Storage is just bits on a disk, right? What's complicated about this? A lot. Storage is a surprisingly complex thing to manage.

I tell you that a volume is 95% full. What does this mean? I tell you that another volume is 70% full. What does this mean? Maybe not what you think...

A volume that is 95% full is unhealthy. There is no getting around this. The algorithms that allocate space on volumes start breaking down after 85% of the space is taken. Fragmentation begins to increase dramatically, and there is not enough free space to reorg the data in place. In other words, the volume is unhealthy. The reaction of most people to this situation is to say that this volume demands immediate attention. Seeing only 5% free space, many a wise storage manager would declare an emergency.

But suppose I also tell you that the volume's size has been stable over time. It neither grows nor shrinks by more than 1%. Now there is no immediate emergency. The volume in question is unhealthy, but not very risky. Should you apply your efforts here, or do something else?

Our other volume is 70% full. At 70%, we know this volume is healthy. But now suppose I tell you that the space used is growing by 10% to 15% a month... This volume is certain to run to be completely out of space in less than 3 months. Sounds pretty risky to me. I might put my attention here and let the other volume slide for a while.

The point of the story is that things - storage management in particular - are rarely one-dimensional. If your model for storage management is over simplified, you might make serious mistakes, or you might be constantly chasing your tail never knowing what is truly important. As you plan to manage your storage cost-effectively, you need more information than just performance and free space. If all you know is performance and free space, your only solution to any issue is add more hardware. Hardware vendors love this solution, but your senior management doesn't. Nor should you.

Tuesday, October 10, 2006 9:52:14 AM (Eastern Daylight Time, UTC-04:00)      Comments [0]  Storage Management