Database Reference
In-Depth Information
Furthermore, I/O transactions that are aligned to the native block size
of the disk offer the best performance, whereas unaligned accesses can
severely impact the effective performance delivered to applications as shown in
Figure 1.4(b). The block alignment issues manifest themselves at all layers of
the storage hierarchy, and even the file system. GPFS is used in this example,
but this is common to many different file system implementations. In addi-
tion, streaming write operations tend to be slower than reads by up to 50%
because of the need to read the disk block into the system memory, modify
its contents, and then write it back to disk. Since read and write operations
are mutually exclusive for the mechanical device (disk heads are in effect half-
duplex), the read-modify-write can impact delivered bandwidth. However, the
impact can be modest (
<
10%) because it is amortized by the track-to-track
seek latencies.
1.3.3 Enterprise and Commodity Disk Technology
The disk storage market is typically divided into three market segments—
enterprise, consumer, and handheld. The enterprise class storage market em-
phasizes performance (reduced failure rates, reduced seek times, and increased
transfer bandwidth) above all other considerations. Enterprise-class disk in-
terfaces tend to employ host interfaces such as SCSI and Fiber Channel that
emphasize reliability, support for multiple outstanding transactions, more ro-
bust error checking and correction, as well as support for addressing a larger
number of devices on the same loop or channel interface. The spindle speeds
tend to be higher than consumer-grade devices, leading to higher power con-
sumption. A typical enterprise class device will have a lower storage capacity
per unit than the consumer class devices, but will spin at up to 15,000 RPMs,
offer seek times of less than 5 milliseconds, and consume 12-20 watts.
The consumer class devices emphasize low cost and high capacity for
volume-driven consumer electronics markets. A typical consumer-grade disk
will offer double the storage capacity of the enterprise-class storage devices,
but spin at a lower spindle rate of 5,400-7,200 RPMs. The drive interface for
these devices tends to rely more on the host computer interface to control the
disks than the enterprise-class devices. Consumer-class devices tend to sup-
port fewer devices on the same interface, and are less tolerant of faults. For
example, IDE devices perform parity checking on the data bus, but not on
the control interface. Interface technology is typically derived from integrated
device electronics (IDE), ATA and Serial ATA (SATA) standards. The power
consumed by such a device is typically limited by applications to less than
10 watts.
The handheld devices favor a design point that emphasizes reduced size and
power consumption for devices such as MP3 players, ultraportable computers,
and handheld movie cameras. The typical capacity for such devices is an
order of magnitude smaller than for enterprise storage devices, but power
consumption is typically on the order of 0.5-2 watts. These devices are also
