[mythtv-users] RAID 5 of different sized disks for mythtv storage?

Alan Chandler alan at chandlerfamily.org.uk
Mon Sep 11 21:19:08 UTC 2006

On Monday 11 September 2006 21:53, Paul V. Gratz wrote:
> Hey all,
>   I'm fairly early in the contemplation stage of setting up RAID for
> my backend storage needs.  Currently I've got LVM setup across three
> drives: part of a 250GB drive, all of a 200GB drive and all of a 300GB
> drive.  Those drives are broken up into 50GB partitions to make it
> easier to shift things around w/ LVM when the time comes.  I just
> bought a new 300GB drive.

Raid 5 partitions all need to be the same size, why not do something like
make one 200G partition on all drives and make it raid 5, one 50G partition on 
the two bigger drives and make them raid 1, and lastly use the remaining 50G 
as a separate partition.

Use LVM2 on top of the raids to make separate logical volumes. That way you 
can easily create logical volumes and move things and change their sizes (be 
careful with filesystems - some can only grow and not shrink)

>   Setting aside the issue of each drive needing to be on its own IDE
> bus (right now they share but I'm planning on picking up a new
> controller card to get to one drive per bus). 

Why don't you use sata, each disk is someone less likely to interfere with the 
others if it fails.  Assuming your motherboard is not too old it should work 
OK (but beware, I have an old celeron 1.6Ghz motherboard where the last bios 
update was about 4 years ago and I couldn't get a new sata pci card to work 
with it)

> Is there a way to use 
> RAID5 on 50GB partitions spread across several disks or is that asking
> for trouble. 

Not sure I understand what you are really saying.  Raid 5 needs two out of the 
3 disks working to not lose data.  If you are trying different combinations 
of 3 disk sets then consider what happens when each disk fails - how many of 
each raid 5 array disks(partitions) do you loose?

> I think I remember someone mentioning having that kind 
> of setup.  Alternatively it sounds like I either need to get one more
> 300GB drive or settle on 250GBs for the RAID setup wasting the last
> 50GB of the two 300GB drives to make this work.  Is this correct?

No see above - put the really valueable stuff on raid1 and the less valuable 
stuff on raid 5. - or join them via LVM (ie pvcreate both /dev/mdX devices 
and then vgcreate/vgextend a single volume group with these two devices)

Alan Chandler

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