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by Zsolt Kerekes,
editor - May
2011 |
when comparing flash SSDs - capacity is not
always equal!
- the flash capacity shown on your invoice
- the flash capacity accessible to your apps
- the flash capacity inside the SSD
These differences tell you a
lot about your SSD.
Although few of us will ever get close enough to
a real iceberg to worry about being the next
Titanic - it's
nevertheless widely known that 9/10 of the mass of these floating mountains is
under the surface of the water.
Have you ever wondered about the flash
memory inside an SSD - and how the advertised storage capacity - which is
shown on your invoice - may be considerably different to the capacity that's
inside the SSD?
There
once was a
time when this difference was small - in the range 3 to 5 percent.
But with the declining cost of memory the clever people who design flash SSDs
have become accustomed to push the boundaries of
performance and
reliability by
leveraging excess flash capacity.
The 3 most popular techniques
are
- overprovisioning
- to ensure an abundant supply of pre-erased blocks which can be written to
almost immediately
- RAID - like
redundancy (called by various trademark names like RAIC, RAISE etc) - to ensure
that data in the SSD can survive the loss of individual blocks or even the loss
of entire flash chips.
- spare blocks - used to replace
bad memory
blocks which arise out of infant process defects and long term
wear-out.
Overprovisioning
for the purposes of getting high
write IOPS
performance has a visible and an invisible side to it too.
In some
designs the excess capacity for overprovisioning is manufactured invisibly
inside the SSD by compression techniques. Compression can also increase the
write throughput. Examples of companies which do this include:-
EasyCo and
SandForce.
SandForce says that overprovisioning by adding additional flash chips means
there are more chips to go wrong and implies it's a bad thing to do.
In
stark contrast - Fusion-io
explicitly enables its oem partners and systems integrators to select precisely
what percentage of its SSDs is deployed for overprovisioning. Typically done by
a low level format the realistic range can be from 20% to over 50%. Fusion-io's
view about reliability is that because their SSDs don't need an additional
processor (the controller
work is done by the host CPU) reliability is improved in that way. (Reliability
is just one of the many SSD
heresies you can read about elsewhere.)
Meanwhile
Texas Memory Systems
provides a guaranteed 30% overprovisioning inside its enterprise SLC flash
SSDs. A blog by the company explains why they think
30%
is a particularly optimal number. BTW this is memory which is additional
to the capacity which the customer sees on their invoice. That's an
additional 3 terabytes in the RamSan-630 - and upto 150GB on the company's
RamSan-20 PCIe card.
2.5" SSDs all
currently use small
SSD architecture controllers. In the Nitro
N2 launched by pureSilicon
in January 2012 - the usable capacity is 1.6TB compared to 2TB of raw flash
inside.
When it comes to RAID-like approaches the percentage of an
SSD's internal flash storage which is lost due to RAID techniques varies from
manufacturer to manufacturer. In some markets the impact is greater than
others. That's why SandForce enables oems to turn-off this feature in the
SF-2200 - for cost sensitive consumer markets because it frees up the
capacity of an entire flash chip in an entry level 8 chip array to give 14%
extra capacity.
As you can see from this briefing note the headline
capacity of a flash SSD isn't the same as the capacity installed inside the SSD.
The difference - which can be double digit percentage points - is something
else to think about when you're shopping around for the cheapest product - and
is just one of the many reasons that simply
comparing the price per
gigabyte doesn't tell you the whole story.
See also:-
StorageSearch talks
SSD with Holly Frost, CEO, Texas Memory Systems - which among other
things - discusses the many different detailed ways they put redundancy in their
no single point of failure rackmount SSDs. Despite all the redundancy in there -
the way they do it (with variable size RAID stripes for example) gives them one
of the lowest costs per
terabyte of systems in that performance class. | | |
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