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Power and performance characteristics of USB flash drives

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Even though their capacities are still orders of magnitude lower than those of hard disks, flash storage systems are rapidly gaining importance in energy-constrained systems. This paper focuses on USB flash drives, which can provide portable storage to mobile systems or storage to systems that otherwise do not have persistent storage opportunities (e.g., low-power sensor devices). The paper presents studies relating to power consumption, energy overheads and benefits, and performance impacts of USB flash drives. The key insights obtained from these experiments are that (i) read/write costs are not significantly greater than idle costs and (ii) the size of the flash itself has only limited bearing on energy consumption.
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978-1-4244-2100-8/08/$25.00
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2008 IEEE
Power and Performance Characteristics of USB Flash Drives
Kyle O’Brien, David C. Salyers, Aaron D. Striegel, Christian Poellabauer
Department of Computer Science and Engineering
University of Notre Dame
Notre Dame, IN 46556 USA
Email:{kobrie5, dsalyers, striegel, cpoellab}@nd.edu
Abstract
Even though their capacities are still orders of mag-
nitude lower than those of hard disks, flash storage sys-
tems are rapidly gaining importance in energy-constrained
systems. This paper focuses on USB flash drives, which
can provide portable storage to mobile systems or stor-
age to systems that otherwise do not have persistent stor-
age opportunities (e.g., low-power sensor devices). The
paper presents studies relating to power consumption, en-
ergy overheads and benefits, and performance impacts of
USB flash drives. The key insi ghts obtained from t hese ex-
periments are that (i) read/write costs are not significantly
greater than idle costs and (ii) the size of the flash itself has
only limited bearing on energy consumption.
1 Introduction
Storage components of mobile wireless systems, includ-
ing laptops, handhelds, and sensor devices, are essential for
temporary and long-term preservation of information. In
the recent past, solid state, or flash storage, has emerged
as a promising alternative to hard disks. While flash mem-
ory and drives offer the potential for very low-power stor-
age, they are still significantly smaller than available disk
drives [7].
USB flash drives provide an opportunity to extend an ex-
isting mobile/wireless device with (additional) storage ca-
pacity; this is particularly important f or small low-power
devices that otherwise do not have any storage opportunities
(e.g., TelosB motes and Crossbow Stargate wireless mesh
routers). With newer areas of research such as Delay Tol-
erant Networking (DTN) [5], on-device flash capacities are
often not enough. Finally, in the absence of designing a
new device solely for their research, researchers are often
faced with employing USB flash drives to meet that need.
Unfortunately, the performance of said devices with regards
to energy and read/write speeds is poorly understood. The
usage of flash for the network researcher is especially criti-
cal as the choice of flash can have a profound effect on the
energy and performance characteristics of the overarching
system or application.
Toward this end, this paper provides a comprehensive as-
sessment of the power, energy, and performance character-
istics of flash drives. Specifically, the contributions of this
work can be summarized as follows:
Comprehensive study of USB flash drive power con-
sumption: This paper experimentally examines the
power consumption characteristics of USB flash drives
across a wide variety of considerations including man-
ufacturer, device size, operating system, read/write
blocks, and USB connection types (1.1 or 2.0).
Comprehensive performance studies: Further, this pa-
per evaluates the energy - performance tradeoff of USB
flash drives, assessing the energy costs and perfor-
mance numbers of various I/O accesses to a USB flash
storage.
2 Experimental Methodology
The measurement results were obtained using a
Fluke189 multimeter and a DSO3202A oscilloscope
(200MHz bandwidth, 2 channels, 1GSa/s sample rate). The
results were imported into and visualized using LabView.
The USB flash drives utilized in the experiments are sum-
marized in Table 1. While there are certainly commonal-
ities between different drive vendors in terms of the actual
underlying flash, our goal was to roughly assess the existing
spectrum of flash drives with selective finer grained assess-
ment within select individual vendors (Kingston, Sandisk).
The flash drives’ power characteristics were studied on
multiple host devices and operating systems:
Crossbow Stargate (Linux:) Crossbow Stargate de-
vices are frequently used as wireless mesh routers or
as powerful sensing devices (Linux 2.4.19, USB 1.1).
