[linux-lvm] Best practice for lvm on raid-5

[linux-lvm] Best practice for lvm on raid-5

[Jens Hoffrichter]

Hi everyone,

I have a LVM vg on top of a softraid 5 consisting of 3 80 GB harddrives.

Currently I have /dev/md0 as my only physical volume, but this will lead
to problems if I am going to expand the raid 5 by adding another 80 GB
harddrive to (as pvresize is not implemented again yet, as I understand).

What is the best practice in this case? To add a partition table on top
of /dev/md0 (but how is it represented in the /dev filesystem) and only
add single partitions as pvs to the vg?

I'm happy for every suggestion :)

CU all,
Jens
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 Jens Hoffrichter / joho@hausboot.org / Joho@IRC / Fon: +43-650-5376989
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Re: [linux-lvm] Best practice for lvm on raid-5

[Nils Juergens]

On Mon, 02.02.04, Jens Hoffrichter <joho@hausboot.org> wrote:

> Currently I have /dev/md0 as my only physical volume, but this will lead
> to problems if I am going to expand the raid 5 by adding another 80 GB
> harddrive to (as pvresize is not implemented again yet, as I understand).

Unless I am mistaken, it is not even possible to expand /dev/md0 by adding
another drive in the first place. And because linux software raid has
no way of knowing which blocks are in use and which are not, i do not see
how this could ever be possible.

If you add another array (raid1 or raid5) as /dev/md1, you can vgextend your
VG.

cheers,

Nils

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Nils Juergens  | nils@muon.de | icq 7090774
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Re: [linux-lvm] Best practice for lvm on raid-5

[Jens Hoffrichter]

On Mon, Feb 02, 2004 at 05:20:25PM +0100, Nils Juergens wrote:

> > Currently I have /dev/md0 as my only physical volume, but this will lead
> > to problems if I am going to expand the raid 5 by adding another 80 GB
> > harddrive to (as pvresize is not implemented again yet, as I understand).
> 
> Unless I am mistaken, it is not even possible to expand /dev/md0 by adding
> another drive in the first place. And because linux software raid has
> no way of knowing which blocks are in use and which are not, i do not see
> how this could ever be possible.
Yes, it should be possible using raidreconf. I have not testet it yet, but
at least the manpage states it. Converting a raid 0 to a raid 5 of same size
worked without problem.

> If you add another array (raid1 or raid5) as /dev/md1, you can vgextend your
> VG.
Yes, I'm aware of this, but for this I have to add another 3 disk to the
system, and I want to avoid this.

CU all,
Jens
-- 
GPG: 1024D/CF884D50  F2E8 F7FC F823 6464 4E9D  EFAB 6EE9 8B9C CF88 4D50
------------------------------------------------------------------------
 Jens Hoffrichter / joho@hausboot.org / Joho@IRC / Fon: +43-650-5376989
    A dream is a wish your heart makes. 
                               (Seen on a parade in Walt Disney World)

Re: [linux-lvm] Best practice for lvm on raid-5

[Chris Cox]

Jens Hoffrichter wrote:
> Hi everyone,
> 
> I have a LVM vg on top of a softraid 5 consisting of 3 80 GB harddrives.
> 
> Currently I have /dev/md0 as my only physical volume, but this will lead
> to problems if I am going to expand the raid 5 by adding another 80 GB
> harddrive to (as pvresize is not implemented again yet, as I understand).
> 
> What is the best practice in this case? To add a partition table on top
> of /dev/md0 (but how is it represented in the /dev filesystem) and only
> add single partitions as pvs to the vg?
> 
> I'm happy for every suggestion :)

Obviously you cannot extend the existing md0 RAID5.  However you can
add another PV to the VG.  That new PV could be another soft RAID5.
It doesn't have to be though... you can basically build quite
a menagerie with LVM if you really want to.