[vdr] Re: OT: LVM

[Emil Naepflein <Emil.Naepflein@philosys.de>]

On Thu, 30 Jan 2003 16:58:44 +0200, Lauri Pesonen <lauri.pesonen@iki.fi>
wrote:

> There was a disk discussion on the list a while back about how to use
> multiple disks. People seemed to prefer either RAID5 or then multiple
> independent disks which all contained complete recordings (as opposed to
> VDRs default way of placing files all over the place). RAID5 is obviously
> the best solution, but you will loose some disk space and you need to buy
> additional hardware (the RAID card). 

You don't need any raid card because there is software raid available
with the md driver.

> Using independent disks requires some
> planning and manual (?) work in organising recordings. 

To reduce the risk of total data loss you should move all files
belonging to a recording to one disk partition. Otherwise you risk all
your permanent recordings on one single failure.

Here is the evolution of my setup:
I have started with 3 80 GB maxtors. I had partitioned them into 7 equal
partitions. I then defined 7 raid5 partitions spanning the 3 disks to
allow easier upgrade. As the mainboard with on-board additional ide
channels (GA-ZXR3.0) wasn't available yet, I used a normal mainboard and
defined 3 data disk and 1 faulty disk and ran the raid5 in degraded
mode. After a few months the mainboard was available and the upgrade was
easy. I just replaced the mainboard and added one additional disk to the
on-board raid controller. All what I had to do was to partition the new
disk and do a "raidhotadd". After this point all the data was protected
by raid5. After some time I ran out of space and I added additional
disks. This was easy because I have planned this from the beginning by
partitioning the disk in 7 parts. Because of this, a upgrade to a
further disk is possible without doing a backup. 

First you partition the additional disk the same way as the old ones.
Then you have to free a partition on the old array by moving data to the
spare partitions on the new disk. Then the raid5 is recreated with the
corresponding partition from the new disk and the filesystem is created.
Then you go to the next partition and move the data to the fresh raid5
with higher capacity. And so on. Of course, the links from the primary
to the secondary are wrong now. But this is easy to fix. I wrote a tool
which recreates the symbolic links for the new location.

I have done this for each of the partitions, except for VDRs primary,
without any downtime. As the amount of data to move is large, this all
takes some time. But this really doesn't matter because VDR could still
run. Only the access to the partition which is recreated is not
possible.

This all wouldn't be possible with a hardware raid5 controller because
the raid5 they create contain the full disks and not single partitions.
Adding additional disks or upgrading to disks with higher capacity step
by step is not possible. You would have to do a backup or loose all the
data when doing a upgrade. What private user can do a backup for 500 GB?
Here software raid5 is much more flexible.

To make the software raid5 in my system more reliable I now have added a
Promise SuperTrak SX6000 raid controller to my system. I don't use the
raid5 functionality because of the upgrade problems mentioned above. I
just defined single disk raid0 and use each of its separate channels as
single disk. Now each of the disks in my system runs on a separate ide
channel and a faulty disk cannot cause going the raid5 down because the
other disk on the same ide channel is also blocked. BTW, such errors are
not really fatal with software raid. You just have to recreate the array
with the same disk sequence and after a fsck everything is back online. 

The next step in my system is to upgrade the 80 GB disks on demand by
replacing them with the new 320 GB Maxtor Maxline. Software raid5 makes
this possible. 

I hope this gives some hints to others which also want to build a server
over the time.

Enjoy,

Emil


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