
> 
> Does anyone here have experience of software RAID?

Yes

> I would like to know what the performance is like and
> how easy it is to rebuild an array after a disk
> replacement.

Performance is normally acceptable (the man pages or something I've read 
claim to have similar performance to a RAID controller that's a few 
years old...). Most RAID controllers are software based now; there are 
relatively few (more higher end ones I think) that support true hardware 
raid.

Array rebuilding depends if you use "mdadm" or "raidtools" to control 
the kernel; neither will affect the performance which is generally 
governed by the I/O bandwidth you have. You may find the system is 
sluggish on rebuilding (just like it would be if you were busy writing 
to the disk... )

When rebuilding, cat /proc/mdstat, which gives a nice little progress 
bar and time / io count etc.

> 
> I am also interested to know whether you can RAID the
> whole disk including the boot partition etc, so that
> in the event of a disk failure you can simply replace
> a disk, reboot and the rebuild the array or whether
> you just get to keep the data on the mirrored disk and
> then have to reinstall on the new disk to boot to a
> poitn where you can rebuild the array.

It can, but it depends upon boot loader (i think!)

LILO will allow you to have everything under RAID1 (i.e. swap, / etc) 
while GRUB doesn't support something (or does now, but didn't not very 
long ago?).

I don't _think_ it's necessary to have a seperate /boot partition with 
grub now (I'm sure we have machines here which don't, yet mine at home 
does).


> 
> Apologies if this is either obvious or an rtfm, none
> of the tutorials I am reading at the mo seem to
> explain this part. To me you need a booted kernel
> before you can have software RAID, so therefore you
> can't RAID your /boot partition. Maybe I'm
> misunderstanding.

It depends if the bootloader (lilo or grub) support software raid. I 
think they support RAID 0 and RAID 1 but nothing else (in which case you 
probably need a seperate /boot).

Most/all vendor kernels support software raid out of the box, so you 
should have few problems. Most installers will configure/set it up for you.

David.

-- 
David Goodwin
w: http://www.clocksoft.co.uk
e: david.goodwin at clocksoft.com
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