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Review: Samsung 840 TLC NAND endurance |
It was my intention to publish this article a couple of
weeks ago, but even the best laid plans can occasionally suffer from unforeseen
circumstances. My plan was to publish this first in a series of articles, which
will examine the predicted write endurance of TLC NAND. As it happened, I was
forced to abandon the tests for a period of nearly two weeks when the heating
system failed in the house during a very cold period of weather. I was still
using the PC occasionally, but not enough to claim that my usage during that
period would be typical.
A new heating system was installed, which entailed a lot of
cleanup work, and while we were waiting on the new heating system, the PC test
suite was just too darn cold to sit in for any length of time. Anyway, enough
of that and back to the article.
Until the Samsung 840 SSD was launched, TLC NAND wasn't
considered to have enough write endurance to be used in an SSD. However, TLC
NAND is not new, and has been used for some time in USB flash drives, where
high writing endurance is not required. A system drive is another matter
altogether, where writes to the drive will increase dramatically compared to writing
the occasional backup data to a USB flash drive. A system drive in a consumer
PC will have to endure somewhere between 5GB to perhaps 20GB of writes per day
depending on usage.
Clearly if TLC NAND is going to be used in a consumer SSD,
it has to have enough write endurance to last the useful lifetime of the SSD,
and this is what this article is all about. Let's first have a quick refresher
on the differences between MLC and TLC NAND.
The NAND MLC vs TLC
MLC NAND, as we know, stores 2 bits of data per cell, meaning
that each cell has four possible states, all of which require a different
voltage to access one of these states. The four possible states for MLC NAND
are as follows.
- 00 (high voltage)
- 01 (medium high voltage)
- 10 (medium low voltage)
- 11 (low voltage)
TLC NAND can store 3 bits of data per cell, and from this
you might expect TLC NAND to have six possible states, but this isn't so. TLC
NAND has eight possible states, and requires eight different voltages to
access, erase, and program the NAND, which makes TLC much more difficult to
program. The eight possible states for TLC NAND are as follows.
- 000 (highest voltage)
- 001 (high voltage)
- 010 (medium high voltage)
- 100 (high medium voltage)
- 011 (low medium voltage)
- 101 (medium low voltage)
- 110 (low voltage)
- 111 (lowest voltage)
With all these different states and voltages required, not
only is TLC more complex, it is also less durable, and it takes longer to erase
the NAND as well.
The NAND used in the Samsung 840 series is Samsung's own SSD
grade 21nm toggle DDR2 TLC NAND, with 400Mbps bandwidth per die.
Samsung's TLC NAND is reckoned to have a write endurance of between
1000 PE/C (program/erase cycles) and 1500 PE/C. When compared to MLC's NAND
write endurance of typically 3000 PE/C, it appears that TLC NAND is a lot less
durable. To enable TLC NAND to last, the SSD controller and firmware must be
optimised to have extremely low write amplification, and thereby make the very
most of the limited write cycles of TLC NAND.
Let's head to the next page where I'll explain the
testing methodology and show the results....
Test machine
For this article I will be using a computer with the
following configuration:
Hardware:
- Motherboard: Asus P8Z77 V Deluxe (Intel Z77 chipset)
- Processor: Intel 3rd generation Core i7 3770K
- RAM: 16GB Samsung Green DDR3 1600MHz (dual channel)
- GFX: Onboard Intel HD 4000
- Sound: Onboard Realtek ALC898 HD audio controller
- Hard disk OS: OCZ Vector 256GB SSD
- Hard disk storage: 1X 500GB Samsung Spinpoint F3, and 1X 1TB
Samsung Spinpoint F1. - Case: Antec Performance One P280
- PSU: Antec True Power modular 550W
- Display: Dell UltraSharp U2412M 24” widescreen IPS LCD (HDCP
compliant) - Operating System: Windows 8 Professional 64bit
The Samsung 840 SSD was connected to the Intel native SATA 6Gbps
(port 0) on the Z77 motherboard of our review PC and all tests on the drive were
carried out with the drive connected to this port.
AHCI mode was also selected for all drives in the UEFI of
our test PC, and all tests were carried out in this mode. The SATA 6Gbps drivers
used on our review PC were the Intel Rapid Storage Technology (RST) Version
11.6.0.1030.
For the sake of consistency, all CPU power saving states
were disabled in the system UEFI.
