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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Anand Lal Shimpi - Monday, August 31, 2009 - link
I believe OCZ cut prices to distributors that day, but the retail prices will take time to fall. Once you see X25-M G2s in stock then I'd expect to see the Indilinx drives fall in price. Resellers won't give you a break unless they have to :)Take care,
Anand
bobjones32 - Monday, August 31, 2009 - link
Another great AnandTech article, thanks for the read.Just a head's-up on the 80GB X-25m Gen2 - A day before Newegg finally had them on sale, they bumped their price listing from $230 to $250. They sold at $250 for about 2 hours last Friday, went back out of stock until next week, and bumped the price again from $250 to $280.
So....plain supply vs. demand is driving the price of the G2 roughly $50 higher than it was listed at a week ago. I have a feeling that if you wait a week or two, or shop around a bit, you'll easily find them selling elsewhere for the $230 price they were originally going for.
AbRASiON - Monday, August 31, 2009 - link
Correct, Newegg has gouged the 80gb from 229 to 279 and the 160gb from 449 to 499 :(Stan Zaske - Monday, August 31, 2009 - link
Absolutely first rate article Anand and I thoroughly enjoyed reading it. Get some rest dude! LOLJaramin - Monday, August 31, 2009 - link
I'm wondering, if I were to use a low capacity SSD to install my OS on, but install my programs to a HDD for space reasons, just how much would that spoil the SSD advantage? All OS reads an writes would still be on the SSD, and the paging file would also be there. I'm very curious about the amount of degradation one would see relative to different use routines and apps.Anand Lal Shimpi - Monday, August 31, 2009 - link
Putting all of your apps (especially frequently used ones) off of your SSD would defeat the purpose of an SSD. You'd be missing out on the ultra-fast app launch times.Pick a good SSD and you won't have to worry too much about performance degradation. As long as you don't stick it into a database server :)
Take care,
Anand
swedishchef - Tuesday, September 1, 2009 - link
What if you just put your photoshop cache on a pair of Velociraptors? Would it be the same loss of benefit?I have the same question regarding uncompressed HD video work, where I need write speeds well over the Intel x25-m ( over 240Mb/s). My assumption would be that I could enjoy the fast IO and App. launch of an SSD and increase CPU performance with the SSD while keeping the files on a fast external or internal raid configuration.
Thank you again for a a brilliant Article Anand.
I have been waiting for it for a long time. Yours are the only calm words out on the net.
Grateful Geek /Also professional image creator.
creathir - Monday, August 31, 2009 - link
Great article Anand. I've been waiting for it...My only thoughts are, why can't Intel get their act together with the sequential business? Why can the others handle it, but they can't? To have such an awesome piece of hardware have such a nasty blemish is strange to me, especially on a Gen-2 product.
I suppose there is some technical reason as to why, but it needs to be addressed.
- Creathir
Anand Lal Shimpi - Monday, August 31, 2009 - link
If Intel would only let me do a deep dive on their controller I'd be able to tell you :) There's more I'd like to say but I can't yet unfortunately.Take care,
Anand
shotage - Monday, August 31, 2009 - link
Awesome article!I'm intrigued with the cap on the sequential reads that Intel has on the G2 drives as well. I always thought it was strange to see even on their first gen stuff.
I'm assuming that this cap might be in place to somehow ensure the excellent performance they are giving with random read/writes. All until TRIM finally shows up and you'll have to write up another full on review (which I eagerly await!).
I can't wait to see what 2010 brings to the table. What with the next version of SATA and TRIM just over the horizon, I could finally get the kind of performance out of my PC that I want!!