[linux-lvm] Extending a LVM2 strip set

[AJ Lewis]

lemons_terry at emc.com writes:

> http://www.tldp.org/HOWTO/LVM-HOWTO/mapmode.html
> <http://www.tldp.org/HOWTO/LVM-HOWTO/mapmode.html>  states:
> 
> "In LVM 2, striped LVs can be extended by concatenating another set of
> devices onto the end of the first set. So you can get into a situation where
> your LV is a 2 stripe set concatenated with a linear set concatenated with a
> 4 stripe set. Are you confused yet?"
> 
> As a matter of fact, I am!
> 
> I need to start with a 2 disk (therefore, 2 stripe) logical volume, that I
> later extend with 2 more disks.  How is this done?

In lvm2 you simply create the lv with a stripe size of 2, then later
when you want to extend it, you lvextend it with a stripe size of 2.
The reason this is explained in so much detail is that in LVM1, you
could not extend a striped set past its original PVs.  In LVM2, the
original striped set can be extended to the end of the original PVs,
and any additional PVs become a striped set that is tacked on to the
end (concatenated).

Maybe an ACSII picture can help ;)

Original striped volumes:
 ___   ___
| 1 | | 2 | 
| 3 | | 4 |
|_5_| |_6_|

After adding two more disks and lvextending:
 ___   ___   ___   ___
| 1 | | 2 | | 7 | | 8 | 
| 3 | | 4 | | 9 | | 10|
|_5_| |_6_| |_11| |_12|


So you have 2 sets of striped segments (with 2 stripes each),
concatenated together.  It does *not* remap all the striping so you
have 1 set of striped segments (with 4 stripes each).

Is that any clearer?
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