The Performance Degradation Problem

When Intel first released the X25-M, Allyn Malventano discovered a nasty corner case where the drive would no longer be able to run at its full potential. You basically had to hammer on the drive with tons of random writes for at least 20 minutes, but eventually the drive would be stuck at a point of no return. Performance would remain low until you secure erased the drive.

Although it shouldn't appear in real world use, the worry was that over time a similar set of conditions could align resulting in the X25-M performing slower than it should. Intel, having had much experience with similar types of problems (e.g. FDIV, Pentium III 1.13GHz), immediately began working on a fix and released the fix a couple of months after launch. The fix was nondestructive although you saw much better performance if you secure erased your drive first.

SandForce has a similar problem and I have you all and bit-tech to thank for pointing it out. In bit-tech's SandForce SSD reviews they test TRIM functionality by filling a drive with actual data (from a 500GB source including a Windows install, pictures, movies, documents, etc...). The drive is then TRIMed, and performance is measured.

If you look at bit-tech's charts you'll notice that after going through this process, the SandForce drives no longer recover their performance after TRIM. They are stuck in a lower performance state making the drives much slower when writing incompressible data.

You can actually duplicate the bit-tech results without going through all of that trouble. All you need to do is write incompressible data to all pages of a SandForce drive (user accessible LBAs + spare area), TRIM the drive and then measure performance. You'll get virtually the same results as bit-tech:

AS-SSD Incompressible Write Speed
  Clean Performance Dirty (All Blocks + Spare Area Filled) After TRIM
SandForce SF-1200 (120GB) 131.7MB/s 70.3MB/s 71MB/s

The question is why.

I spoke with SandForce about the issue late last year. To understand the cause we need to remember how SSDs work. When you go to write to an SSD, the controller must first determine where to write. When a drive is completely empty, this decision is pretty easy to make. When a drive is not completely full to the end user but all NAND pages are occupied (e.g. in a very well used state), the controller must first supply a clean/empty block for you to write to.

When you fill a SF drive with incompressible data, you're filling all user addressable LBAs as well as all of the drive's spare area. When the SF controller gets a request to overwrite one of these LBAs the drive has to first clean a block and then write to it. It's the block recycling path that causes the aforementioned problem.

In the SF-1200 SandForce can only clean/recycle blocks at a rate of around 80MB/s. Typically this isn't an issue because you won't be in a situation where you're writing to a completely full drive (all user LBAs + spare area occupied with incompressible data). However if you do create an environment where all blocks have data in them (which can happen over time) and then attempt to write incompressible data, the SF-1200 will be limited by its block recycling path.

So why doesn't TRIMing the entire drive restore performance?

Remember what TRIM does. The TRIM command simply tells the controller what LBAs are no longer needed by the OS. It doesn't physically remove data from the SSD, it just tells the controller that it can remove the aforementioned data at its own convenience and in accordance with its own algorithms.

The best drives clean dirty blocks as late as possible without impacting performance. Aggressive garbage collection only increases write amplification and wear on the NAND, which we've already established SandForce doesn't really do. Pair a conservative garbage collection/block recycling algorithm with you attempting to write an already full drive with tons of incompressible data and you'll back yourself into a corner where the SF-1200 continues to be bottlenecked by the block recycling path. The only way to restore performance at this point is to secure erase the drive.

This is a real world performance issue on SF-1200 drives. Over time you'll find that when you go to copy a highly compressed file (e.g. H264 video) that your performance will drop to around 80MB/s. However, the rest of your performance will remain as high as always. This issue only impacts data that can't be further compressed/deduped by the SF controller. While SandForce has attempted to alleviate it in the SF-1200, I haven't seen any real improvements with the latest firmware updates. If you're using your SSD primarily to copy and store highly compressed files, you'll want to consider another drive.

Luckily for SandForce, the SF-2500 controller alleviates the problem. Here I'm running the same test as above. Filling all blocks of the Vertex 3 Pro with incompressible data and then measuring sequential write speed. There's a performance drop, but it's no where near as significant as what we saw with the SF-1200:

AS-SSD Incompressible Write Speed
  Clean Performance Dirty (All Blocks + Spare Area Filled) After TRIM
SandForce SF-1200 (120GB) 131.7 MB/s 70.3 MB/s 71 MB/s
SandForce SF-2500 (200GB) 229.5 MB/s 230.0 MB/s 198.2 MB/s

It looks like SandForce has increased the speed of its block recycling engine among other things, resulting in a much more respectable worst case scenario of ~200MB/s.

Verifying the Fix

I was concerned that perhaps SandForce simply optimized for the manner in which AS-SSD and Iometer write incompressible data. In order to verify the results I took a 6.6GB 720p H.264 movie and copied it from an Intel X25-M G2 SSD to one of two SF drives. The first was a SF-1200 based Corsair Force F120, and the second was an OCZ Vertex 3 Pro (SF-2500).

I measured both clean performance as well as performance after I'd filled all blocks on the drive. The results are below:

6.6GB 720p H.264 File Copy (X25-M G2 Source to Destination)
  Clean Performance Dirty (All Blocks + Spare Area Filled) After TRIM
SandForce SF-1200 (120GB) 138.6 MB/s 78.5 MB/s 81.7 MB/s
SandForce SF-2500 (200GB) 157.5 MB/s 158.2 MB/s 157.8 MB/s

As expected the SF-1200 drive drops from 138MB/s down to 81MB/s. The drive is bottlenecked by its block recycling path and performance never goes up beyond 81MB/s.

