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No With
Lookaside Lookaside Win
100 Mb/s 14 (0.5) 13 (2.2) 7.1%
10 Mb/s 39 (0.4) 12 (0.5) 69.2%
1 Mb/s 317 (0.3) 12 (0.3) 96.2%
100 Kb/s 4301 (0.6) 12 (0.1) 99.7%
This table shows the resume latency (in seconds) for the CDA
benchmark at different bandwidths, with and without looka-
side to a USB flash memory keychain. Each data point is the
mean of three trials; standard deviations are in parentheses.
Figure 7. Resume Latency
No With
Lookaside Lookaside Win
100 Mb/s 173 (9) 161 (28) 6.9%
10 Mb/s 370 (14) 212 (12) 42.7%
1 Mb/s 2688 (39) 1032 (31) 61.6%
100 Kb/s 30531 (1490) 9530 (141) 68.8%
This table gives the total operation latency (in seconds) for
the CDA benchmark at different bandwidths, with and with-
out lookaside to a DVD. Each data point is the mean of three
trials, with standard deviation in parentheses. Approximately
50% of the client cache misses were satisfied by lookaside on
the DVD. The files on the DVD correspond to the image of a
freshly-installed virtual machine, prior to user customization.
Figure 8. Total Operation Latency
correctness. A concerned user could, of course, carry
his own DVD.
Figure 8 shows that lookaside caching reduces to-
tal operation latency at all bandwidths, with the reduc-
tion being most noticeable at low bandwidths. Fig-
ure 12 shows the distribution of slowdown for indi-
vidual operations in the benchmark. We define slow-
down as (T
BW
− T
NoISR
)/T
NoISR
, with T
BW
being
the benchmark running time at the given bandwidth
and T
NoISR
its running time in VMware without ISR.
The figure confirms that lookaside caching reduces the
number of operations with very large slowdowns.
5.3 Trace Replay
5.3.1 Benchmark Description
Finally, we used the trace replay benchmark de-
scribed by Flinn et al. [6] in their evaluation of data
staging. This benchmark consists of four traces that
were obtained from single-user workstations and that
range in collection duration from slightly less than 8
Number of Length Update Working
Trace Operations (Hours) Ops. Set (MB)
purcell 87739 27.66 6% 252
messiaen 44027 21.27 2% 227
robin 37504 15.46 7% 85
berlioz 17917 7.85 8% 57
This table summarizes the file system traces used for the
benchmark described in Section 5.3. “Update Ops.” only refer
to the percentage of operations that change the file system
state such as mkdir, close-after-write, etc. but not individual
reads and writes. The working set is the size of the data ac-
cessed during trace execution.
Figure 9. Trace Statistics
hours to slightly more than a day. Figure 9 summa-
rizes the attributes of these traces. To ensure a heavy
workload, we replayed these traces as fast as possible,
without any filtering or think delays.
5.3.2 Experimental Setup
The experimental setup used was the same as that
described in Section 5.1.2.
5.3.3 Results
The performance metric in this benchmark is the
time taken for trace replay completion. Although no
think time is included, trace replay time is still a good
indicator of performance seen by the user.
To evaluate the performance in relation to the
portable device state, we varied the amount of data
found on the device. This was done by examining the
pre-trace snapshots of the traced file systems and then
selecting a subset of the trace’s working set. For each
trace, we began by randomly selecting 33% of the files
from the pre-trace snapshot as the initial portable de-
vice state. Files were again randomly added to raise
the percentage to 66% and then finally 100%. How-
ever, these percentages do not necessarily mean that
the data from every file present on the portable stor-
age device was used during the benchmark. The snap-
shot creation tool also creates files that might be over-
written, unlinked, or simply stat-ed. Therefore, while
these files might be present on the portable device, they
would not be read from it during trace replay.
Figure 10 presents our results. The baseline for
comparison, shown in column 3 of the figure, was the
time taken for trace replay when no lookaside device
was present. At the lowest bandwidth (100 Kb/s), the
win due to lookaside caching with an up-to-date device
was impressive: ranging from 83% for the Berlioz trace
(improving from 1281.2 seconds to 216.8 seconds) to
8