Manufacturer Size(s)
Axiom, Entec 1 GB
Edge, PNY 2 GB
EP 128 MB
Imation, Kanguru 512 MB
Kingston 512 MB, 1 GB, 2 GB, 4 GB
Memorex 256 MB
OCZ, Transcend 1 GB
Patriot 4 GB
Sandisk 512 MB, 2 GB, 4 GB
Simpletech, Smartmod 256 MB
Table 1. USB flash drives used in study
Mac mini (Mac OS X), Gateway Tablet (Windows XP),
Sun x2100 Workstation (Linux 2.6): For USB 2.0 per-
formance across various operating systems and sus-
pend behavior, the listed array of systems were em-
ployed.
3 Power Consumption
Figure 1 captures the power consumption of the respec-
tive USB flash drives under USB 1.1 accessed via the Linux
2.4 kernel on a Stargate Crossbow device. The figure cap-
tures the average power consumed under idle conditions,
reading a significant file, and writing a significant file.
With regards to the cost of read/write over idling, the an-
swer appears to clearly be that writing to the respective flash
device when done in a bulk write (64kB blocks) does not
incur a serious cost over normal read and write operations.
The notable anomaly is the case of the Memorex flash de-
vice whereby power consumption is actually improved over
the idle case. In short, the action of reading and/or writing
for the Memorex device changes the main LED from a con-
tinual on state to one that flashes on and off, thus reducing
power significantly.
In contrast, the Sandisk devices exhibit the opposite ex-
treme of the behavior, representing a jump in total power
that cannot be solely attributed to the cost of the reading
and/or writing the device. Notably, the Sandisk devices pos-
sess the largest and brightest LEDs of the surveyed devices
representing a notable power cost for USB 1.1 interactions.
The Kingston set of devices were some of the most efficient
devices in terms of average power consumption.
While the limited impact of read and write operations
as well as the nearly indistinguishable cost of writing over
reading were certainly interesting, the most striking aspect
of the devices was the lack of engaging the low power sus-
pend mode when not in use. Per the USB specification
[1, 2], if a device does not see activity in the last 3.0 ms,
Figure 1. Power consumption of all devices -
idle, read, write under USB 1.1
the device has 7.0 ms to enter a low power suspend mode
where the device may use a maximum of 0.5 mA of cur-
rent. Devices with remote wakeup may draw up to 2.5 mA
of current. Upon further investigation of t he data line itself,
a 2-byte keep alive signal is sent to the flash drive devices
continually, thus preventing the device from ever entering
the low power mode. This keep-alive message will persist
for the duration of the host running with the only exception
being when the “Safely remove hardware” operation is per-
formed under Windows. Unmounting the drive under Linux
or ejecting the drive under Mac OS X did not push any of
the drives into suspend mode despite repeated attempts and
configurations.
3.1 Examining Write Power Consumption
Further
Figure 3 examines the effect of block size averaged
across all devices. In the figure, the same size file is writ-
ten out using different block sizes for each respective write
call. While the graph itself is not unexpected, the key transi-
tion points are interesting. A block size of 4 kB yields little
performance difference versus block sizes of up to 18 kB.
Once the 20 kB threshold is passed, the net write perfor-
mance is quickly saturated leaving minor gains (if any) for
pushing the write block size even higher. An interesting re-
search case would be to examine the effects of writing with
regards to reliability although most modern flash devices
scatter writes across the flash to avoid continually rewriting
the same area. We note that the 20 kB nicely captures the
Figure 2. Write speed - USB 1.1 - 100 MB file
Figure 3. Effect of block size USB 1.1
static content of many popular websites for the purposes of
on-device caching in the flash itself [6].
3.2 Varying Device Size
Figure 4 examines the effect of device size on the overall
power consumption of a specific series of devices, namely
variations within the Kingston devices. Due to the signif-
icantly larger power consumption of the Sandisk devices,
only the Kingston devices are highlighted in the graph. In
fact, the Sandisk devices demonstrated power savings when
switching to larger devices (i.e. gains in efficiency) over
smaller devices.
While there is a slight general increasing trend for idle
power, the read/write power consumption is relatively in-
conclusive until the 4 GB device. The cost of increasing
size appears to be largely masked and only of significant
consequence with the largest device size. Hence, from the
perspective of the network researcher, the selection of a 2
GB device has little energy cost with significant flexibil-
ity improvements over a 512 MB or 1 GB device. Further
study is warranted regarding if the trend continues for newly
emergent 8 GB and larger devices.
