SATA, Raid 1, LVM and controller question.

[Robin Laing]

Christofer C. Bell wrote:
> 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!
> 

Well as I have been on holidays, I finally get a chance to respond. 
It went so easy that I was surprised.  I gave myself a few hours to do 
the change and expected major headaches.  None occurred.

As you stated, the mdadm accepted the drives after moving between the 
controllers.  First good sign.

Added the LVM with the system-config-lvm but the extra drive didn't 
work as expected.  A quick search found that I had to use the CLI and 
I did lvextend and resize2fs.  All worked like a dream.  Had a beer 
and enjoyed the added disk space.

Now that I know it is easy, I am not afraid to add two more drives and 
extend the partition again.  At least until I build the RAID NAS for 
my home.

Man media files take allot of space.