|
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.
All
2.5" SSDs and
3.5" SSDs which
were in production upto the middle of
2012 used
small SSD
architecture controllers - which is inherently less
efficient in its use
of capacity than large controller designs.The industry's first large controller
designs in 2.5" form factors were shown in prototype models by
Fusion-io (2.5" PCIe) and
beta models by Bitmicro
(SAS SSDs). The
combination of architecture
and adaptive
R/W DSP IP means that the differences between the best and worst 2.5"
SSDs in usable to raw flash capacity
ratios will be much
bigger in 2013 than before.
Going back to classical SSD architecture
for now- in the small SSD architecture 2.5"
Nitro N2 launched by
pureSilicon in January
2012 - the usable capacity was 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:-
SSD efficiency (in
design and business), and
adaptive R/W
flash IP. | | |
| . |
 |
| . |
| Over-provisioning flash
capacity in SSDs - article by LSI |
Editor:- January 8, 2013 -
Understanding
SSD over-provisioning - is the title of a new article published in EDN and written by Kent Smith, Sr. Director of
Product Marketing at the SSD controller part of LSI.
Kent's
article describes the trade-offs between performance, the percentage of
over-provisioned flash capacity and the useful impact of compressible data -
which inside SandForce controllers is leveraged to create additional
over-provisioning.
The interaction between write amplification
counter-measures and the benefits of using TRIM commands on performance are also
noted. ...read
the article
Editor's comments:- there wasn't anything
new for me in this article - which covers similar ground to my 2011 article -
flash SSD
capacity - the iceberg syndrome - which shows how SSD makers leverage
capacity to tweak reliability and performance.
But - having said that -
I learned about over-provisioning by 10 years of talking about it - with
many SSD companies. And some of the things I put in my own article had been
gleaned from past conversations with Kent Smith himself when he was at
SandForce - as well as various other people in
Violin,
Texas Memory Systems
and Adtron.
I'm
guessing that what Kent would have liked to say on OP may have been "trimmed"
by a word count limit in his latest EDN article.
So here are some
other suggestions for more substantial and ideas packed articles I recommend
- which Kent Smith has written in the past for other publications, and which
cover SSD controllers from other angles:-
| | |
| . |
|
|
| . |
|
| |
.. |
 |
| Nothing suprised the
penguins. |
| .. |
|
|
|
| . |
|
|
| . |
|
|
.. |
 |
... |
 |
| ... |
| "Some SSD companies
can build the same usable capacity, performance and reliability for the user's
app - even when using 20, 30, 40% and even 50% less chips as their
competitors..." |
| SSD efficiency | | |
. |
 |
. |
| For new apps to work better
it will more important to know the SSD software than to know the OS.
|
| thinking inside
the box | | |
. |
| On what basis are users
going to compare value for money when buying new enterprise flash storage arrays
and hybrids? SSD marketers don't understand the technology in their
products. User needs keep changing. Most people aren't confident that what
they can measure and model today will predict the results they'll see in the
upwardly scaled application experience. |
| Exiting the
Astrological Age of Enterprise SSD Pricing | | | |
