Storage Systems - SQL Server 2012 Internals and Troubleshooting - page 98

FIGURE 4-12
The hard drive assembly is serviceable only inside a high-technology clean room. Opening the
cover on a hard disk drive will void the warranty. The drive platter rotates around a spindle and
is powered by a spindle motor. A drive is made up of several platters that are stacked. Each platter is
double-sided and coated with a magnetic oxide.
Data is physically read and written by a hard drive head. Each drive platter has a dedicated drive head.
An actuator arm houses all the drive heads, and a magnetic actuator moves the arm. You can think of
a hard drive as a record player. The platter spins and the head reads and writes data. Unlike a record
player, however, the disk drive head can move back and forth. The head actually rides just above the
disk surface on a cushion of high-pressure air that is created when the platter spins at high speed.
SATA disk drives provide commodity storage. They offer much larger capacity than FC or SAS drives.
At the time of this writing, SATA drives are available with a capacity of three terabytes (3TB). SATA
drives spin at lower speeds — 5,400 to 7,200 RPM. They are sold for both the consumer and the
enterprise markets. Enterprise drives are designed for more continuous use and higher reliability.
Both FC and SAS drives are considered enterprise-class disk drives. They are available in 7,200,
10,000, and 15,000 RPM models. With the exception of Nearline SAS (NL-SAS) drives, these disk
drives are generally lower in capacity than SATA disk drives. SAS 6GB/s drives are displacing Fibre
Channel 4GB/s drives in the marketplace.
Modern SAS drives are manufactured in a 2.52 form factor, unlike the traditional 3.52 form factor
of Fibre Channel and SATA drives. This smaller drive enables more disk drives to be housed in a
given space. NL-SAS drives offer a high-reliability enterprise SATA drive with a SAS interface.
The logic board governs how the disk operates. Each disk contains buffers, and some disk drives con-
tain cache. For the proper operation of SQL Server write-ahead logging, volatile cache must be bypassed
for write operations. Most array vendors will guarantee cached data with battery backing. When using
disks directly in a JBOD, it is important to ensure that they meet SQL Server reliability requirements.
A disk drive is made of both electronic and mechanical components. When data is read sequentially,
the drive can read it as fast as the drive spins. When the data needs to be accessed out of order, the
head needs to move to the appropriate track. Head movement, or seeking, is not instantaneous.