The SSD Relapse: Understanding and Choosing the Best SSD
by Anand Lal Shimpi on August 30, 2009 12:00 AM EST- Posted in
- Storage
The Instruction That Changes (almost) Everything: TRIM
TRIM is an interesting command. It lets the SSD prioritize blocks for cleaning. In the example I used before, a block is cleaned only when the drive runs out of places to write things and has to dip into its spare area. With TRIM, if you delete a file, the OS sends a TRIM command to the drive along with the associated LBAs that are no longer needed. The TRIM command tells the drive that it can schedule those blocks for cleaning and add them to the pool of replacement blocks.
A used SSD will only have its spare area to use as a scratch pad for moving data around; on most consumer drives that’s around 7%. Take a look at this graph from a study IBM did on SSD performance:
Write Amplification vs. Spare Area, courtesy of IBM Zurich Research Laboratory
Note how dramatically write amplification goes down when you increase the percentage of spare area the drive has. In order to get down to a write amplification factor of 1 our spare area needs to be somewhere in the 10 - 30% range, depending on how much of the data on our drive is static.
Remember our pool of replacement blocks? This graph actually assumes that we have multiple pools of replacement blocks. One for frequently changing data (e.g. file tables, pagefile, other random writes) and one for static data (e.g. installed applications, data). If the SSD controller only implements a single pool of replacement blocks, the spare area requirements are much higher:
Write Amplification vs. Spare Area, courtesy of IBM Zurich Research Laboratory
We’re looking at a minimum of 30% spare area for this simpler algorithm. Some models don’t even drop down to 1.0x write amplification.
But remember, today’s consumer drives only ship with roughly 6 - 7% spare area on them. That’s under the 10% minimum even from our more sophisticated controller example. By comparison, the enterprise SSDs like Intel’s X25-E ship with more spare area - in this case 20%.
What TRIM does is help give well architected controllers like that in the X25-M more spare area. Space you’re not using on the drive, space that has been TRIMed, can now be used in the pool of replacement blocks. And as IBM’s study shows, that can go a long way to improving performance depending on your workload.
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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.