Information Technology Reference
In-Depth Information
0.35
no−exchange
30
60
90
120
0.3
0.25
0.2
0.15
0.1
0.05
0
0
500
1000
1500
2000
Time (hour)
Fig. 5.
Ratio of access of disks in low-power mode
with a Dual Xeon 3.60 GHz CPU, 2 GB memory, and a single 36 GB SCSI disk.
For our prototype, owing to the limitation of our experimental environment (i.e.,
the bandwidth of different servers), we evaluated the response time of data access
by measuring the time from sending a request until the data were loaded into
the memory of the server. In addition, no underlying lookup service to access
data was implemented in our prototype. Thus, in the experiments, the data were
accessed by their storing server.
Parameters and Settings.
In our experimental environment, although the real
capacity of a disk was 36 GB, we assumed that the capacity was 500 GB, which
was emulated by only accessing one file. In addition, because it was dicult to
spin up or spin down disks in the current system configuration, we realized these
actions by letting the server wait before accessing the disk. The two parameters
were spin-up time of 5 seconds and idle time before spin-down of 60 seconds.
(According to simulation analysis, this configuration was the best in terms of
response time and power consumption.)
Response Time.
Figure 6 shows the change in response time. The figure shows
that the overall average response time is 133 ms. However, we observed that
some responses were delayed and required more than 5 seconds after 15 hours,
among which the worst took 18,307 ms. This result means that some accesses
were of the spin-down disk and had to wait for the disk to spin up or for the
completion of another file access.
Migration Cost.
Table 1 shows the migration cost evaluated by measuring the
number of files that migrated per data exchange and the elapsed time. Note that
this result was for 48 hours and the migration cost would decrease gradually after
48 hours. In addition, owing to the limitation of our experimental environment,
times in Table 1 are predicted values. This result shows that the required time
