Disk Storage: How Much is that Doggie in the Window? The traditional approach of purchasing storage is broken. Comparing the purchase cost per terabyte of one storage array to another misses over 70% of the potential costs and leads to two classic storage management pitfalls. Managers who purchase new storage often focus too much of their attention on the upfront cost of buying disk and inherit a cost structure that requires ever-increasing expenditures on advanced functions, storage software, and support and maintenance. Perhaps even more at risk are those managers who elect to stay with their existing environment, unaware that a more cost-effective storage solution is available to them. This second predicament is exacerbated further by the fact that the financial savings from consolidation or migration do not show up in traditional storage expenditure categories such as disk drives, disk expansion units, and disk arrays.

Disk Storage Utalization Rates: Short stroking utilization rate is simply the percentage of usable storage that end up being used for data. Older systems that do not have thin provisioning, for example, end up wasting space because storage is allocated in larger blocks than can be managed effectively. Also, to achieve required performance, older systems often either were short-stroked or extra capacity and drives were purchased. As a result, utilization tends to run as low as 40% in the case of NAS solutions and 50-55% for older block arrays. Utilization in todays most advanced systems can run as high as 82% for block and even as high as 85% for unified options, depending on performance needs.

For example, in the case of a database, physically positioning files on the outer edges of the disks is difficult if not impossible in modern RAID systems, and therefore short-stroking seems to be the common response. However, one has to take into account the operating system and the RAID selected. You must tune your disk architecture to support the expected IO profile and must tune the database system to take advantage of the disk architecture. For example, some databases have different IO characteristics depending on whether they are reading or writing data and what type of read or write is being done, while some databases have fixed read/write sizes. You must determine the IO profile for your database and then use that IO profile to determine the maximum and minimum IO size. The IO profile will tell you what percentage of IO is large IO and what percentage is small IO, and it will also give you the expected IO rate in IO/second (IOPS). Once you have the IO per second, you can determine the IO capacity (number of drives) needed to support your database and then you can determine a reasonable utilization rate.