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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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!!