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| When flash
SSDs aren't fast enough! |
RAM based SSDs are the
original type of
solid state disk and have
been around for
decades.
They rely on batteries to retain data
when power is
lost. Most models also include internal
hard disk drives to
which data is saved under battery power, so that data is not lost when the
battery runs down. This hybrid technology means that RAM based SSDs are more
bulky than
flash counterparts
and RAM SSDs are unable to operate in the same range of
hostile environments.
RAM based SSDs are mostly used in enterprise server speedup
applications. The fastest RAM SSDs are faster than the fastest
flash SSDs. But
for many server speedup applications flash SSDs are fast enough.
Unlike
flash SSDs, RAM based SSDs never had restrictions on the number of write
cycles. That made them more popular in enterprise acceleration applications in
the past. But write
endurance problems may be a thing of the past for flash.
Like hard
disks - RAM SSDs have symmetric read/write IOPS. That's another big
difference between
RAM and flash SSDs.
The fastest flash SSDs available in 2009 had
achieved parity between random read and write IOPS.
But that's not how
transaction based applications work. The important differentiator here is repeat again write
IOPS. If you compare that between RAM and flash based SSDs - the RAM
SSDs are upto 100x faster - even when the datasheets suggest they look
the same.
On the other hand - in some enterprise applications - like
IPTV servers - the
random write IOPS rarely repeats in the same memory space during milli-second
timeframes - and in these video server apps - flash really does perform as well
as RAM - and is much cheaper.
Latency figures quoted by many flash SSD
products can also look very similar to those for RAM SSDs. But low random write
latency doesn't mean that the data has actually hit the flash media yet - as
you'll find if you try to read back the data and rewrite to the same block.
There
are also some non volatile memory products such as
PRAM,
FRAM and RRAM which are replacing flash in industrial applications - and
which already offer 1 to 1 read/write performance. But their capacity is 2
orders of magnitude too low to be of use in server applications.
RAM
SSDs cost about 3x as much as SLC flash SSDs for similar capacity in FC
SAN rackmount systems - (based on pricing data 2011.)
The ideal
choice of SSD depends on the specific server and application environment and
cost / benefit analysis.
Not everyone needs or can afford the
fastest SSDs. Some
environments do. Others don't.
Identifying the right choice of SSD in
the right place is a complex decision - which requires a high degree of
SSD education and
trust in the vendor.
More articles about the problems and solutions
related to accelerating enterprise server apps can be seen on the
SSD ASAPs page. | | |
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| RAM vs
flash SSDs decision tipping point |
Editor:- in December 2010 - I interviewed Jamon Bowen,
Director of Sales Engineering for Texas Memory Systems
and asked him about the use of SSDs in financial applications like banks and
traders - a market which he said accounts for most of their RAM SSD sales.
The company which started in RAM SSDs
over 30 years
ago - now sells more flash SSDs than RAM SSDs (even though the product
brand for both types of SSD is
confusingly called
RamSan.) Bowen said that flash is 70% of their business.
Jamon
Bowen said that in many bank applications RAM SSDs are actually cheaper than
flash - because of the small size of the data. TMS still sell a lot of 16GB RAM
SSDs.
Production bank systems are typically shared by many hosts and
get a lot of write IOPS / capacity. To achieve the same
reliability and
latency with
flash would
require over
provisioning which would drive the cost up.
He suggested a simple
rule of thumb for intensive IOPS bank SSDs on the SAN
- < 128GB capacity - RAM SSDs cheaper
- 128GB to 4TB capacity - middle ground could be either - or determined by
other constraints
- > 4TB - flash SSDs cheaper
Jamon Bowen said that the analysis
side of operations in banks is different. That tends to have much larger data
sets and is more read than write intensive. In these apps - flash SSDs are
usually more economic. | | | |
Fusion-io's 1 billion IOPS demo narrows
latency gap between flash and RAM SSDs
Editor:- January 6, 2012 -
in a historic
demo
yesterday showing the capabilities of its latency reducing Auto Commit
Memory (ACM) extension Fusion-io
announced it had exceeded 1 billion IOPS (64 byte data packets) in a
configuration which used 8 HP servers each configured with 8x
ioDrive2 Duo PCIe
SSDs.
Editor's comments:- although we're used to thinking about
SSD IOPS in terms of bigger packets - such as 4kB - instead of the very small
packet size in this demo -
IOPS is simply
a convenient and not always reliable way of comparing the relative
performance of storage products.
In real life - users don't have a
choice of what size the R/W operations are which take place in their apps. They
occur at all sizes (mostly smaller than 4kB) and when these R/W operations take
place in traditional storage architecture systems - which internally impose
their own restrictions on the minimum size of atomic data packets - that's where
latencies and performance become discontinuous compared to the value of the data
update due to amplification
and packetization effects.
In my view - the important thing about this
demo - is that the same PCIe SSD product which can perform useful work as a
storage device - can also be deployed as a super scaler memory device - when it
is running the appropriate software.
The difference is that with
traditional storage software - you might expect that a 64x PCIe SSD system might
hit 64M IOPS or some similar figure (regardless of the small size of the data
packet). Instead the demo shows that apps developers can get 16x more
performance in small R/W transactions if they are willing to invest the
effort to make their apps work with FIO's new APIs.
It's that order of
magnitude difference which is the attraction for some markets - because it
closes the gap in performance between
RAM SSDs and flash
SSDs. And when you can run apps 10x faster than other flash competitors at the
same price - or support 10x bigger data sets than competitors using RAM SSDs -
that create new markets. See also:-
Record Breaking
Storage
Kove snapshots financial markets 12x faster |
Editor:- October 14, 2011 - STAC (a specialist in
testing low latency platforms used in financial markets) has released
audited benchmarks for Kove's
XPD2 - a
RAM SSD - in a setup
configured with InfiniBand
adapters.
This solution stack set several new official records. For
example, the Market Snapshot benchmark was over 12x the previous best
published speed. See also:-
the fastest SSDs,
record breaking
storage |
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| the Problem with
Write IOPS in flash SSDs |
Random "write IOPS"
in many of the fastest
flash SSDs are now similar to "read IOPS" - implying a
performance symmetry which was once believed to be impossible.
So why
are flash SSD IOPS such a poor predictor of application performance? And why
are users still buying
RAM SSDs which cost an
order of magnitude more than SLC? (let alone
MLC) - even
when the IOPS specs look superficially similar?
This article
tells you why the specs got faster - but the applications didn't. |
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And why competing SSDs with
apparently identical benchmark results can perform completely differently.
...read the
article | | | |
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