Storage Area Networks For Dummies®

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Par t II: Designing and Building a SAN
Here’s a big assumption that I now must face: To have multiple HBAs see the
same LUNs and load balance and fail-over, you need a software or OS feature
that allows this functionality. Using multiple paths from your server to the
LUN is called (well, yeah) multipathing; I’m assuming that your servers have
multipathing software installed or built into the operating system. That way,
if the same LUN appears across two different HBAs, the OS will know it’s looking
at the same LUN. Operating systems are dumb, and they don’t usually
expect to see the same disk on two different host adapters.
Just keep pretending that your OS will handle multipathing for you and not
allow you to see four LUNs when you have only one with four paths.
Unix servers
Log in to the Unix system first. In this pretend Unix system, the OS is Solaris
and it contains a few key files that you need to be aware of before you expect
to see any disk devices appear.
Persistence pays off
First, make sure that the HBA’s configuration file has the proper entries. A
few paragraphs from here is a segment of a typical configuration file called
/etc/hba.conf. I removed all the mumbo-jumbo entries for timeouts and
the like; they’re standard, and your SAN vendor will tell you what you
specifically need to put in for them.
The important line is the one starting with hba-bind-WWNN. That line
controls persistent binding, which is used to make sure that when a server
reboots and scans its HBAs for disk devices, it always finds the same LUNs
that it used before in the same place. You see, computers are funny — when
they start up and go out initializing their devices (such as HBAs), they don’t
always remember where things were the last time they were used. If you
don’t use persistent bindings, the LUNs that you had on HBA card #1 might
suddenly show up on HBA card #2 the next time around. This is not good
because your applications expect to find your files on a specific device, and if
they aren’t there, the applications will fail to run.
When multiple HBAs are in a server, the same driver for each HBA loads multiple
times: each one called an instance. An instance name typically starts with
a shortened name or abbreviation for the HBA card manufacturer (here it’s the
generic hba) and then a number, usually starting with zero and incrementing
with each physical HBA card found in the server. With this section of the configuration
file, you can associate each instance of the HBA driver to a specific
port on the storage array. Use the World Wide Port Name (WWPN — same as
the WWN of the storage port) to bind each instance to a specific card and SCSI
target # ID. This will make all the devices appear in the same place every time
upon a server reboot.