[linux-lvm] Extending a LVM2 strip set

[lemons_terry at emc.com]

So, when I add the two additional disks, which I want to be striped, I would
do:

lvextend --stripes 2 -- extents {nnnnn}{volumegroup} /dev/system/lvol0

And I'll end up with two 2-stripe sets, concatendated together?

So, when data is written to these extending LV, will the data fill the first
two striped disks before writing to the second two striped disks, or will
writes be sent across all four disks?

Thanks
tl

-----Original Message-----
From: linux-lvm-bounces at redhat.com [mailto:linux-lvm-bounces at redhat.com] On
Behalf Of AJ Lewis
Sent: Thursday, September 23, 2004 2:46 PM
To: LVM general discussion and development
Subject: Re: [linux-lvm] Extending a LVM2 strip set

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?
-- 
AJ Lewis Voice: 612-638-0500 Red Hat Inc.  E-Mail: alewis at redhat.com
720 Washington Ave. SE, Suite 200 Minneapolis, MN 55414
   
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