It seems that online resizing of a multipath device is actually impossible:
http://www.redhat.com/archives/dm-devel/2007-August/msg00205.html

This is my personal offline how-to:
# Rescan devices (for all 4 paths)
echo "1" > /sys/bus/scsi/drivers/sd/<h>\:<b>\:<t>\:<l>/block/device/rescan
...
# Check /var/log/messages
# Trespass LUN by Navisphere
# Rescan devices (for all 4 paths)
echo "1" > /sys/bus/scsi/drivers/sd/<h>\:<b>\:<t>\:<l>/block/device/rescan
...
# Check /var/log/messages
# Umount filesystem
# Deactivate VG
vgchange -an vg_LUN
# Delete and recreate multipath device
multipath -F
multipath -v2
multipath -ll
# Activate VG
vgchange -ay vg_LUN
# Remount filesystem
# Extend PV
pvresize /dev/mapper/<long-multipath-identifier>
# Check VG
vgdisplay -v vg_LUN
# Extend LV
lvextend -l +256 /dev/vg_LUN/lvol...
# Extend FS
ext2online /dev/vg_LUN/lvol...

Alternatively, you can create an additional LUN and add it to the VG
(online)

If someone is able to find a better method, please post it!

--
Domenico Viggiani


> -----Original Message-----
> From: [EMAIL PROTECTED] 
> [mailto:[EMAIL PROTECTED] On Behalf Of Collins, 
> Kevin [Beeline]
> Sent: Monday, July 21, 2008 6:31 PM
> To: Red Hat Enterprise Linux 5 (Tikanga) discussion mailing-list
> Subject: RE: [rhelv5-list] RE: multipathing in RHEL5
> 
> What about if the LUN is an LVM physical volume (PV) already? 
> Can you use pvresize?
> 
> Thanks,
> 
> Kevin
> 
> -----Original Message-----
> From: [EMAIL PROTECTED]
> [mailto:[EMAIL PROTECTED] On Behalf Of Domenico Viggiani
> Sent: Monday, July 21, 2008 1:28 AM
> To: 'Red Hat Enterprise Linux 5 (Tikanga) discussion mailing-list'
> Subject: [rhelv5-list] RE: multipathing in RHEL5
> 
> We are currently using native Red Hat multipath with an EMC 
> Clariion CX700 disk-array. No problem at all, you need only 
> to be well-disciplined and methodical with 
> creating/destroying LUNs. Only real BIG problem is that you 
> cannot resize a LUN online (unless you add PV to an existing 
> VG, using LVM).
> 
> Bye
> --
> Domenico Viggiani
> 
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