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.

Why Does My 80GB Drive Appear as 74.5GB? Understanding Spare Area Tying it All Together: SSD Performance Degradation


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  • Mr Perfect - Tuesday, September 01, 2009 - link

    Probably demand. When I saw that price, I shopped around to see what was going on. Answer? Everyone else seems to be out of stock. Reply
  • Naccah - Tuesday, September 01, 2009 - link

    I've been waiting to get an SSD till Win 7 released hoping that the prices would have stabilized somewhat by that time. The recent price fluctuation is disturbing as well as the availability of the X25 G2. When the G2 first hit Newegg I was surfing the site and could have grabbed one for $230, but like I said I was content to wait. Now I'm having second thoughts! and wondering if I should grab one if the price goes down again. Reply
  • gfody - Tuesday, September 01, 2009 - link

    That doesn't explain the 160gb - it's not even in stock yet. I have been waiting a month for this drive to be in stock and here they more than double the price one day before the ETA date! It's an outrage.. if I'd known the drive was $1000 I would have bought something else.

    Way to screw your customers Newegg
  • araczynski - Tuesday, September 01, 2009 - link

    A) your intro has the familiar smell of tomshardware, you'd do to be without that, its unbecoming.

    B) your final words smell of the typical big corp establishment mentality; bigger, faster, more expensive, consumers want! while if the market is any indication, is completely the opposite of the truth. people want 'good enough' for cheap, as the recent Wired magazine article more or less said. granted, Wired isn't the source for indepth technical reading, but it is a good source sometimes of getting the pulse of things...sometimes, still, more often than anything coming out of the mouths of the big corps.

    C) everything in between A and B is great though :) Please leave the opinions/spins to the PR machines.

    Personally, the cost of these things is still more than i'm willing to pay for, for any speed increase. the idiotic shenanigans of firmwares and features only present after special downloads/phases of the moon make me just blow off the whole technology for a few more years. I'll revisit this in say 2 or 3 years, perhaps the MLC's will finally die off and the SLC's (unless i have the 2 backwards) or something better rolls out with a longer lifespan.
  • Anand Lal Shimpi - Tuesday, September 01, 2009 - link

    A) My intention with the intro was to convey how difficult it was for me to even get to the point where I felt remotely comfortable publishing this article. I don't like posting something that I don't feel is worthy of the readership's reception. My sincere apologies if it came off as arrogant or anything other than an honest expression of how difficult it was to complete. I was simply trying to bring you all behind the scenes and take you into the crazy place that's my mind for a bit :)

    B) I agree that good enough for cheap is important, hence my Indilinx recommendation at the end. But we can't stifle innovation. We need bigger, better, faster (but not necessarily more expensive, thank you Moore's Law) to keep improving. I remember when the P3 hit 1GHz and everyone said we don't need faster CPUs. If we stopped back then we wouldn't have the apps/web we have today since developers can count on a large install base of very fast processors.

    Imagine what happens in another decade when everyone has many-core CPUs in their notebooks...

    Take care,
  • DynacomDave - Tuesday, September 29, 2009 - link

    First - Anand thanks for the good work and the great article.

    I too have an older laptop that has a PATA interface that I'd like to upgrade with an SSD. I contacted Super Talent about their MasterDrive EX2 - IDE/PATA. Their response was; We only use Indilinx controller for SATA drives, like UltraDrive series. We use Phison controller for EX2/IDE drives.

    I want to improve performance not degrade it. I don't know if this will perform like the Indilinx or like the old SSDs. Can anyone help me with this?
  • bji - Tuesday, September 01, 2009 - link

    There are a few more smaller players in the SSD controller game that don't ever show up in these reviews. They are Silicon Motion and Mtron. The reason I am interested in them is because I have a laptop that is PATA only (it's old I know but I love it and I want to extend its life with an SSD), and I am trying to get an SSD that works in it.

    Turns out the Mtron MOBI SSDs are not compatible with this laptop. I have no idea why. So I have put an order into eBay for an SSDFactory SSD and am crossing my fingers that it will work.

    Mtron makes SATA SSD drives so they could be included in these reviews, and I don't know why they are excluded. It would be interesting to see how their controllers stack up. I personally own two Mtron SSD drives (both 32 GB SLC drives) that I tried to get to work in my laptop and failed to - so one is now the system disk in my desktop and it is very fast (at least compared to platter drives, maybe not compared to newer SSDs). The other one I am still trying to find a use for.

    The only Silicon Motion controller drives I have seen are PATA drives so they clearly are a different beast than the SATA drives typically reviewed in these articles. But I would still be interested in seeing the numbers for the Silicon Motion controller just to get an idea of how well they stack up against the other controllers, especially for the 4K random writes tests. The PATA interface ought not to be the limiting factor for that test at least.
  • paesan - Tuesday, September 01, 2009 - link

    I see NewEgg has a Patriot Troqx and a Patriot Torqx M28. What is the difference in the 2 drives. Reply
  • paesan - Tuesday, September 01, 2009 - link

    After reading thru the Patriot forum I found the differences. The M28 has 128MB cache compared to 64MB cache on the non M28. The biggest difference is the M28 uses a Samsung controller instead of the Indilinx controller on the non M28. I wonder why they switched controllers. Reply
  • valnar - Tuesday, September 01, 2009 - link

    It seems to be that using trim would make a "used" SSD faster, no doubt, but is it required? Would it be okay to buy an SSD for a Windows XP box and just set and forget it? Even used and fragmented, it appears to be faster than any hard drive. My second question is longevity. How long would one last compared to a hard drive? Reply

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