The SSD Relapse: Understanding and Choosing the Best SSD
by Anand Lal Shimpi on August 30, 2009 12:00 AM EST- Posted in
- Storage
A Wear Leveling Refresher: How Long Will My SSD Last?
As if everything I’ve talked about thus far wasn’t enough to deal with, there’s one more major issue that directly impacts the performance of these drives: wear leveling.
Each MLC NAND cell can be erased ~10,000 times before it stops reliably holding charge. You can switch to SLC flash and up that figure to 100,000, but your cost just went up 2x. For these drives to succeed in the consumer space and do it quickly, it must be using MLC flash.
SLC (left) vs. MLC (right) flash
Ten thousand erase/write cycles isn’t much, yet SSD makers are guaranteeing their drives for anywhere from 1 - 10 years. On top of that, SSD makers across the board are calling their drives more reliable than conventional hard drives.
The only way any of this is possible is by some clever algorithms and banking on the fact that desktop users don’t do a whole lot of writing to their drives.
Think about your primary hard drive. How often do you fill it to capacity, erase and start over again? Intel estimates that even if you wrote 20GB of data to your drive per day, its X25-M would be able to last you at least 5 years. Realistically, that’s a value far higher than you’ll use consistently.
My personal desktop saw about 100GB worth of writes (whether from the OS or elsewhere) to my SSD and my data drive over the past 14 days. That’s a bit over 7GB per day of writes. Let’s do some basic math:
| My SSD | |
| NAND Flash Capacity | 256 GB |
| Formatted Capacity in the OS | 238.15 GB |
| Available Space After OS and Apps | 185.55 GB |
| Spare Area | 17.85 GB |
If I never install another application and just go about my business, my drive has 203.4GB of space to spread out those 7GB of writes per day. That means in roughly 29 days my SSD, if it wear levels perfectly, I will have written to every single available flash block on my drive. Tack on another 7 days if the drive is smart enough to move my static data around to wear level even more properly. So we’re at approximately 36 days before I exhaust one out of my ~10,000 write cycles. Multiply that out and it would take 360,000 days of using my machine the way I have been for the past two weeks for all of my NAND to wear out; once again, assuming perfect wear leveling. That’s 986 years. Your NAND flash cells will actually lose their charge well before that time comes, in about 10 years.
This assumes a perfectly wear leveled drive, but as you can already guess - that’s not exactly possible.
Write amplification ensures that while my OS may be writing 7GB per day to my drive, the drive itself is writing more than 7GB to its flash. Remember, writing to a full block will require a read-modify-write. Worst case scenario, I go to write 4KB and my SSD controller has to read 512KB, modify 4KB, write 512KB and erase a whole block. While I should’ve only taken up one write cycle for 2048 MLC NAND flash cells, I will have instead knocked off a single write cycle for 262,144 cells.
You can optimize strictly for wear leveling, but that comes at the expense of performance.
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Jedi2155 - Monday, August 31, 2009 - link
Anandtech has always been known for its in-depth analysis, you're just looking for a simple review list. I much prefer these detailed articles than just hearing the list of performance and simple recommendations that most people can write if provided the proper hardware.I love how Anand always writes excellent, very well detailed articles that are still SIMPLE to understand. A number of other sites may offer some similar levels of detailed but are sometimes a bit too difficult to comprehend without a background in the same field.
KommisMar - Sunday, April 4, 2010 - link
Anand,I read your long series of articles on SSDs today, and just wanted to say thanks for writing the most informative and interesting series of tech articles I've read in years. I've been avoiding SSDs because my first experience with one was horrible. The sustained transfer rates were no better than a traditional hard drive, and the system halting for several seconds on each random write operation was too much for me to stand.
I was so sick of the SSD coverage that I was reading on other websites because none of them seemed to answer my biggest question, which was "Which SSD won't bring my system to a screeching halt every time it needs to write a little data?"
Thanks for answering that question and explaining what to look for and what to avoid. It sounds like it's a good time for me to give SSDs another shot.
jamesy - Thursday, April 22, 2010 - link
That about sums it up: disappointment. Although this was a top-caliber SSD article, like i have come to love and expect out of anand, this article didn't make my buying decision any easier as all. In fact, it might have made it more complicated.I understand Intel, Indillinx, and Sandforce are good, but there are so many drives out there, and most suck. This article was amazing by most standards but the headline should be changed: removing the "Choosing the Best SSD."
Maybe "Choosing the right controller before sorting through a hundred drives" would be an appropriate replacement.
