SAN management

Checklist: How to troubleshoot your LUN

By  Rick Cook

SearchStorage.com

What you will learn from this tip: A few things that could be going wrong with your LUNs, and how to fix them. Need a downloadable copy? Go here.

LUN issues are difficult to diagnose, because they usually arise from specific features of a specific system's hardware and software. The most common problem is that one or more LUNs aren't visible, but this can have a whole host of reasons.

Because LUNs sit at the crossroads of physical and virtual storage, they can be affected by both hardware and software. Everything from HBAs to operating system configuration to storage management software can cause problems with LUNs. If your LUNs are acting up, follow these steps to diagnose the problem:

How to troubleshoot your LUN

1. Read the manuals!

If you have LUN problems, the place to start is the documentation. It should tell you what each device or software service is expecting from the LUNs. Almost invariably these are configuration issues rather than hard limits. But to fix them, you have to understand what is expected.

For example, does your hardware auto-configure? Most hardware does, but there are a few items that don't. Another possible problem, with a Windows boot-from-SAN installation, occurs when the storage controller doesn't follow the SCSI-3 standard of assigning LUN 0 to the controller rather than a disk. To boot from SAN Windows requires that LUN enumeration adhere to the SCSI-3 standard.

2. Get the right drivers.

Make sure you're using the latest drivers in HBAs and other parts of the system.

3. Check your LUN masking policies. 

One of the most common problems is an inappropriate LUN masking policy, especially if LUN 0 is masked from hardware or software that needs it. Although LUN masking is basic to storage security, you need to check your policies make sure that everything that needs to see LUN 0 can find it.

4. Make sure that your system is configured for the right number of LUNs.

Many kinds of hardware and software assume you have eight LUNs or less. If you have more LUNs than that you may have to change configuration parameters.

5. Determine whether some LUNs are visible.

If you can see some, but not all, LUNs you can assume that the problem isn't fabric-specific.

6. If all else fails, drop back five yards and punt.

If worse comes to worst, determine what you know (and what you don't know) and then follow the LUN creation process from the beginning to see where it fails. Make sure your system finds the physical disks and then work through setting up the LUNs. Try setting up LUN 0 separately and then adding LUNs -- and don't forget to re-read the manuals looking for obscure sections relating to LUNs. (Better yet, if you have the docs on CD sometimes it helps to search them for "LUN" and see what turns up.)

For more information:

Topics: LUNs

Storage Management Survival School: Operations

Advice: What is a LUN, and why do we need one?

About the author: Rick Cook has been writing about mass storage since the days when the term meant an 80 K floppy disk. The computers he learned on used ferrite cores and magnetic drums. For the last twenty years he has been a freelance writer specializing in storage and other computer issues.

25 Mar 2005

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