Test methodology
The methodology used in this article is a very simple one. I
installed the Samsung 840 SSD as my system drive and I used Acronis True Image
Home 2013 to clone my normal everyday Windows 8 Pro installation, including all
the applications I use for everyday computing. My normal everyday Windows 8 Pro
installation only requires approximately 54GB of space, and this may not be a
typical install. Keep in mind that the less free space there is on an SSD, then
typically the more write amplification will increase, simply because there is
less available free NAND blocks for the controller to work with. With this in
mind, I decided to add a good amount of static data to the Samsung 840 before I
started testing the write endurance of the TLC NAND.
The Samsung 840 250GB SSD has approximately 233GB available
to the user after the SSD is formatted with an MBR NTFS partition. The added
static data brought the used space to approximately 199GB, leaving
approximately 33.2GB of space free on the Samsung 840 SSD (see the screenshot
below), or 13.8% of the drive free.
Used and free space available on the Samsung 840
Once I had the drive filled with data I was ready to begin
the tests. I would just use the PC normally, and log the amount of data that
was being written to the drive on a daily basis. I had planned on doing this for
a period of one month, but as I explained on the previous page, this plan was
abandoned due to unforeseen circumstances. So instead of a one month period,
the test cycle was extended to 46 days.
Whilst there is some fairly complex maths available to
compute NAND wear, I will instead use a much simpler method, which all users
will be able to use with the minimum of effort. If you have purchased a Samsung
840, then you are able to make full use of the Samsung Magician
software, which is specifically designed for Samsung SSDs.
The homepage of the Magician software will give an overview
of the SSD, including space used, free space available, and more importantly
for this article, an overview of the amount of data that has been written to
the Samsung 840 SSD.
I will also use S.M.A.R.T. data to show the MWI (media wear
indicator) to give an approximate value of how fast the NAND is being worn out.
When I
reviewed the Samsung 840, the SSD took a hammering, and this accounted for
4.1TB of writes during the review period (see the screenshot below). At that
time I also had the opportunity to gather some data on how fast the NAND was
being burned. From 4.1TB of data written, and keeping in mind that this data
was mostly written on an empty drive, I was able to see that the MWI had
dropped by two percentage points from 100% to 98%.
Total writes before beginning this article stood at 4.1TB (March the 6th 2013).
Now let's look at the total amount of writes that the
Samsung has endured over the last 46 days whilst used as a system drive in a
normal consumer environment.
Total writes 46 days into our write endurance tests (21st of April 2013).
According to the Samsung Magician software, over the last 46
days the Samsung 840 has been subjected to 500GB of writes. The actual figure
was 564.21 Gigabytes. From this figure I can state that the Samsung 840 has
endured an average of 12.265 Gigabytes of writes per day.
Now let's take a look at the media wear indicator from
S.M.A.R.T.
MWI from S.M.A.R.T.
We can see that the MWI is still at 98%, and of course as I
expected it has not gone down a single percentage point during the 46 days
simply because not enough data has been written to the Samsung 840 during this
period.
So what does this tell us about the endurance of the TLC
NAND onboard the Samsung 840?
Well at the moment, not a great deal. In this first article
I'm only setting the bar, so we have some data to work with over the follow up
articles. The data does allow me to make a very early prediction though.
At this stage, we can guess that if we go on to experience a
linear (or roughly linear) rate of decline, then the rate of wear out is greater
than (4.6/3) 1.53 Terabytes and less than (4.1/2) 2.05 Terabytes per percentage
point change in MWI
Whilst I can't assume anything, if the MWI drops by one
percentage point for every 1.53 to 2.05 Terabytes of data written, and my daily
writes don't increase by a large amount, then I can predict the following.
- 12.265 Gigabytes per day for one year equals 4474.88
Gigabytes per year, or 4.47 Terabytes. - MWI expected percentage drop per year would be in the
range of approximately 2.2% to 2.9%.
If MWI values prove to be accurate, and I continue to write
somewhere in the region of 12 Gigabytes per day, then the TLC NAND onboard the
Samsung 840 could potentially last from 35 to 45 years, which I think you will
agree is no cause for concern. It is however prudent to mention that wear to
the NAND may not be linear, therefore this is only a very early prediction.
I intend to continue these tests until I've been running the
Samsung 840, as my main system drive, for a period of one year, and I will make
quarterly reports on how the NAND is behaving. So look out for the follow up
articles.
So far it would appear that the early worries which
circulated around the Internet about using TLC NAND in an SSD are at least, for
the moment, unfounded.
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