SSD - MLC vs SLC

I know I know... this has probably been asked many times before... But I'd like to just focus mainly on SLC's weaknesses..

Here is quote from Bit-tech...

"Unlike with mechanical or MLC drives where data can be stored in multiple states, the SLC memory only modulates between written and unwritten. This means that once the drive has written to each cell, rewriting to them means the drive must first set the cell from written, to unwritten and then back to written in accordance with the new data, doubling the write times the second time the drive needs to write to that particular cell."

But here... TheTechReport states.. "However, that drive [X25-M] is going to run out of write-erase cycles much quicker than the X25-E, which should last ten times longer thanks to the higher endurance of SLC memory."

And here Tom's Hardware is saying...

"Our benchmark cycle alternated traditional throughput and I/O benchmarks three times, and added three more throughput test runs to see whether or not the SSDs are capable of returning from degraded throughput levels to the sequential performance levels you actually paid for. As expected, all SSDs showed a performance decrease, but only the two products based on MLC flash exhibited significant performance drops. The impact on I/O performance is typically small and acceptable, while throughput on the two MLC flash SSDs by Intel and Samsung suffered quite a bit [As opposed to the Intel X25-E (SLC)]."

Throughout all three reviews, their opinions on SLC vs MLC differ drastically.
Bit-tech are the only one against SLC however the other two favor SLC.

Which really is the better? (Money aside of course) xD

SLC is better in almost every way, but it is very limited capacity wise and expensive. The biggest performance benefit is that you can read + write at the same time a few times faster than a comparable MLC SSD (however you will not be doing much of this on a normal PC). Of course the endurance of SLC is 10x more than MLC.

"Unlike with mechanical or MLC drives where data can be stored in multiple states, the SLC memory only modulates between written and unwritten. This means that once the drive has written to each cell, rewriting to them means the drive must first set the cell from written, to unwritten and then back to written in accordance with the new data, doubling the write times the second time the drive needs to write to that particular cell."
This is partially false, and partially misrepresented.

Both SLC and MLC SSDs have to erase a page before it can be re-written. SLC (single level cell) flash has two states, 1 and 0, and thereby 1 bit per cell. MLC has multiple, normally 2 or 3 bits pr cell.
Like one herz says, the main difference is SLC is faster, has higher endurance, but comes in lower capacities and costs more pr GB (normally 2,5-3x more).
SLC has a great advantage at write speeds, since it only has two bit states and a lower charge delta, and thereby is faster to program.
MLC has higher charge delta to be able to accurately distinguish bit states, and takes longer to set an accurate charge.
For reads, sequential performance is about the same, but responsetime is a bit lower for SLC.

Not sure what Bit-tech are talking about. :shrug:

SLC nand programs typically 2x - 3x faster than MLC with some minor benefits for both read and erase speed.

Micron’s enterprise 34nm process nand can currently achieve 30,000 write/ erase cycles with MLC and 300,000 write/ erase cycles with SLC.

(I think I read somewhere that the Gen 2 X25-E will shift to 34nm MLC nand.)

Whilst the SLC write/ erase cycles are much better that MLC the added durability of SLC based ssd’s is also boosted by a much higher percentage of spare blocks. These spare blocks are not seen as available capacity because they are used for wear levelling. The added cost of SLC is therefore higher manufacturing costs plus more spare blocks.

SLC does not degrade like MLC but it does degrade. (One_Hertz will know more).

i'll just add here a few things:

Quote:

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the SLC memory only modulates between written and unwritten.

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for one, an SLC usually capable of x10 writes vs the MLC (~100k vs ~10k), so, even if the drive is using 2 cycles per write, it still has much longer potential life.
an SSD erases a full block before rewriting to it, it doesn't erase single cells,
it cannot of course just paste the data, as some cells would still contain the eldest one.
it must then erase a full block and rewrite it and u have write amplification added to it.
it doesn't seem to work that way as bit-tech described, u should read the SSD relapse at AT to figure it.
erasing each cell separately, doesn't seem right, it isn't DRAM.

as for TR, these drives aren't exactly replaced every month, or even year, they're quite resilient, of course depending on the controller and quality of the product.
again, have a look at AT article, he got quite in dept explanations at that,
(havn't had the time opportunity yet to unfold this technology entirely).

it's really not about which drive is better, it's more about what u need.
SLC's are generally faster, more robust, and so expensive.
if u don't mind losing a bit of speed, u shouldn't be losing any from using any decent MLC.

SLC basically, are less for mainstream users and more for demanding work load enterprise use, where drive robustness is really important as it's life time, price is less worrying and speed is always a benefit.

