IEEE Consumer Electronics Magazine - July 2015 - 91

Touch Rate Versus Response Time

100,000
More Accessible

10,000

100
10

in
as

Transaction

Near Line
Fast Enough for Disk Stack
Semiactive

Inc

Limit of Human Response Tolerance
Cold-Active
Faster

One Day = 86,400 s
One Hour = 3,600 s

µs

0
10

s
1

s
m

s
0

0
,0
0

1,
00

Inactive

0.1%

/T
S$

10%

0,
00

Hi IOPS

Touch/Y

1,000

response time measures the time to act
on a single object, while the touch rate
relates to the time to act upon the data
set as a whole. Figure 1 shows such a
chart that includes some indications of
various sorts of applications (performance regions) and general tradeoffs
that are important to the system design
indicated. Note that this is a log-log
chart with log base-10 scales on both
the vertical and horizontal axes.
The chart shows the touch rate as the
log touch per year on the vertical axis
and the log response time on the horizontal axis with faster response times on
the left. A shorter response time means
that the data can be accessed more
quickly, increasing its value. A higher
touch rate means that more data can be
processed in a given time period,
increasing the value that can be extracted. Thus, the data value increases for
objects to the upper right. However, the
system costs also tend to increase to the
upper right. Note that the total value of
the data includes the amount of data, so
most of the data value could be at lower
performance regions for a system with a
lot of stored content.
We have defined six performance
regions as shown in Table 1 and Figure 1.
While the boundaries are somewhat arbitrary, they are representative of the performance bounds for many workloads.
The Hi IOPS region is for storage
system performance beyond the capabilities of hard disk drives (HDDs) and
may require flash memory or even volatile RAM. The touch rate lower bound
for the Hi IOPS region is near three per
day. Note that many mobile consumer
devices use flash memory because the

Response Time

FIGURE 1. Touch rate versus response time, indicating various types of uses.

fast read speeds help reduce power use
in these mobile devices.
The transaction region is for transaction data, and the performance limits are those of high-performance
enterprise disks. The touch rate lower
bound for the transaction region is
about one every three days (or 0.33
per day).
The near-line region has performance limits matching the characteristics of high-capacity HDDs. The touch
rate limit is one per month.
The semiactive region is defined as
having a response time fast enough to
not have time-outs when used with an
HDD software (SW) stack (typically
about 10 s). A system in this region
would then be functionally plug-compatible with an HDD-based system,
albeit slower. The touch rate lower limit
for the semiactive region is one per year.

The cold-active archive region is
characteristic of online archives with
human interaction. Thus, the response
time limit is 60 s, which is about the
human tolerance for an I/O operation.
If the I/Os take longer than this, most
humans would assume the I/O has
failed or wander off and do something
else. The touch rate lower bound here is
3% per year. Everything slower and
below 3% touch/Y is in the inactive
archive region (the slowest response
times in Figure 1).

TECHNOLOGY REGIONS
Figure 2 shows where various storage
technologies lie in the touch rate chart
as shown in Figure 1. Flash, HDD,
and tape are used to address workloads in different regions. These storage technology regions have a hockey
stick shape because of the way the

TABLE 1. Six performance regions for a discussion of storage system design.
region

minimum response time

minimum touch/Y

High (Hi) input output operations per second (IOPS)

1 ms

1,000 (~three per day)

Transaction

10 ms

100 (~one per three days)

Near line

300 ms

12 (one per month)

Semiactive

10 s (HDD SW stack)

One

Cold-active

60 s (human tolerance)

Inactive

>60 s

<3%

july 2015

IEEE ConsumEr ElECtronICs magazInE

Table of Contents for the Digital Edition of IEEE Consumer Electronics Magazine - July 2015