I Come Not to Praise Disks, but to Bury Them

Disk based systems have gone from 5000 RPM and 30 or less megabytes to 15K RPM and terabytes in size in the last 30 years. The first Winchester technology drive I came in contact with in the early 1980’s had a 90 megabyte capacity (9 times the capacity that the 12 inch platters that I was used to had) and was rack mounted, since it weighed over 400 pounds! Now we have 3 terabyte drives in a 3-1/2 inch form factor. However as information density increased, the bandwidth of information transfer didn’t keep up at the hard drive level. Most modern disk drives can only accomplish 2 to 3 times the transfer rate of their early predecessors, why is this?

The speed at which a disk drive can access information is limited by 3 things:

1.      The number of independent heads

2.      The speed of the disk actuator mechanism

3.      The speed of the disks rotation

While most disks have multiple platters and multiple read/write heads, the read/write heads are mounted to a single actuator mechanism. By mounting the heads on a single actuator mechanism you may increase the amount of data capable of being read/written at the same time, but you do not increase the maximum random IOPS. Because of these physical limitations most hard drives can only deliver 2-5 millisecond latency and 200-300 random IOPS.

Figure 1: HDD Armature and Disks

Modern SAN and NAS system are capable of delivering hundreds of thousands of IOPS, however, you must provide enough disk spindles in the array to allow this. To get 100,000 IOPS assuming 300 IOPS per disk you would need 334 disk drives at a minimum, more if you want to serve that 100,000 IOPS to multiple servers and users. In an EMC test, they needed 496 drives to get to 100,000 IOPS. Of course, the IOPS latency would still be from 2-5 milliseconds or greater. The only way to reduce latency to nearer to the 2 millisecond level is to do what is known as short-stroking.

Short stroking means only utilizing the outer, faster, edges of the disk, in fact usually less than 30% of the total disk capacity. That 496 disks for 100,000 IOPS at 5 milliseconds just became 1488 or more to give 100,000 IOPS at 2 millisecond latency.

Disks have fallen dramatically in cost per gigabyte.  However their cost per IOPS has remained the same or risen. The major cost factors in a disk construction are the disk motor/actuator and technology to create the high density disks. This means that as disk technology ages, without major enhancements to the technology, their price will eventually stall at around 10 times the cost of the raw materials to manufacture them.

Figure 2: Disk Cost Per GB RAW

So where does all this leave us? SSD technology is the new kid on the block (well, actually they have been around since the first memory chips) now that costs have fallen to the point where SSD storage using Flash technology is nearly on par with enterprise disk costs with SSD cost per gigabyte falling below $40/gb. A recent EMC representative presentation quoted $17K/TB of storage.

SSD technology using Flash memory utilizing SLC based chips provides reliable, permanent, and relatively inexpensive storage alternatives to traditional hard drives. Since each SSD doesn’t require its own motor, actuator and spinning disks, their prices will continue to fall as manufacturing technology and supply-and-demand allows. Current prices for a fully loaded 24 TB SSD using eMLC technology sit at around $12K/TB, less than for enterprise level HDD based SAN. This leads to a closing of the gap between HDD and SDD usage modes as shown in Figure 3.

Figure 3: Usage Mode Changes for HDD and SSD

In addition to price to purchase, operational costs (electric, cooling) for SSD technology is lower than the costs for hard disk based technology. Combine that with the smaller footprint per usable capacity and you have a combination that sounds a death knoll for disk in the high performance end of the storage spectrum. Now that SSDs are less expensive then enterprise level disks at dollars (or Euros) per terabyte and when a single 1-U chassis full of SSDs can replace over 1000 disks and costs are nearing parity, it doesn’t take a genius to see that SSDs will be taking over storage.

Figure 4: Comparison of IOPS verses Latency

A SSDs full capacity can be used for storage, there is no need to “short-stroke” them for performance. This means that rather than buying 100 terabytes to get 33 terabytes you buy 33 terabytes. SSDs also deliver this storage capacity with IOPS numbers of 200,000 or greater and latencies of less then 0.5 milliseconds. With the various SSD options available the needed IO characteristics for any system can be met as is shown in Figure 5.

Figure 5: Response and IOPS for Various SSD Solutions

These facts are leading to tiering of storage solutions with high performance SSDs at the peak, or tier zero, and disks at the bottom as archival devices or for storage of non-critical data. Rest in peace disk drives.