Xsan 2.3 or later: Deactivating a storage pool (Apple KB)

It may be necessary to deactivate (down) an Xsan data storage pool in a volume, but retain access to data on other storage pools within the volume. You may need to do this if there is a LUN issue in an Xsan storage pool and Xsan clients are unable to mount the Xsan volume, for example.

Read more: http://support.apple.com/kb/HT4656