3.3 USB 1.1 vs. USB 2.0
Although support for USB 2.0 in small scale embed-
ded devices is less common, the increased speed for USB
2.0 (480 Mb/s) versus USB 1.1 (12 Mb/s) has significant
appeal as wireless network speeds are continually increas-
ing. While the performance of USB 2.0 is considerably
better than USB 1.1 in terms of write performance (16.97
MB/s peak write performance versus roughly 700 KB/s un-
der USB 1.1), the average power consumption is well over
double during the write operation itself as well as during
idle periods.
Table 2 illustrates the power consumption of the Patriot
4 GB drive across various combinations of platforms, oper-
ating systems, and USB connectivity. Interesting observa-
tions from the graph are that the power cost at idle time or
during a read or writ e (read is omitted due to its similar per-
formance to a write operation) are consistent despite varia-
tions in platform, CPU/motherboard, and operating system.
While this is to be expected given that USB is a well un-
derstood standard, the tightness of the numbers across plat-
forms was somewhat surprising.
Although USB 2.0 performance is significantly in-
creased over USB 1.1, the idle power consumption is quite
troubling given the fact that suspend is never engaged de-
spite long idle periods on the device itself (barring cases
where the OS hibernates or sleeps). Unless the applica-
tion strictly needs the improved performance (i.e. caching
content from an 802.11 draft n link versus an 802.11g or
802.11b link), the idle power consumption of USB 2.0
makes it quite costly relative to USB 1.1.
4 Related Work
In the direct area of USB flash drives, very little previous
work exists in characterizing the power of the devices. To
the best of our knowledge, ours is the first work to directly
assess USB flash drive performance with regards to energy
characteristics. We comment briefly on several of the more
recent related works that consider flash for the purpose of
energy savings.
The notion of energy-awareness with regards to multi-
media and mobile computing is a concept that has seen
considerable research [8]. In [7], Singleton, Nathuji, and
Schwan explore algorithms to manage flash allocation as a
Figure 4. Effect of varying flash size - USB 1.1
cache for disk access. Similarly, Bisson and Brandt in [3]
examined similar aspects to allow for improved disk spin
down for reduced energy consumption. Chen, Jiang, and
Zhang propose SmartSaver in [4] for using flash as a pre-
fetch and cache for web content for mobile nodes. These
works simply represent a small subset of the works exam-
ining the applicability of flash. Notably, the end result in
nearly of the work is that to actually provide the flash itself,
a USB flash drive is used whose characteristics are assumed
to roughly mirror actual on-device flash.
Specifically, the emergence of Delay Tolerant Network-
ing [5] as a subject of much research is especially notable.
Critically, if a researcher assumes that the flash device oper-
ate in a manner as power efficient as normal OS operations
(i.e. the drive will suspend if not used), the end result will
be a distorted picture of the true energy savings offered by
flash. Moreover, the characterization of the true costs of re-
suming and suspending the flash device make the develop-
ment of better performing suspend/resume operations a nec-
essary component for the research community as a whole.
5 Conclusion
In conclusion, this work represented one of the first
works to comprehensively address the power consumption
of USB flash drives across a multitude of combinations of
manufacturer, connecting host, and connection type. No-
tably, the most striking results are that: (i) reads and writes
while more expensive are not significantly more expensive
than normal idle operation, (ii) raw performance often does
not translate to overall energy savings (faster write perfor-
mance vs. overall energy efficiency, USB 1.1 idle vs. USB
2.0 idle, etc.).
Platform - OS USB Idle (mA) Write (mA)
Stargate /
Linux 2.4 1.1 22.37 34
x86 Workstation /
Linux 2.6 2.0 50.88 100.5
x86 Tablet /
Windows XP 2.0 50.84 105.5
PPC Mac Mini /
Mac OS X 1.1 22.4 34.5
x86 Mac Mini /
Mac OS X 2.0 50.95 110.0
Table 2. Comparison of USB 1.1 vs. USB 2.0
for various OS / platforms for the Patriot 4 GB
flash drive
This work was partially supported by the National Sci-
ence Foundation as part of an REU project through grant
CNS03-47392. Equipment support was provided in part by
a DURIP grant and a Notre Dame Faculty Grant.
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