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Flexing SSD Strengths for High-Reliability
Data Recorders
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A Cost Scenario
Let us a take look at one particular instance where the cost of
operating a data recorder is estimated using tape drives as media.
A good example of a data recording application is an aerial reconnaissance
mission, capturing imagery. This mission may either be military
(enemy detection and surveillance) or civilian (mapping, traffic
monitoring, science and geological survey) in nature. Each military
mission normally lasts three to four hours and may cost millions
of dollars each. These flights require data recorders with a sustained
write rate of up to 230 Mbytes/s.
To meet this spec, tape-based data recorders will need high-speed
magnetic tape costing well over $1,000 per reel. Each reel can only
be used once because recording quality degrades if used more than
once.
Assuming that one reel has a recording time of 15 minutes, then:
60 minutes (1 hour) = 4 reels per hour
15 minutes (per reel)
If 1 mission = 4 hours
Then 4 reels per hour x 4 hours = 16 reels
Multiplying the figure by the cost per reel (about $1,000), the
cost of tapes alone could reach $16,000 per mission. If 100 missions
were performed in a year, about $1.6 million would be spent on tape
media.
There are other disadvantages to tapes as well. The tape controller
is expensive and may be very bulky and heavy. Since each tape reel
can only record data for 15 minutes, a new reel has to be loaded
four times per hour. This means that there is a gap between reloads
when the data recorder will not be able to take pictures. In these
cases, reconnaissance planes have to cover the same area twice,
increasing their exposure to enemy fire.
SSD Technology
The solid-state disk (SSD) emerged as an alternative medium during
the early 1980s as data storage solution providers searched for
a reliable device that could withstand hostile environments (shock,
vibration and temperature extremes) in military and industrial applications.
A good example is the commercial airline industry. The National
Transportation Safety Board has set stringent standards for flight
data recorders ("black boxes") installed in aircraft (Table 1).
Another aspect to consider is recording speed. As time and technology
move forward, there is a greater need to capture higher resolution
information. Today's sensors are able to collect much more data
in a shorter amount of time. This means that data storage must be
faster and more reliable than ever before. To ensure that no data
is lost during collection, the storage subsystem must always be
able to outperform the data collection subsystem, even when the
data collection is at its fastest. Otherwise, valuable information
will be lost.
Today, many recorders capture data to a RAID stripe on multiple
disk drives. This is effective, but the reliability of disks can
be a problem in harsh environments. Another aspect to consider is
that traditional RAID controllers are managed by operating systems,
and their disks frequently hold system files as well as recorded
data. Thus, disk space and performance for "real-time" recording
are shared with other applications and services in the system. For
this reason, it can be difficult for a RAID subsystem to guarantee
instantaneous performance at any given time as is required in real-time
applications.
These demanding requirements are starting to dictate the need for
a new storage medium, and SSD is an ideal technology to answer the
call. SSDs offer a lot of features that makes them desirable for
use in data recorders. First, an SSD is much more robust than a
disk or a tape media as it has no moving parts. Second, the speed
of an SSD is much faster than all other storage devices available,
in terms of both data throughput and access speed. This is because
there are no platters to spin nor heads that seek, resulting in
immediate access to any data located in the SSD.
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