The SF-2000 however doesn't drop in performance. Brand new performance is at 157MB/s and post-torture it's still at 157MB/s. What's interesting however is that the incompressible file copy performance here is lower than what Iometer and AS-SSD would have you believe. Iometer warns that even its fully random data pattern can be defeated by drives with good data deduplication algorithms. Unless there's another bottleneck at work here, it looks like the SF-2000 is still reducing the data that Iometer is writing to the drive. The AS-SSD comparison actually makes a bit more sense since AS-SSD runs at a queue depth of 32 and this simple file copy is mostly at a queue depth of 1. Higher queue depths will make better use of parallel NAND channels and result in better performance.

Sequential Read/Write Speed AnandTech Storage Bench 2011: Much Heavier
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  • HangFire - Friday, February 18, 2011 - link

    As a software engineer, I can tell you that temp files are used over in-memory storage because the s/w was originally written that way, and no bug report concerning them will ever reach high priority status because it is ranked as a system configuration issue that can be fixed by the user.

    In other words, inertia of the "good enough" file writing code (written when RAM was sparse) will prevent software from being re-written to more optimal in-memory usage. The long backlog of truly important bugs taking precedence insures that.

    You have a good point about ramdisks competing with disk caching. What is optimal depends on your application load, and to some extent your storage subsystem.
    Reply
  • cdillon - Thursday, February 17, 2011 - link

    The idea of moving the page-file to a RAM disk makes my head hurt. That's just retarded. You'd do better to turn off paging entirely, but that's also of questionable benefit because paging isn't really that hard on your SSD.

    Putting the temp directory along with browser caches and other non-critical frequently-written data is not a bad idea as long as you don't over-do it. The only problem with putting the temp drive on a volatile software-based RAM drive is that any software installation you do that requires a reboot with intermediate installer files kept in the temp directory which are expected to be there on the next reboot is going to fail.
    Reply
  • Qapa - Saturday, February 19, 2011 - link

    Hi Anand,

    I second this request :)

    A few changes though:
    - DISABLE page file
    --- no matter if you have SSD or HDD, windows writes to the page file even if only using 10% of RAM), so you decrease writes to disk which does 2 thing: increase life of disk and increase speed of system. possibly both only marginally, but that's what benchs would show;
    - browser caches
    --- for sure this is one of the most wasteful disk writing and it should be more and more a great amount of writes since we are ever more on the web
    - temporary folders
    --- as someone else mentions you could come into problems if you need a install-reboot-finish_install kind of instalation
    --- and I agree, with the sw engineer - if it works it won't get changed, so programs will put stupid stuff to files just because that was the way they did it at some point in time

    I think a 1-2Gb RAM Disk is more than enough for browser and temp files, considering an initial starting RAM size of 4-8Gb of RAM. And yes, I do believe this improves system performance.

    Can you do the benchs?

    Thanks for the site - all reviews - and hope you can add this request as another review.
    Reply
  • shawkie - Thursday, February 17, 2011 - link

    I notice that the Intel SSD 510 has just started to appear on some retailer websites. It looks like it is SATA 6Gbs and comes in 120GB and 250GB versions. Pricing looks pretty high at this point. Reply
  • BansheeX - Thursday, February 17, 2011 - link

    Color me unexcited. SSD is fast and reliable enough for people to want it. The price per GB isn't coming down anywhere near as fast as other technologies. I paid $200 1.5 years ago for an 80GB SSD drive that goes for $180 today. Reply
  • chrysrobyn - Friday, February 18, 2011 - link

    Maybe 80GB for $200 is good enough for you, but I need twice that capacity, and I'm unwilling to pay more than $200. The next generation of SSDs that are coming out between now and May are going to come far closer to that price point for me. Reply
  • seapeople - Friday, February 18, 2011 - link

    The point is that 1.5 years ago the OP purchased a SSD for $2.5/GB which had anywhere from a 2x-30x performance improvement over its predecessor (HD's), and here we are in 2011 reading a review about the next generation SSD which uses smaller, cheaper flash with half the available write-cycle life which is going to sell for... $5/GB and get a 1.2x-3x performance improvement over its predecessor (initial SSD's).

    What's next? A solid state drive that reads and writes at 2,000 GB/s and sells for $10,000 for the 1 TB model? Oh I can't wait for that.
    Reply
  • ABR - Saturday, February 19, 2011 - link

    I have to agree. Year after year we see more and more mind-boggling performance improvements over regular HDDs, but little or no price drop. Perhaps the materials costs are just insurmountable and the replacement of HDDs won't be happening after all. SSDs will be like SLR digital cameras -- premium and professional use only, and pricing a previous generation of amateur users out of a market they used to be in. Reply
  • FunBunny2 - Saturday, February 19, 2011 - link

    From what I see: as each feature size drop in the NAND, the controller has to get increasingly more byzantine, needs more cache, and so on just to maintain performance. Word is that IMFT 25nm includes an ECC engine on die!!! Reply
  • Aernout - Saturday, February 19, 2011 - link

    Maybe we wil hear more of the hybride disks like the momentus XT from seagate in the future, for 'standard' users they can offer a lot.
    now they have a 4 gb flash with 500 gb but its 10 months old.
    I think lots of people are hoping they will multiply those specs.
    I'm thinking of getting one for my laptop, but then on the otherside i am not sure if i will use 500 gig on my laptop, maybe i should buy a 64 ssd in stead.
    Reply

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