Information Technology Reference
In-Depth Information
Size
Platters/Heads
2/4
Capacity
320 GB
Performance
Spindle speed
7200 RPM
Average seek time read/write
10.5 ms/ 12.0 ms
Maximum seek time
19 ms
Track-to-track seek time
1 ms
Transfer rate (surface to buffer)
54{128 MB/s
Transfer rate (buffer to host)
375 MB/s
Buer memory
16 MB
Power
Typical
16.35 W
Idle
11.68 W
Figure12.3: Hardware specifications for a laptop disk (Toshiba MK3254GSY)
manufactured in 2008.
cause the disk starts attempting to read data before the disk arm has completely
settled, but it must wait a bit longer before it is safe to write.
When transferring long runs of contiguous sectors, the disk's bandwidth is
54-128 MB/s. The bandwidth is expressed as a range because the disk's outer
tracks have more sectors than its inner tracks, so when the disk is accessing
data on its outer tracks, sectors sweep past the disk head at a higher rate.
Once the data is transferred off the platter, the disk can send it to the main
memory of the computer at up to 375 MB/s via a SATA (Serial ATA) interface.
Random v. sequential performance. Given seek and rotational times
measured in milliseconds, small accesses to random sectors on disk are much
slower than large, sequential accesses.
Example: Random access workload.
Question: For the disk described in Figure 12.3, consider a workload con-
sisting of 500 read requests, each of a randomly chosen sector
on disk, assuming requests are serviced in FIFO order. How long
will servicing these requests take?
Answer: Disk access time is seek time + rotation time + transfer time.
Seek time. Each request requires a seek from a random starting
track to a random ending track, so the disk's average seek time
of 10.5 ms is a good estimate of the cost of each seek.