Do i still go with the intel 160 X-25m G2?
Do I get the addon Sata 6g card and get the C300?
Do i save the money, and get an indillinx drive? Is the extra money worth the Intel/C300 drive?
These are the main questions enthusiasts have, and while this article contained a great overview of the market in Q3 2009, SSD Tech has progressed dramatically. Only now, i think, are we getting to the point that we could publish a buying guide and have it last a few months.
I trust Anandtech, i just wish they would flat-out make a buying guide, assign points in different categories (points for sequential read/write, points for random read/write, points for real-life performance or perceived performance, points for reliability, and points for price.). Take all of these points, add em up, and make a table pls.
A few graphs can help, but the 200 included in each article is overwhelming, and does nothing to de-complicate or make me confindent in my purchase.
It's great to know how drives score, how they perform. But it's even important to know that you bought the right drive.
mudslinger - Monday, June 28, 2010 - link
This article is dated 8/30/2009!!!!It’s ancient history
Since then newer, faster SSD’s have been introduced to the market.
And their firmware have all been updated to address known past issues.
This article is completely irrelevant and should be taken down or updated.
I’m constantly amazed at how old trash info is left lingering about the web for search engines like Google to find. Just because Google lists an article doesn’t make it legit.
cklein - Monday, July 12, 2010 - link
Actually I am trying to find a reason to use SSD.1. Server Environment
No matter it's a webserver or a SQL server, I don't see a way we can use SSD. My SERVER comes with plenty of RAM 32G or 64G. The OS/start a little bit slow, but it's OK, since it never stop after it's started. And everything is loaded into RAM, no page file usage is needed. So, really why do we need SSD here to boost the OS start time or application start time?
For SQL server database, that's even worse. Let's say I have a 10GB SQL server database, and it grows to 50GB after a year. Can you image how many random writes, updates between the process? I am not quite sure this will wear off the SSD really quick.
2. For desktop / laptop, I can probably say, install the OS and applications on SSD, and leave everything on other drives? And even create page file on other drives? As I feel SSD is only good for readonly access. For frequent write, it may wear off pretty quick? I am doing development, I am not even sure I should save source code on SSD, as it compiles, builds, I am sure it writes a lot to SSD.
So over all, I don't see it fits in Server environment, but for desktop/laptop, maybe? even so, it's limited?
someone correct me if I am wrong?
TCQU - Thursday, July 29, 2010 - link
Hi peopleI'm up for getting a new Macbook pro with ssd.
BUT i heard somthing about, that the 128gb ssd, for apples machines, was made by samsung. I was ready to buy it, but now that i've heard that first of all "apples" ssd's is much slower that they others on the marked. Then i read this. So now i'm really confused.
What shoud i do?
buy apples macbook pro with 128gb ssd
or should i buy it without and replace it with an other ssd? thoughts? plzz help me out
thanks
Thomas
TCQU - Thursday, July 29, 2010 - link
Hi peopleI'm up for getting a new Macbook pro with ssd.
BUT i heard somthing about, that the 128gb ssd, for apples machines, was made by samsung. I was ready to buy it, but now that i've heard that first of all "apples" ssd's is much slower that they others on the marked. Then i read this. So now i'm really confused.
What shoud i do?
buy apples macbook pro with 128gb ssd
or should i buy it without and replace it with an other ssd? thoughts? plzz help me out
thanks
Thomas
marraco - Friday, August 13, 2010 - link
Why Sandforce controllers are ignored?I’m extremely disappointed with the compiler benchmark. Please test .NET (With lot of classes source files and dependencies). It seems like nothing speeds up compilation. No CPU, no memory, no SSD. It makes nonsense.
sylvm - Thursday, October 7, 2010 - link
I found this article of very good quality.I was looking for a similar article about express card SSDs using PCIe port, but found nothing about their performance for rewrite.
The best I found is this review http://www.pro-clockers.com/storage/192-wintec-fil... saying nothing about it.
Expresscard SSDs would allow good speed improvement/price compromise : buying a relatively small and cheap one for OS and softwares, while keeping the HDD for data.
Has anyone some info about it ?
Best regards,
Sylvain
paulgj - Saturday, October 9, 2010 - link
Well I was curious about the flash in my Agility 60GB so I opened it up and noted a different Intel part number - mine consisted of 8 x 29F64G08CAMDB chips whereas the pic above shows the 29F64G08FAMCI. I wonder what the difference is?-Paul