IEEE Consumer Electronics Magazine - July 2015 - 92

Technology Application Regions
Flash

100,000
10,000

Performance HDD
Hi IOPS
Capacity HDD

100

Transaction
Near Line

10
1

Semiactive

Tape

10%
1%

µs
10

µs

s
m

s
1

1,
00

,0
0

Inactive

0.1%

0,
00

I/O OBJECT SIZE CURVE

Cold-Active

Touch/Y

1,000

removable media library, increasing
the ratio of drives to storage media
similarly increases the touch rate.
The touch rate chart can be extended
up beyond flash, and we anticipate that
new nonvolatile storage technologies
(such as STT-MRAM) will enter this
region in the not-too-distant future.

Response Time

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

FIGURE 2. Digital storage technologies regions overlaid on the touch rate/response time chart.

lower-performing technology is usually
more cost effective.
Moving the performance of a storage technology to the upper right,
beyond its native capability, is difficult
and is usually a very expensive proposition. It frequently involves reducing
the capacity of each storage unit by
overprovisioning. For example, shortstroking HDDs can reduce the response
time and increase the touch rate, but
they also significantly increase the
US$/TB. If the capacity is limited to
25% of the total available HDD capacity, the touch rate will go up by four
times, but so will the US$/TB. In a

performance limits impact the system
performance. On the right side of a
region, the performance for small
objects is dominated by the access time
(i.e., the time to get to the first byte of
data). On the left side, the performance
for larger objects is dominated by the
data transfer time.
A given technology is most cost
effective at the upper-right boundary
(i.e., the knee of the storage technology
curve), where its full performance can
be extracted. It becomes less cost-effective as it is used more to the lower left
of this knee. At some point, as the
requirements move to the left, a

1 TB

64 TB 4 GB 256 MB 16 MB
Capacity HDD

100
10

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

µs
10

µs

10
0

s
1

10
0

Inactive

0,
00
0
s
10
,0
00
s
1,
00
0
s
10
0
s

4 kB

Cold-Active

Transaction
Near Line 64 kB

Cap. HDD

10%

0.1%

Hi IOPS
1 MB

Semiactive

Touch/Y

1,000

10,000

4-TB Capacity HDD Touch Rate Versus Response Time

m
s

100,000

Response Time

FIGURE 3. Touch/Y and response time for 100% random I/O in a 4-TB capacity HDD.

92 IEEE ConsumEr ElECtronICs magazInE

july 2015

The object size used by an application
influences both the touch rate and the
response time. Thus, we will get a curve
for the touch rate versus response time
as a function of the object size for various storage technologies. The object
size is indicated along this curve. The
curve shown in Figure 3 is for 100%
random I/O at the given object sizes for
a typical 4-TB capacity HDD system,
such as an HDD used in a DVR or STB.
The touch rate versus response time
curve for this capacity HDD has a shape
similar to that shown for technologies in
the technology regions chart (Figure 2).
At a 4-kB object size, the response time
is 13 ms and the touch rate is 2.4/Y.
While this is a near-line drive, this small
object size falls outside the near-line performance region. An object size associated with transaction workloads is 4 kB,
and the curve shows why capacity HDDs
are not favored for such workloads.
At a 64-kB object size, however, the
touch rate has grown to 37, and the
response time has only increased slightly
to 13.4 ms. As a result, this is a better
object size for this class of HDD and is a
more typical object size for near-line
workloads. At a 1-MB object size, the
touch rate has reached 412, and the
response time has increased to 19.1 ms.
By this object size, we are reaching the
point where the data transfer time component of the response time is becoming
important. At a 64-MB object size, the
touch rate has saturated at 1,240, but the
response time has now grown to 400 ms.
Beyond this, the touch rate is essentially
constant, and the response time grows linearly with the object size. We use the term
saturation touch rate to refer to this region
with a constant touch rate over a wide
range of response times.
When reading the touch rate chart, the
object size chosen should be suitable for

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