Disks are getting slower - it's true!

Okay, maybe the title should be "Disks are not getting faster, as a linear function of capacity growth". That just doesn't roll of the tongue, nor is it very catchy!

Unless you live under a rock, you're aware that Seagate and Hitachi are now shipping 1TB SATA hard disks in quantity. You're probably also aware that these disks feature SATA-II 3.0Gb/s disk interfaces. What you probably don't know is that these classes of hard disks use the same disk technologies as the last two generations of Seagate drives, and the last three generations of drives from HGST.

Hard disks rely on heads to read and write blocks on the physical disk platters, then buffers the data in small caches, typically 2MB to 16MB. This cache then puts blocks on the SATA bus, traveling to the host - we call transfers data at this step "buffer-to-host". In 2005, the Barracuda line average 65MB/s transfer buffer-to-host; in 2007, this is now 72MB/s. In 2005, the largest Barracuda was 250GB; now it's 1TB. What does that mean?

Under the best circumstances, it now takes longer to empty or fill your 1TB disk, relative to a 500GB or 250GB drive. For enterprises that are moving to disk-based backups, that implies that the aggregate throughput of disk-based backups will not increase as the number of (higher-capacity) disks increases. This information is not new - it's the same reason that high-performance, transaction-based environments use lots and lots of small, fast spindles. But in transaction environments, you goal is to achieve more IOPs - executing more, small transactions, not more bandwidth - transfer streams of larger files/blocks.

With 6Gb/s interface specifications coming in the next year, we're on the cusp of a significant "loss" in performance, that is, a tremendous increase of capacity without an equivalent increase in transfer rate to/from disk. Users expecting their high-capacity storage environments to scale will be sorely disappointed.