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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nemitech - Monday, August 31, 2009 - link
opps - not ebay - it was NEWEGG.Loki726 - Monday, August 31, 2009 - link
Thanks a ton for including the pidgin compiler benchmarks. I didn't think that HD performance would make much of a difference (linking large builds might be a different story), but it is great to have numbers to back up that intuition. Keep it up.torsteinowich - Monday, August 31, 2009 - link
HiYou write that the Indilinx wiper tool collects a free page list from the OS, then wipes the pages. This sounds like a dangerous operation to me since the OS might allocate some of these blocks after the tool collects the list, but before they are wiped.
Have you received a good explanation for Indilinx about how they ensure file system integrity? As far as i know Windows cannot temporarily switch to read-only mode on an active file system (at least not the system drive). The only way i could see this tool working safely would be by booting off a different media and accessing the file system to be trimmed offline with a tool that correctly identifies the unused pages for the particular file system being used. I could be wrong of course, maybe windows 7 has a system call to temporarily freeze FS writes, but i doubt it.
has407 - Monday, August 31, 2009 - link
It: (1) creates a large temporary file (wiper.dat) which gobbles up all (or most) of the free space; (2) determines the LBA's occupied by that file; (3) tells the SSD to TRIM those LBA's; and then (4) deletes the temporary file (wiper.date).From the OS/filesystem perspective, it's just another app and another file. (A similar technique is used by, e.g., sysinternals Windows SDelete app to zero free space. For Windows you could also probably use the hooks used by defrag utilities to accomplis it, but that would be a lot more work.)
cghebert - Monday, August 31, 2009 - link
Anand,Great article. Once again you have outclassed pretty much every other site out there with the depth of content in this review. You should start marketing t-shirts that say "Everything I learned about SSDs I learned from AnandTech"
I did have a question about gaming benchmarks, since you made this statement:
" but as you'll see later on in my gaming tests the benefits of an SSD really vary depending on the game"
But I never saw any gaming benchmarks. Did I miss something?
nafhan - Monday, August 31, 2009 - link
Just wanted to say awesome review.I've been reading Anandtech since 2000, and while other sites have gone downhill or (apparently) succumbed to pressure from advertisers, you guys have continued to give in depth, critical reviews.
I also appreciate that you do some real analysis instead of just throwing 10 pages of charts online.
Thanks, and keep up the good work!
zysurge - Monday, August 31, 2009 - link
Awesome amazing article. So much information, presented clearly.Question, though? I have an Intel G2 160GB drive coming in the next few days for my Dell D830 laptop, which will be running Windows 7 x64.
Do I set the controller to ATA and use the Intel Matrix driver, or set it to AHCI and use Microsoft's driver? Will either provide an advantage? I realize neither will provide TRIM until Q4, but after the firmware update, both should, right?
Thanks in advance!
ggathagan - Wednesday, September 16, 2009 - link
From page 15 (Early Trim support...):Under Windows 7 that means you have to use a Microsoft made IDE or AHCI driver (you can't install chipset drivers from anyone else).
Mumrik - Monday, August 31, 2009 - link
but I can't live with less than 300GB on that drive, and SSDs in usable sizes still cost more than high end video cards :-(I really hope I'll be able to pick up a 300GB drive for 100-200 bucks in a year or so, but it is probably a bit too optimistic.
Simen1 - Monday, August 31, 2009 - link
This is simply wrong. Ask anyone over 10 years if they think this mathematical statement is true or false. 80 can never equal 74,5.Now, someone claims that 1 GB = 10^9 B and others claim that 1 GB is 2^30 B. Who is really right? What does the G and the B mean? Who defines that?
The answers is easy to find and document. B means Byte. G stands for Giga ans means 10^6, not 2^30. Giga is defined in the international system of units, SI.
No standardization organization have _ever_ defined Giga to be 2^30. But IEC, International Electrotechnical Commission, have defined "Gi" to 2^30. This is supposed to be used for digital storage so people wont be confused by all the misunderstandings around this. Misunderstandings that mainly comes from Microsoft and quite a few other big software vendors. Companies that ignore the mathematical errors in their software when they claim that 80GB = 74,5 GB, and ignore both international standards on how to shorten large numbers.