[ale] new to raiding

[Danny Cox]

Just a couple of clarifications/enhancements:

On Thu, 2002-05-09 at 23:04, Denny Chambers wrote:
> Cade Thacker wrote:
> > 
> > I have mirrored two 1 gig drives at /home and my wife, via samba, is
> > saving her Grad school papers onto that share.
> > 
> > So my question is,
> > 
> > 3) how do I replace a drive, will it automagically pick it up, and bring
> > it up to speed?
> No, not that I recall. I think you will need to manually create a
> partition on the drive that is the same size or larger than the
> partition that you are trying to mirror, then you will need to do
> raidhotadd of the partition back into the raid set. Then the mirror will
> sync back up.  

	The syntax is: "raidhotadd /dev/md0 /dev/hdd1".  Stand well back. 
There are two files in /proc: /proc/sys/dev/raid/speed_limit_min and
speed_limit_max which determine how fast it will re-mirror (or, in the
case of RAID-[45] reconstruct).  Changing the min (set at 100) to
something larger will greatly speed up the re-mirror, at the expense of
other operations.  On the other hand, you can proceed with your regular
work during the re-sync.

> > 4) Can I boot with a mirrored drive unplugged?
> Sure! Just as long as your boot partition is not on the unplugged drive.
> The md device will just come up in a de-graded mode, which for a mirror
> is not very de-graded as far as performance is goes (No more overhead
> having to write to two drives), but you will not have you mirrored
> protection.

	Another small point, which you may already know: a RAID-1 is slower in
writing, since it must schedule a write for each disk, but it is usually
faster reading.  The driver keeps track of the last block accessed for
each drive, and uses the "closer" drive to service the request.

	Also: make sure you're using the "persistant-superblock 1" parameter in
/etc/raidtab.  This is the preferred method, and allows the driver to
"recognize" it's own.

-- 
kernel, n.: A part of an operating system that preserves the
medieval traditions of sorcery and black art.

Danny


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