Information Technology Reference
In-Depth Information
know that the request covers a significant fraction of a track. This
means that there is a good chance that after the seek and settle
time, the disk head will be in the middle of the region to be read.
In this case, the disk will immediately read some of the track into
the track buffer; then it will wait for the first track to rotate around;
then it will read the remainder of the desired data.
We can estimate that for the outer track, there is a one in four
chance that the initial seek and settle will finish while the head is
within the desired range of sectors, and that when that happens,
we read an average of
1
8
th of the desired data before we arrive
at the first desired sector. So, for the outer track, this overlap will
save us
1
4
1
8
=
1
32
of a rotation for the average transfer. This
effect slightly reduces the average access time: 16:7 ms
1
32
8:3 ms = 16:4 ms.
Similarly, for the inner tracks, there is about a one in two chance
that the initial seek will settle in the middle of the desired data,
saving on average
1
2
1
4
=
1
8
. This reduces the average access
time: 19:5 ms
1
8
8:3 ms = 18:5 ms.
So, we estimate that such an access would take between 16.4 ms
and 18.5 ms.
Notice that the sequential workload takes vastly less time than the random
workload (less than 20 milliseconds v. 5.5 seconds). This orders of magnitude
disparity between sequential and random access performance influences many
aspects of file system design and use.
Still, even for the 500 sector request, a non-trivial amount of the access time
is spent seeking and rotating rather than transferrring.
Example: Effective bandwidth.
Question: For the transfer of 500 sequential sectors examined in the pre-
vious example, what fraction of the disk's surface bandwidth is
realized?
Answer: The effective bandwidth ranges from
(500 sectors)(512
bytes
1
18:5 ms
)(
1000000 bytes
)(
1000 ms
1 MB
sector
)(
) = 13:8 MB/s
s
bytes
sector
)(
1
16:4 ms
)(
1000000 bytes
)(
1000 ms
1 MB
(500 sectors)(512
) = 15:6 MB/s
s
13.8 MB/s
54 MB/s
= 26% to
15.6 MB/s
This gives us a range of
128 MB/s
= 12%
of the maximum bandwidth from the inner to the outer tracks.
