SATA, Raid 1, LVM and controller question.

[Christofer C. Bell]

On 12/15/05, Robin Laing <Robin.Laing at drdc-rddc.gc.ca> wrote:
>
> Now with the present setup, I have the md0 drives on the same SATA
> controller chip and I notice that when large files are written to the
> array, there is a pause in the operation of the workstation.  Anything
> being done is affected.
>
> I was wondering if moving the drives to different controllers would
> benefit in throughput as putting IDE drives on different controllers?

Accepted practise is to run mirrors across controllers for reasons of
performance (ie; avoding the situation you're seeing) and for fault
tolerance (loss of a controller).  If you have a correctly configured
/etc/mdadm.conf file, then you should (in theory) be able to simply
plug the drive into the other controller and have it work.

The md subsystem does not look at, for lack of a better term, the
"hardware path" of the drives it's assembling into an arry, it looks
at the UUID that's tagged to each disk.  This UUID is unique per RAID
volume regardless of where the disks are physically located in your
machine.

You can see this UUID by running the following command:

# mdadm --detail --scan | grep ARRAY

In fact, you can put the output of that command in /etc/mdadm.conf and
have a more or less working configuration.  You may choose to add
"auto=md" at the end each line to force the md software to create the
relavent md entries in /dev at boot time should they be found to not
exist.

You'll also need to add the following two lines on top of each ARRAY entry:

DEVICE partitions
MAILADDR root

The first line tells mdadm to use /proc/partitions when determining
what it should scan for md plexes.  You can specifically enumerate
partitions or devices here instead as outlined in the manpage for
mdadm.conf.  The second line tells mdadm who to email should a plex in
an md device fail or otherwise experience problems.

Best of luck!

--
Chris

"I trust the Democrats to take away my money, which I can afford.  I
trust the Republicans to take away my freedom, which I cannot."