In RHEL 4.8 it is possible to do online resizing of multipath devices. The steps to do this are:
1. Resize your storage
2. Rescan your devices
3. run multipath
multipath will notice the new path size, and correctly resize the multipath device.
However, on some storage devices, it is necessary to unmap a LUN in order to resize it. In that case, any IO that is issued while the LUN is unmapped will fail, unless your multipath device is set to queue IO when no paths exist.
To avoid this, you can run
# dmsetup suspend --noflush <devicename>
before doing step 1, and then
# dmsetup resume <devicename>
after doing step 1.
This will ensure that no IO goes to the device
while the LUN is unmapped.