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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jengeek - Wednesday, September 2, 2009 - link
As of 09-02-09 from Toshiba Direct:80GB = $243
160GB = $473
http://www.toshibadirect.com/td/b2c/adet.to?poid=4...">http://www.toshibadirect.com/td/b2c/adet.to?poid=4...
http://www.toshibadirect.com/td/b2c/adet.to?poid=4...">http://www.toshibadirect.com/td/b2c/adet.to?poid=4...
gfody - Thursday, September 3, 2009 - link
nice thank you, ordered mine from herescrew Newegg! :D
jengeek - Wednesday, September 2, 2009 - link
Both are G2, in stock and ship the next dayBoth are retail box including the installation kit
Best price I've found
ARoyalF - Sunday, September 13, 2009 - link
Thank you posting that!I was going to wait out that awful price hike over at the egg.
You rock
ElderTech - Tuesday, September 1, 2009 - link
It's difficult to imagine the amount of time and effort that went into this article, Anand. Just the clean installs of Win7 took a fair amount of extra effort, let alone the other detailed diagrams and testing involved. From an old technology advocate over many years of working to keep pace with Moore's Law in a variety of research environments, your site provides the most satisfying learning experience of all. A sincere thank you!PS: As for the availability of the G2, it pops in and out of stock at a variety of online retailers, including Newegg, of course, as well as MWave. Both had it available for a short while at $249, Newegg on Friday and MWave today, Monday. However, it's out of stock presently as of midnight, EST 9-1-09 at both, with MWave still at $249 but Newegg going from there to $279 over the weekend and now at an amazing $499! OUCH. Sounds like supply and demand gouging if the price holds when they are next available! There is also some stock available in the distributor channel from small Intel Partners, as I confirmed by calling around the Chicago area. You might give this a try tomorrow. Good luck!
blyndy - Monday, August 31, 2009 - link
You really got performance anxiety because some high-profile people/sites liked your article and linked to it? It's hardly like it got printed in some prestigious science journal and the publishers are waiting on a follow-up.It was just the first time that SDD operation had been detailed in plain english from a reputable website.
Enough of this 'anthology' nonsense, I don't care if it's 1 page or 20, just tell me how some of the new SSDs perform (eg OCZ, Western Digital). You've already detailed how they work so now I want to know which ones do/will support TRIM and some details on the controller. Nothing to get anxious about.
Anand Lal Shimpi - Monday, August 31, 2009 - link
Indeed I did get performance anxiety after the last one, I even got it after the first X25-M. It's not so much the linkage, but the feedback from all of you guys. I received more positive feedback to the last SSD article than any one prior. More than anything I don't want to let you all down and I want to make sure I live up to everyone's expectations.As far as your interests go, all three manufacturers (Indilinx, Intel and Samsung) have confirmed support for TRIM. When? I'd say all three before December.
Take care,
Anand
cacca - Thursday, September 3, 2009 - link
Dear Anand i really thank you for your SSD articles, the improvements in this area seem tangible.Can I ask you to test Fusion-IO & ioXtreme, i am really curious to see how this other approach performs.
I know that isn't a perfect apple to apple comparison but at least we could compare the per $ performance.
Best regards
Ca
vol7ron - Monday, August 31, 2009 - link
Good article.I have a follow-up question regarding your size suggestion.
In more words you say, "get the size you need," but don't these drives perform that much better in a RAIDed system?
The cost per GB isn't that much more if you're looking at getting a 160GB Intel drive, to get the 2x 80GB instead.
SSDs are more reliable than HDs and you have the benefit of more RAM. 2x 32MB for an SSD in RAID0.
Curious to hear your thoughts,
vol7ron
StraightPipe - Tuesday, September 1, 2009 - link
Since RAID cards aren't going to support TRIM commands for a while, I'd stick with a large, single SSD.Anybody ahve any experience running these cards in RAID? I'd love to put some of these in my server, but i'm terrified of lossing data through the complexities of RAID combined with SSD.
I'd love to do a simple RAID1 setup, but it looks like i may be better of waiting too.
In the mean time, these look like a mean machine for an OS disk.