The introduction of “enterprise SATA” disks a few years ago was an excellent solution for all the companies craving storage space. With capacities up to 1TB per drive, “RAID Enabled” SATA disks offer huge amounts of magnetic disk space with decent reliability. Magnetic disks have been a very cheap solution if you want storage space, with prices at 20 cents per gigabyte (and falling). Performance is terrible, however, with seek times and latency adding a few milliseconds over faster and more expensive alternatives (i.e. SCSI). That’s 10,000 to 100,000 times slower than the speed of CPUs and RAM, where access times are expressed in nanoseconds. Even worse is the fact that seek times and latency have been improving at an incredibly slow pace. According to several studies[1], the time (seek time + latency) to get one block of random information has only improved by a factor 2.5 over the last decade while bandwidth has been improved a tenfold. Meanwhile, CPUs have become over 60 times faster! (As a quick point of reference, ten years ago state-of-the-art servers were running 450MHz Xeon processors with up to 2MB of L2 cache.)
The result of this lopsided performance improvements is a serious performance bottleneck, especially for OLTP databases and mail servers that are accessed randomly. A complex combination of application caches, RAID controller caches, hard disk caches, and RAID setups can partially hide the terrible performance shortcomings of the current hard disks, but note the word “partially”. Caches will not always contain the right data and it has taken a lot of research and software engineering to develop database management systems that produce many independent parallel I/O threads. Without a good I/O thread system you would not even be able to use RAID setups to increase disk performance.
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