Re: What is the point of RAID?
On Fri, 07 Nov 2008 17:15:08 -0500
Ken Heard <ken@heard.name> wrote:
> The installer does not allow / (root), /boot or swap to be part of a
> RAID array.
It should allow that --- I haven't tried it with software raid, but you
can boot from the raid array when you have a hardware raid controller.
The problem of needing a module or the support compiled into the
kernel for the raid is pretty much the same with hardware raid as it is
with software raid, meaning that it should be possible.
> Now, I assume that the purpose of RAID is to guard against loss of
> data if one of the drives in the array dies. In that case the data
> on the good drive will serve until the dead drive is replaced.
That depends on the raid mode. You can mirror one disk to another, make
several disks appear as if it was one larger disk and run a number of
disks in parallel to distribute the data across the array. One reason
to do this is to increase protection against data loss and to keep the
system running running even if disks have failed. Another reason is
increasing speed, as several disks can be a lot faster than one.
However, none of the raid modes is a substitute for backups in any way.
> If however the dead drive contains any of the / (root), /boot or swap
> partitions, the box would be unusable; as access to the good drive
> would be impossible. In such a situation I would not want to
> contemplate the hoops I would have to jump through to salvage the
> data from the good drive.
You can boot from a CD to rescue your data and/or to repair an
unbootable system. If the boot disk has failed, you can replace it and
reinstall on another disk without deleting the data on the disks you
have. If you use the raid to mirror your data and only one of the disks
in the array is broken, you can recover it from the disk that is still
working.
> The only way I can think of to avoid such a situation in this
> particular box where there are only two drives in the RAID array, it
> to have a third drive with everything but the /home partition stored
> in it, and use the RAID array only for /home. If so, I wonder why I
> should bother with RAID, as all data in the /home partition-directory
> needs to be backed up periodically in any event.
Well, sort of ... What you describe is a good way to use a raid 1
(mirroring). The advantage is that it is much easier to recover data
from the hard disk than it is to recover it from a backup --- and the
data on the disk is likely to be more recent than the backup. The other
advantage is that the system keeps running --- I've had a crappy Ausus
board (don't buy Asus!) that would loose connection to one of the SATA
disks in irregular intervals; but fortunately, I happened to be using a
raid 1 ...
Besides, if the third disk you have installed the system on fails, you
are looking at the same problems you have if you installed the system
on one of the disks in the raid array on a non-raid partition.
Now when you have three disks, you can run a raid 5. In case one of the
disks fail, all you need to do is to replace the broken one.
And always use raid when you can. I've seen too many disks fail in too
short time for not to use it.
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