as ssds or solid-state drives have
evolved
some of the components have become much
more sophisticated
like the controllers on board that have
gone from rudimentary single core
affairs to
multi-core processors with huge amounts
of ram
and complex algorithms built into their
firmware but
the foundation of nearly every ssd
the nand flash memory that actually
stores your data
has actually gotten progressively worse
in some big ways we went from high
speed and super reliable slc flash
which draws only a single one or zero to
each cell
to dual air mlc to triple layer tlc
to finally today this
is the intel 660p the first consumer ssd
with qlc flash which is notable for its
ability to store
4 bits per cell that means 16
separate voltage levels this gives it
fantastic
affordability especially for an nvme ssd
but my mama always told me if it seems
too good to be true
it probably is so let's take a look at
the pros
and cons
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let's start with the bad stuff first up
is that qlc nand has lower endurance
than tlc which means that all other
things being equal
it doesn't last for as long why
well i'm glad you asked because it's
science time
with pictures this is really important
every time a cell gets written to a
voltage pulse is sent through the
control gate which creates an electric
field
which agitates the electrons causing
them to move through the silicon dioxide
layer towards the floating gate
that their layer wears out a tiny bit
every time a program erase command is
sent to the cell
causing some of these shifting electrons
to get
stuck inside of it which means that it
will have
and more importantly continue
accumulating a progressively
more negative charge now this gets
compensated for
by applying ever slightly higher
positive voltage to the cell
to get it to the desired voltage state
now at some point
the voltage levels start bordering those
required by the adjacent states to the
point where it takes
too long to distinguish what's what and
that block will get taken out behind the
barn
old yeller style now the fewer layers
that there are
inside of a cell the wider the spare
voltage is
in between the states so then as you can
imagine
with qlc the point of cannot deal with
this anymore gg cell
comes sooner than with tlc and
especially mlc
or slc much sooner
second bad stuffs qlc is also slower
which like wait a second why why is that
denser processor transistors are better
increasing the aerial density of a hard
drive platter is better
shouldn't more data density in nand
flash be better
unfortunately no it's actually the
opposite
you see with hard drives cramming more
data
into the same surface area increases the
read
and the write speeds because the platter
rotates
at a constant speed usually 5400 to 7200
rpm or so
which means that the more densely the
bits are packed
the more of them pass under the head in
a given amount of time
more bits is more data so denser in this
case
is better that is as long as you can
keep your read error rate under control
but that's a whole separate discussion
back to ssds for now when an ssd cell is
accessed
a distinction between the multiple
voltage levels
needs to be made and the number of
states
that you need to sift through goes up
exponentially with the number of layers
that it holds
so the more layers the more states and
the longer it takes to get a reading
for example it takes 25 microseconds to
read for
slc 50 for mlc 75 for tlc
and 100 for qlc and it's the same story
with rights
except the performance drop-off is even
worse
finally big problem number three
here we're getting into this drive
specifically the
advertised write speeds are kind of
hacks
because there isn't really a right way
to do it you see
this drive treats part of its qlc cells
as slc flash as a kind of cache
so remember slc that's the fastest kind
and the exact size of this cache scales
depending on how much you've filled up
your drive
so so then by this point in the video
you're probably thinking
all right well then qlc has a severe
case of the no buenos
and this here 660p drive is a nomi gusta
but uh no
everything that we just said was in
theory now it's time for
a little bit of reality so to put those
hacks right numbers to the test and see
just how bad the whole
slc cache with slower qlcnn behind it
deal affects things we hit this drive
with a full range right through hd tune
pro
and guess what halfway through the
performance
plummeted to below that of a hard drive
and then stayed there for consecutive
runs after a
brief spike on each run but then
we observed this behavior only after
more than
half of the drive's capacity was written
to something that was supposed to happen
earlier on in the test which got us
thinking maybe the caching algorithm was
actually
working in the background shuffling data
over to the slower qlc cells
so we tried running the cache flusher
utility from intel's ssd toolbox
during the test and observed its
progress bar actually going
backwards while our benchmark was
showing throughput spikes
when we ran the cash flush after doing a
full capacity right
it took about 20 minutes total and then
restored the drive to its full write
speed
now when we tested intel's own higher
tier and noticeably more expensive
760p we also saw right performance drop
during the first run after it ran out of
cash and dropped further on consecutive
runs
but as you'd expect it remained much
faster in the same scenarios than its
lesser sibling
okay so that's synthetic tests but what
about a real life
but edge case let's copy a 250 gig steam
folder
over to the 660p there we go
same thing it starts off fast then it
dips dips dips and
plummets like occ's stock before their
bankruptcy in 2013.
when compared to a similarly priced 840
evo
that thing managed to maintain a steady
485 megabytes per second
and completed the copy in only eight
minutes and 45 seconds
this thing took a staggering 23 and a
half minutes
remember when i said qlc is slow without
a cache
well behold the worst case scenario
slower than a hard drive
meanwhile the tlc equipped 760p after
starting off strong
dropped to a more stable 560 megabytes
per second
and then maintained that completing the
same transfer in 8 minutes and 20
seconds
now let's get more realistic all of the
theory crafting we've done so far
failed to have any tangible effect on
our other tests
game load times were about the same
between all three drives
and when we ran the pc mark 8 storage
subsystem bench
we had to double check that we didn't
accidentally test the same drive twice
so a typical ltt video takes up about 24
gigs for
seven and a half minutes which takes
about 20 minutes to render
so just over one gigabyte per second of
right
so we wouldn't have any performance drop
unless we were filling up our 660p to
the brim leaving
no room for slc caching which by the way
we don't recommend filling up any ssd
also keep in mind that during normal
usage your ssd
is idle the vast majority of the time
so that cash flush algorithm will have
all the time it needs to quietly restore
your ssd in the background
so now that we've covered the worst case
for qlc performance
and how the slc cache can give it boosts
of higher performance
let's talk longevity when compared to
intel's own tlc
760p drive this thing is rated for one
third the longevity but
looking closer this actually assumes
about a hundred gigabytes of writes per
day
that is basically equivalent to
rendering out a couple of ltt
videos and installing doom on your
computer
every day it's not a realistic use case
for the average consumer
so to sum things up if you were to
install a 660p ssd in your system
unless you're using it for something
that it wasn't designed for
like as a cache for your nas or your
hard drive
all things considered you would save a
few bucks since
this is intel we're talking about
they're not generally known for their
aggressive pricing
this thing out competes other nvme
drives on price
and you'd probably never notice that
you're running qlc
because of the slc cash and you'd have a
five year warranty to give you peace of
mind
unless of course the drive hits its
total drive rights limit
before that five years runs out so
the enthusiast in me has some serious
misgivings about this move
for the nan storage industry but
this product wasn't made for me that's
optane
and i haven't seen anything about this
drive to suggest that it won't do a good
job
for its intended audience thanks to the
firmware and controller trickery that we
alluded to
earlier i guess that's okay
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