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Reason #3: Reliability

There’s More To An Enterprise Drive
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However, while interface type may not have a major impact on sustained throughput, it does affect I/O performance. According to a feature breakdown created by Intel (www.intel.com/support/motherboards/server/sb/CS-031831.htm) as a guideline for server storage, a typical SATA drive is capable of 77 I/O operations per second (IOPS). A SAS drive more than quadruples this amount, averaging 319 IOPS.

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Sidebar: Key Terms

Latency. In the context of hard drives, latency refers to the duration of time (in milliseconds) required for the drive to position the read/write heads over a given physical location on the underlying media. The head must be positioned over the appropriate track, then the media must rotate in order to position the desired location under the head. Latency is often noted as “access time” or “seek time.” This is particularly important with high-volume, highly random workloads.

Random read/write. Some applications entail writing small amounts of data at various positions across a drive’s media. For example, the first block of data might be in the inner platter rings while the next block is in the platter’s center. Database queries and OLTP exemplify this behavior. When reading a stream of “random” data, the heads must hop from one location to the next, incurring latency. 

Rotational vibration (RV). The periodic motion of a spinning body outside of that body’s condition of equilibrium. Ideally, an operating hard drive platter should spin with no vibration, but this is impossible under real world conditions.

RPM, or revolutions per minute. This refers to the number of times a drive’s magnetic platters revolve every 60 seconds. The slowest consumer drives tend to be 5400 RPM. Enterprise SATA drives generally start at 7200 RPM, with mid-range models landing at 10,000 RPM, and the highest-end units reaching 15,000 RPM. Generally speaking, the faster the spin rate, the more data passes under the read/write heads in a given amount of time, thus increasing the throughput speed.

SAS, or Serial-Attached SCSI. A point-to-point SCSI drive architecture using at least four data channels operating simultaneously. Currently, SAS drives are available in 3 Gb/s and 6 Gb/s versions. All SAS controllers are backward compatible with SATA drives.

SATA, or Serial ATA. SATA is the dominant drive type for modern consumer hard disks. SATA drives are single-channel. Most models still offer 3 Gb/s interfaces, although 6 Gb/s models are increasingly common at the high-end, especially in enterprise/nearline implementations. Expect 6 Gb/s models to gradually pervade the SATA market as the industry has decided to cap SATA's performance at this rate.

Sequential read/write. Whereas random read/write operations will “hop” around the surface of a hard disk platter because the pieces of a file are non-consecutive on the media, sequential reads/writes proceed in order, with one file block existing contiguously after another. Because this eliminates seek time between block, sequential operations tend to perform at higher speeds than random operations. Large file processing, surveillance, and on-demand video are two common sequential data applications.

Sustained transfer rate. As opposed to a “burst rate,” which is the theoretical maximum transfer speed for a drive, a sustained rate reflects the average data throughput (usually sequential) between a drive and its host system over a continuous transfer period.