By Kelly Cash
There's been a lot of talk in the storage industry recently about
solid-state disks (SSDs) and their ability to dramatically speed
up a computing environment's performance. One thing that is rarely
discussed is the difference between different types of solid-state
technology. There are two basic types of memory used by SSDs: DRAM
and flash memory. A general perception in the computing industry
is that only DRAM is robust enough for enterprise use. That sentiment
doesn't give enough credit to flash memory. As with any two different
technologies, each has its advantages. This paper explains the differences
in effort to help determine which technology is best suited for
one's IT environment.
Speed
It's common knowledge that writing to flash memory is much slower
than writing to DRAM. Isn't speed the main purpose of installing
an SSD in the first place? Yes. However, to say that flash technology
is "slower than DRAM" is to sell it short. First, reading
data from flash memory is very similar to the speed of reading from
DRAM. Second, the better manufacturers of flash SSDs incorporate
a DRAM cache in the drives to speed up writes. The best of those
manufacturers have algorithms inside the devices which are able
to flush that data from cache to flash in the background without
impacting performance. If we graph the relative performance of the
two types of SSD and a traditional rotating disk, it looks like:
This graph is close to scale- one pixel width represents 10 microseconds
(µs). Typical access times are: DRAM SSD: 10-50µs, Flash
SSD: 35-100µs, Rotating Disk: 5000-10000µs (5-10ms).
We can see from the above graph that the DRAM-based SSD is indeed
faster than the flash-based SSD; it may even be three times as fast.
However, we must ask the question: "Is that performance difference
significant?" Considering how much faster each SSD technology
is than rotating disk, the answer may well be "No." Chances
are good that other differentiators below will be more important
to many IT environments.
Longevity/Lifespan
Unlike DRAM, flash memory chips have a limited lifespan. Further,
different flash chips have a different number of write cycles before
errors start to occur. Flash chips with 300,000 write cycles are
common, and currently the best flash chips are rated at 1,000,000
write cycles per block (with 8,000 blocks per chip). Now, just because
a flash chip has a given write cycle rating, it doesn't mean that
the chip will self-destruct as soon as that threshold is reached.
It means that a flash chip with a 1 million Erase/Write endurance
threshold limit will have only 0.02 percent of the sample population
turn into a bad block when the write threshold is reached for that
block. The better flash SSD manufacturers have two ways to increase
the longevity of the drives: First, a "balancing" algorithm
is used. This monitors how many times each disk block has been written.
This will greatly extend the life of the drive. The better manufacturers
have "wear-leveling" algorithms that balance the data
intelligently, avoiding both exacerbating the wearing of the blocks
and "thrashing" of the disk: When a given block has been
written above a certain percentage threshold, the SSD will (in the
background, avoiding performance decreases) swap the data in that
block with the data in a block that has exhibited a "read-only-like"
characteristic. Second, should bad blocks occur, they are mapped
out as they would be on a rotating disk. With usage patterns of
writing gigabytes per day, each flash-based SSD should last hundreds
of years, depending on capacity. If it has a DRAM cache, it'll last
even longer.
Data Integrity
Most flash SSD makers employ error-checking algorithms and are
able to correct a few bytes in a 512-byte block. Some of the less-robust
error-checking will miscorrect three byte errors about 20% of the
time. The best flash SSD providers can correct six random byte errors
(and detect nine) in a 512-byte block. They will also never miscorrect
a three-byte error. This level of error-checking gives security
that the data integrity of the drive will last much longer than
we as IT professionals will have to worry about it.
Volatility
Unlike DRAM, flash is inherently non-volatile. There's an old axiom
which states that "a computer's attention span is only as long
as its power cord." This definitely holds true for DRAM as
well. While flash memory will retain its data beyond 10 years without
power, little more than 10 milliseconds without power will give
DRAM a most annoying case of amnesia. To prevent this, DRAM-based
SSD makers must add batteries and disks to keep the data from being
lost during a power failure. Though rechargeable, these batteries
must be maintained (replaced) on a regular basis (maintenance cycles
vary; consult the SSD's manufacturer) to ensure their ability to
completely backup the data in the SSD. The batteries maintain power
to the memory and disk(s) long enough to transfer the data from
DRAM to the non-volatile storage. Two things to consider are: Some
power failures happen in rapid succession- This may cause the backup
operation of the SSD to start over, which essentially drains the
batteries prematurely. This may mean that the batteries will not
retain enough power to complete a backup cycle. Second, backup and
restoration of the data takes time. It can take 30 to 60 minutes
or more to backup and restore the data. The backup time usually
isn't painful, but the restoration can cause extended downtime.
Consider the scenario of a power failure and successful data backup
to disk. When power returns, the server(s) can be up and ready long
before the SSD's data is restored from its backup disk. This can
mean that the server will be unavailable for an extra hour or so.
Depending on the application, this could range from a mere annoyance
to a business-threatening outage.
Form Factor
Most DRAM-based SSDs are large, rack-mount devices. They require
large internal power supplies, fans, batteries and disk drives to
provide non-volatility. In comparison Flash-based SSDs are much
smaller, usually the same form factor as a conventional disk.
Flexibility
Because the form factor of flash-based SSDs is so much smaller,
they are inherently more flexible in their use. They can often be
used in place of traditional disks in storage arrays or in a server's
internal disk bays. Embedded applications or mobile systems often
require the much smaller footprint of a flash-based solid state
drives.
Reliability
Both types of SSD are quite reliable since there are few, if any,
moving parts. Even the backup disks of the DRAM-based SSDs are typically
spun down during normal operation. This means that both types of
SSDs are much more reliable than a traditional disk. However, for
more demanding environments the smaller, more rigid flash-based
SSDs are often more desirable. They typically withstand greater
vibration and temperature ranges than DRAM-based SSDs. Some flash-based
SSDs are even considered "ruggedized" by NASA and the
U.S. Military. These drives will withstand intense extremes that
would reduce a rack-mount box to rubble.
Power Consumption/Heat Dissipation
One benefit of flash memory is that is uses much less power than
DRAM chips. Because of this, flash-based SSDs generate much less
heat than their DRAM counterparts. This also means that they don't
need cooling fans, whereas the DRAM-based SSDs do. Again, fans take
space and require power themselves, which in turn generates heat
and noise.
Cost
No IT department would purchase a solution without looking at its
price tag. While DRAM chips and flash memory chips are similar in
price, the overall cost per megabyte is generally lower for flash-based
SSDs. This is due to the simpler design, and the lack of need for
backup batteries and disks, and the enclosures in which to hold
them. Some of the cost of the DRAM-based SSDs is the extra sheet
metal for holding the batteries and disks, as well as the labor
involved in assembling it all.
SUMMARY
Now that we've taken an in-depth look at the different functionality
and features, it should be clear to see that flash memory is useful
for more than just consumer devices- it's also well-suited for the
enterprise. Of course, one type of SSD will likely be better suited
to the needs of your applications and IT environment than the other.
Clearly though, it can be seen that flash memory has quite a list
of capabilities that make it a "superstar" technology
for many IT organizations.
Kelly Cash is the Technical Evangelist for BiTMICRO Networks,
a leading supplier of flash-based solid-state disk storage solutions.
He has been in the systems performance arena over 15 years, specializing
in server and storage optimization. During his career he's been
an engineer focusing on performance for such companies as Sun Microsystems,
Data General, and AIM Technology. He's also held positions in the
IT space, such as the Chief UNIX IT Technologist for Cadence Design
Systems.
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