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XLC Disk
XLC Disk, Inc., founded in 2005 and based in Boulder, CO, is a fabless
semiconductor company currently in stealth mode. The company anticipates
unveiling products based on its patent pending revolutionary XLC flash
technology in Q2 08 with shipments in Q1 09. |
| Unveiling
XLC Flash SSD Technology - March 2008 |
editor's comments:- from my
limited contacts with the founders of this company I expect XLC Disk's products
to be flash SSDs - probably in
1.8" and
2.5" form factors.
Looking
at their patent applications - it appears that XLC is a multi level cell flash
technology which offers 4x the capacity of SLC and 2x the capacity of MLC for
the same chip area.
It has been technically impossible for
researchers in long established chipmakers like
SanDisk or
Samsung to
commercialize reliable flash memory with more than 4 levels so I wondered -
how is a well funded
start-up like XLC going to do better than that?
XLC's founder refers
to MLC as "dumb first generation multi level flash" and XLC as the "multi
level flash of the future."
From what I can gather XLC's
technology goes to the core of the multi level cell problem and the problems of
logic level discrimination and
data storage
reliability.
Unless you're deeply versed in the internals of chip
design - these notes won't help you. But it's very hard making reliable MLC
flash - even when you control the whole process.
MLC
flash chip companies get over the limitations by using extensive redundant
data blocks and ECC. Until now no one has succeeded in storing more than 2
bits of data in a high density flash chip - and the reason lies in physics.
When the stored charges are related back to voltage levels - there's a wide
variation - not only over the whole area of the chip - but also in the same
part of the chip over time - due to the accumulation (or changes) in charge in
adjacent parts of the chip which may have occurred since the data was written.
In simple language - you don't always read out the same digital
value that you wrote in. Because the variation and tolerances in the end
to end read write process are greater than the thresholds between the logic
levels.
The problem is mitigated in today's MLC SSDs - because there
are only 4 levels to discriminate between. But when you try to increase this -
to say 8 - then the unreliability of the read back data - is very high. Adding
more ECC doesn't help - because you get into a situation where the unique data
capacity (after ECC) is lower than that achieved with traditional MLC.
XLC's
solution is design distributed discriminator circuits in zones throughout the
chip - and instead of relying on just traditional ECC - they add redundant
calibration cells in these zones which are prewritten with known data each time
a block erase write cycle is done.
When a read cycle occurs - the
converted voltage is compared - not to a fixed level or percentage of the rail -
but to adjacent levels written by the calibration controller. Variations in
charge and voltage in that zone of the chip affect djacent zones in a similar
way. The discriminator circuits compare the address selected data cell with
nearby calibrated cells. That means even if the voltage varies by 20% (which is
more than the difference in logic levels) the loc state is read back correctly
- because the read is effectively differential - the difference between
adjacent known calibrated cells.
Although the internal read and write
processes are slower than in SLC or MLC flash - the amount of data in a single
cell can be much more. For example an 8 level XLC can store 3 bits of data in a
single cell - which gives 2x the capacity of MLC and 4x as much as SLC. So
from the external chip point of view the data throughput is similar (or more)
than SLC even though the internal chip latency is slower. One by product of the
process is better endurance than MLC - approximately 2-3 times better (but still
much lower than SLC).
XLC won't say yet how many levels for XLC the
first commercial devices will have. They are characterizing the technology
across a range of foundry technologies. And that's another difference. XLC is
less process dependent than MLC.
XLC holds the promise of shattering
the cost per gigabyte gap which has separated
flash SSDs from
hard disk drives. I
look forward to the first unveiling of this technology, here on
STORAGEsearch.com April 1, 2008. |
- ...Later:- the first draft of the article above (written in March
2008) was meant to be a work of fiction - and a spoof April 1st news story.
Although I had the sneaking feeling that some of the ways I had "invented"
of solving the multi level data problem might work at some time in the future...
To
add authenticity to the spoof - I contacted Jim Handy at
Objective
Analysis to elicit an analyst's comment on the new technology.
You
can imagine my delight when he said
SanDisk was already
working on something which sounded very similar. That shows that the SSD
industry is programmed on a "fast forward" rate that can surprise us
all - even editors who are embedded in the market. other references
to 3 / 4 bit MLC flash
"SanDisk
believes that the introduction of 3-bits-per-cell technology with a write
speed comparable to existing MLC memory positions SanDisk well to respond to the
growing market demands for higher density flash memory for a wide range of
applications and at a significantly lower cost compared to 2-bits-per-cell MLC
on the same technology."
An article by Lane Mason (Denali
Software, Inc) looks forward to
"...an
8-12x price reduction, in solid state storage, by the move to 4-bit MLC cells,
by 2012," | |
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