[mythtv-users] LVM RAID 5 partition schemes

[Milan Andric]

On Apr 7, 2005 1:58 AM, David <myth at dgreaves.com> wrote:
> Milan Andric wrote:
> 
> >Newbie is getting a fuzzy signal here.  Can someone please confirm that
> >using RAID 5 and LVM one cannot grow a filesystem by adding
> >a new disk/partition?
> >
> >
> It's never that easy I'm afraid.
> 
> Some terms:
> A hard disk has lots of data blocks so it's called a block device.
> 
> LVM creates 'volumes' (cf partitions) on top of multiple block devices.
> It doesn't do raid5 so, on it's own it doesn't offer raid5 redundancy.
> It's easy to 'grow' lvm volumes.
> 
> md provides a virtual block device by RAIDing other block devices.
> It's hard to grow a raid5 md device.
> It can be done in only 3 ways (AFAIK):
> 1 backup, destroy, rebuild bigger and restore
> 2 use raidreconf (not 100% succesful - gulp)
> 3 use EVMS (not 100% succesful - but supposedly much better than raidreconf)
> 
> You can grow an lvm on top of a raid5 by creating another raid5 device
> and growing the lvm onto it.
> 
> If you just grow the lvm onto another disk as Jim Turpin suggested then
> if that single disk fails you lose the entire lvm - thus removing the
> benefit of doing raid5 anyway.
> 
> Finally, there is a complex idea suggested by Dan Christensen:
> 
> Create multiple partitions on each device:
> sda1,sda2,sda3,sda4
> sdb1,sdb2,sdb3,sdb4
> etc
> 
> create 1 raid5 device for each partition
> md0=sda1+sdb1+sdc1
> md1=sda2+sdb2+sdc2
> 
> finally lvm all these together.
> 
> now to add a drive you must:
> * add a backup device to the lvm
> for each md device
>  * lvm move the blocks of 1 md device to a backup device
>  * remove md device from lvm
>  * destroy + rebuild the md device
>  * add back to lvm
>  * lvm move the blocks of the backup device
> * remove backup device from lvm
> 
> So, in summary, you *can* grow a RAID5 device - but it's hard.
> If you think you will want to grow it I'd suggest starting with EVMS -
> it's not that hard to use if you spend some time reading their docs.
> Dan's solution (proper credit!) is probably the safest but is time
> consuming and possibly human-error prone.
> 
> David
> 

David,

This doesn't look to bad at all after you defined the steps.
then again, i'm sure lvm/raid hell will manifest during implementation, but
i'm used to that.  i think i will try to implement this rather than use
yet another tool ...

thanks!
--
Milan