Quote:

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Originally Posted by audienceofone

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SLC nand programs typically 2x - 3x faster than MLC with some minor benefits for both read and erase speed.
...
Whilst the SLC write/ erase cycles are much better that MLC the added durability of SLC based ssd’s is also boosted by a much higher percentage of spare blocks. These spare blocks are not seen as available capacity because they are used for wear levelling. The added cost of SLC is therefore higher manufacturing costs plus more spare blocks.
...

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SLC is faster in write, but not 2x or 3x as fast.. in reality its closer to 1.5x

Well, the second statement is applicable for the Intel X-25E drives. All SSDs use overprovisioning (user capacity is less than NAND capacity) for wear leveling, IOPS/write optimization, and for replacing blocks that go bad.
Typically this is in the 5%-8% range for consumer SSDs.
The Intel, being an enterprise centric product, uses 20% spare blocks to maintain consistent performance.
This is only settable from the factory firmware initialization- I have set/tested drives with up to 50% spare block area...

Quote:

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Originally Posted by onex

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it's really not about which drive is better, it's more about what u need.
SLC's are generally faster, more robust, and so expensive.
if u don't mind losing a bit of speed, u shouldn't be losing any from using any decent MLC.
...
SLC basically, are less for mainstream users and more for demanding work load enterprise use, where drive robustness is really important as it's life time, price is less worrying and speed is always a benefit.

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Yep, SLC is faster but really only valid in a very small percentage of work loads. MLC is a better solution for most cases. Or even using MLC in SLC mode approaches 80% of the specs of SLC, at 1/3rd the cost. I would imagine this is what Intel is doing on X-25E G2..

Quote:

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Originally Posted by zads

SLC is faster in write, but not 2x or 3x as fast.. in reality its closer to 1.5x

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I got those figures directly from an Intel nand engineer, but regardless SLC is faster. :up:

Quote:

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Originally Posted by onex

i'll just add here a few things:

for one, an SLC usually capable of x10 writes vs the MLC (~100k vs ~10k), so, even if the drive is using 2 cycles per write, it still has much longer potential life.
an SSD erases a full block before rewriting to it, it doesn't erase single cells,
it cannot of course just paste the data, as some cells would still contain the eldest one.
it must then erase a full block and rewrite it and u have write amplification added to it.
it doesn't seem to work that way as bit-tech described, u should read the SSD relapse at AT to figure it.
erasing each cell separately, doesn't seem right, it isn't DRAM.

as for TR, these drives aren't exactly replaced every month, or even year, they're quite resilient, of course depending on the controller and quality of the product.
again, have a look at AT article, he got quite in dept explanations at that,
(havn't had the time opportunity yet to unfold this technology entirely).

it's really not about which drive is better, it's more about what u need.
SLC's are generally faster, more robust, and so expensive.
if u don't mind losing a bit of speed, u shouldn't be losing any from using any decent MLC.

SLC basically, are less for mainstream users and more for demanding work load enterprise use, where drive robustness is really important as it's life time, price is less worrying and speed is always a benefit.

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Well right now I'm debating whether I should go with an Intel X25-E 32GB or a Patriot TorqX 128GB. I know... the Patriot has more space but really the SSD will only be used as a boot drive while everything else will be in a Samsung F3 1TB HDD. I was also considering the X25-M G2 but it's write speeds were way too low for my taste.

Right now my budget for a SSD is 200-350. I don't want to spend to much because SSD's in my opinion are still in their infacy and not worth laying out 1K+. Anyways, SLC looks to be BETTER than MLC if money is not an object. In this case it is but really capacity doesn't seem to matter when you have a large 1TB HDD for DATA. --- Dunno bout the drawbacks going with this method though..

maybe u should indeed try the X25-E, 369$ or the kingston SSDnow SLC 32GB 339$,
it should be about the same as the x25-e, same controller, and slightly cheaper from intel's drive.
if u'r using the drive only as a boot drive, a 32GB one, should be generally, enough on XP,
and even window 7, depends on u'r work flow and of course needs.
Linux, with it's spacing,
see how much u need for the OS, estimate the spacing for all the other apps, and keep some room for the drive's regeneration,
generally, 30 should be well enough...

if u still doubt on the torqx 128 for extra space,
u can always get 2-3 X25-V's and raid them,
people got magnificent results,
u leave some 20% room for controller's free space, and it should be fine from there,
the only deficit is u'r losing TRIM.
i might be confusing or losing some things here, though in general, all 3 options (4 with the kingston), sounds generally fine,
i think anyone should make his choices by his understanding and intuition,
i try to stay away generally from advising any particular SSD as to not harm any company effort,
they all deserve some kudos and should all exist :up:.