How do you persuade prospects to buy? Paint an attractive picture. Motivation trumps accuracy. Selling add-on storage by getting customers to focus on an impressive bandwidth number scratches many customer’s itches. The truth is different.
Bandwidth is a simple number that people think they understand. The bigger the number, the faster the storage. Nope. Leaving aside that many consumer bandwidth numbers are bogus — link speed is not storage speed — actual performance is rarely dependent on pure bandwidth. Bandwidth is a convenient metric, easily measured, but not the critical factor in storage performance. What most storage performance tools measure is the bandwidth with large requests. Why? Because small requests don’t use much bandwidth. The following graphic, produced with a tool from ATTO storage, illustrates this. On the X axis is the bandwidth and on the Y axis is the access size. The correlation is obvious: small requests don’t use much bandwidth. But why doesn’t the CPU issue more I/O requests to soak up that unused bandwidth? Because every I/O takes time and resources — context switches, memory management and metadata updates, and more — to complete. There a lot of small requests even if you’re editing huge video files. That’s because behind the scenes, the CPU’s Memory Management Unit (MMU) is constantly swapping out least-used pages and swapping in whatever data or program segments your workload requires.
