For a SAN environment, you may have many SAN devices and SAN hosts. We'll say that SAN hosts are servers that have a fibre connection to the Fibre switch while SAN devices are the big boxes of hard drives and raid arrays. A SAN device can be shared amongst several SAN hosts which can lead to problems if one share from a SAN node connects to multiple SAN hosts.
Let's go with an easy example:
So from the above picture, we have a little SAN environment. The red box is a RAID Array and the blue circles are some servers which are both zoned to the RAID array.Now without host filtering, when we set up a LUN, both servers will see it:
In the above setup, both servers are able to format the disk and mount the LUN as a disk. In some cases, if you try to mount the LUN in two places, both SAN Hosts may panic! Now if we apply the host filtering, using the WWN (world wide name) of the Fibre HBA on SAN Host Z, we'll get something like this:
Now our LUN can't be accidentally deleted from SAN Host Y! Hooray!