Just a few years ago, everyone wanted disk drives that spun at 15,000 rpm, commonly known as “15k disk”. Why did people want these? Well, the faster the spindle turned, the shorter the seek times, the less latency and the faster the writes to that disk. Since I never worked at any of the drive manufacturers, I can’t really speak to the truth of this, but I do take it on faith. So when everything on a storage array was spinning disk, why did people want “15k spindles” in the line up? And since SSD has become so popular, why don’t I really see them anymore?
Why do I want expensive, small disks?
The reason that everyone wanted 15k disk drives was pretty straightforward. The disk themselves were fairly small in capacity (600GB being a standard size) and expensive on a GB/$ ratio. But they were FAST. If there was a target IOPs for a storage array, it was easier to balance out size and speed with a ratio of 15k disk, 10k disk, and standard 7.2k SATA drives. Speed from the smaller drives and space from the slower drives. While everything was acceptable ON AVERAGE, the laws of physics still applied to the different speeds of disk. There was a bit of balance that had to happen. You could put your fast access volumes on 15k, but you still needed the SATA drives for the larger storage requirements. This solution worked, but was expensive – and a bit “hands-on”.
There were even a few manufacturers that started to offer ILM with these systems. This means that “hot” or active data writes to the 15k disk drives since theoretically the write speed on these is fastest. Your storage appliance now writes more across the aggregate of your SAN environment. Once this data is written to the fastest disk on your SAN or NAS, it stays there for a bit. This logic being that it also has the fastest read times and therefore the best performance when you wanted to recall that data. These ILM vendors then move the data off of the fastest tier of disk to a slower tier as that data becomes less active or “ages”. This allowed you to store older, less accessed data on the slower and less expensive tiers of storage. Because the database has to run quickly, but who cares if it takes accounting a week to get the data for their year-end reporting, right? Remember that the next time you need that expense report reimbursed!
The next step
Then SSD entered the market. At an affordable price, that is. Not only could manufacturers use SSD as caching, but they were large enough that manufacturers could also use them as the fastest tier of data storage in an ILM strategy. And the form factor of SSD disks allows them to be used in the existing storage appliance enclosures – JUST LIKE spinning disk. Now, instead of expensive 15k disks, you could put in units in the same form factor that would read and write several hundred times faster than disk. With the speed and storage capability of SSD, it became unnecessary to use 15K disk in storage appliances for speed.
But I still see some 15k disk out there…
You will still see 15k disk used in local solutions. A 15K SAS disk RAID 5 array is quite speedy when used in a local physical server. Virtualization solutions, or database servers will often use 15K spindles for disk targets. They need sizable storage capacities and quick access. However, the cost of SSD is coming down. This allows the justification for installation of SSD disk or arrays in physical servers. Seagate has stopped development of new models for their 15k disk. Previously storage technology leapt from Tape to HDD for large data storage like disaster recovery. Now storage acceptance from high speed disk to SSD will likely accelerate. Technology to increase access speed, reduce manufacturer costs, and increase storage capacity will accelerate this change. So long 15k disk, we hardly knew ya!