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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CList - Tuesday, September 1, 2009 - link
Don't be disgusted at Newegg, be disgusted at the people who are willing to pay the premium price! Newegg is simply playing a reactionary role in the course of natural free-market economics and cannot be blamed. The consumers, on the other hand, are willing participants and are choosing to pay those prices. When no one is left who is willing to pay those prices, Newegg will quickly lower them.Cheers,
CList
gfody - Tuesday, September 1, 2009 - link
I don't understand how consumers have any control over what Newegg is charging for the 160gb that's not even in stock yet.If Newegg wants to get the absolute most anyone is willing to pay for every piece of merchandise they may as well just move to an auction format.
DrLudvig - Tuesday, September 1, 2009 - link
Yeah, if you look at intel's website, http://www.intel.com/cd/channel/reseller/asmo-na/e...">http://www.intel.com/cd/channel/reselle...na/eng/p..., you will se that the R5 includes "3.5" desktop drive bay adapter to 2.5" SSD adapter bracket, screws, installation guide, and warranty documentation.Why on earth Newegg is charging that much more for it i really don't know, here in denmark the R5 retails for about 15 bucks more than the C1.. Which really isn't that bad..
Mr Perfect - Tuesday, September 1, 2009 - link
Whoa. That's it? An adapter kit? With that kind of price difference, I expected it to be the D0 stepping of SSDs or something.Thanks for clearing that up.
NA1NSXR - Monday, August 31, 2009 - link
The reason not being that performance or longevity is not good enough, but because improvements are still coming too quickly, and prices falling fast still. Once the frequency of significant improvements and price drops slow down, I will more seriously consider an SSD. I suppose it depends on how much waiting on the I/O you do though. For me, it is not so much that a Velociraptor is intolerable.bji - Tuesday, September 1, 2009 - link
Perhaps this is what you meant, but you should really clarify. It's still not time for YOU to buy an SSD. SSDs represent an incredible performance improvement that is well worth the money for many people.DragonReborn - Monday, August 31, 2009 - link
say i wanted to go crazy (it happens)...should i get two 80gb intel g2's or the 160gb intel g2? same space...is the RAID 0 performance worth it?i have all my important data backed on a big 2tb drive so the two ssd's (or 1 160gb) will just hold my OS/progs/etc.
thoughts?
kensiko - Monday, August 31, 2009 - link
I would say that in real world usage, you won't notice a huge difference between RAID and not RAID, SSD are already fast enough for the rest of the system. Also, TRIM may not work for now in RAID configuration.Just look at Windows Start up, no difference between Gen2 SSD!
Gc - Monday, August 31, 2009 - link
This is a nice article, but the numbers leave an open question.What is Samsung doing right? Multiprocess/multithread performance?
The article finds Samsung drives performance is low on 2MB reads,
(new 2MB sequential reads not given, assume same as 'used')
used 2MB sequential reads (low rank, 79% of top)
good on 2MB writes:
new 2MB sequential writes (middle rank, 89% of top)
used 2MB sequential writes (2nd place, 91% of top)
and horrible on 4KB random files:
(new 4KB random reads not given, assume same as 'used')
used 4KB random read (bottom ssd ranked, only 36% of top)
new 4KB random write (low rank, only 9% of top)
used 4KB random write (bottom ssd ranked, only 3% of top, < HD)
Yet somehow in the multitasking Productivity test and Gaming test, it was surprisingly competitive:
multitasking productivity (mid-high rank, 88% of top)
gaming (mid-high rank, 95% of top)
The productivity test is described as "four tasks going on at once, searching through Windows contacts, searching through Windows Mail, browsing multiple webpages in IE7 and loading applications". In other words, nearly all READS (except maybe for occasionally writing to disk new items for the browser history or cache).
The gaming test is described as "reading textures and loading level data", again nearly all READS.
Q. Given that the Samsung controller's 2MB read performance and
4KB read performance are both at the bottom of the pack, how
did it come out so high in the read-mostly productivity test
and gaming test?
Does this indicate the Samsung controllers might be better than Indilinx for multiprocess/multithreaded loads?
(The Futuremark pdf indicates Productivity 2 is the only test with 4 simultaneous tasks, and doesn't say whether the browser tabs load concurrently. The Gaming 2 test is multithreaded with up to 16 threads. [The Samsung controller also ranks well on the communications test, but that may be explained: Communications 1 includes encryption and decompression tasks where Samsung's good sequential write performance might shine.])
Since many notebooks/laptops are used primarily for multitasking productivity (students, "office"-work), maybe the Samsung was a reasonable choice for notebook/laptop OEMs. Also, in these uses the cpu and drive are idle much of the time, so the Samsung best rank on idle power looks good. (But inability to upgrade firmware is bad.)
(The article doesn't explain what the load was in the load drive test, though it says the power drops by half if the test is switched to random writes; maybe it was sequential writes for peak power consumption. It would have been helpful to see the power consumption rankings for read-mostly loads.)
Thanks!
rcocchiararo - Monday, August 31, 2009 - link
Your prices are way off, newegg is charging ludicrous ammounts right now :(also, the 128 agility was 269 last week, i was super exited, then it went back to 329, and its now 309.