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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  • FCss - Thursday, February 17, 2011 - link

    "My personal desktop sees about 7GB of writes per day." maybe a stupid question but how do you check the amount of your daily writes?
    And one more question: if you have a 128Gb SSD and you leave let's say 40Gb unformated so the user can't fill up the disk, will the controller use this space the same way as it would belong to the spare area?
    Reply
  • Quindor - Thursday, February 17, 2011 - link

    I use a program called "HDDLED" for this. It shows you some easily accessible leds on your screen and if you hover over it, you can see the current and total disk usage since your PC was booted up. Reply
  • FCss - Thursday, February 17, 2011 - link

    thanks, a great software Reply
  • Breit - Thursday, February 17, 2011 - link

    isn't the totally written bytes to the drive since manufacturing be part of the smart data you can read from your drive? all you have to do then is noting down the value when you boot up your pc in the morning and subtract that from the actual value you read there the next day. Reply
  • Chloiber - Thursday, February 17, 2011 - link

    Or you can just take the average.. Reply
  • marraco - Thursday, February 17, 2011 - link

    Vertex 2 takes advantage of unformated space. So OCZ advices to leave 20% of space unformated , (although to improve garbage collection, but it means that unformated space is used) Reply
  • 7Enigma - Thursday, February 17, 2011 - link

    Comon Anand! In your example you have 185GB free on a 256GB drive. I think that is the least likely scenario that paints an overly optimistic case in terms of write life. Everyone knows not to completely fill up their drive but are you telling me that the vast majority of users are going to have 78% of their drive free at all times? I just don't buy it.

    The more common scenario is that a consumer purchases a drive slightly larger then needed (due to how expensive these luxuries still are). So that 256GB drive probably will only have 20-40GB free. Do that and that 36 days for a single use of the NAND becomes ~5-8 days (no way to move static data around at this capacity level). Factor in write amplification (0.6X to 10X) and you lower the time to between 4-25 years for hitting that 3000X cap.

    Still not a HUGE problem, but much more relevant then saying this drive will last for hundreds of years (not counting NAND lifespan itself).
    Reply
  • 7Enigma - Thursday, February 17, 2011 - link

    Bah I thought the write amplification was 1.6X. That changes the numbers considerably (enough that the point is moot). I still think the example in the article was not a normal circumstance but it seems to still not be an issue.

    <pie to face>
    Reply
  • mark53916 - Thursday, February 17, 2011 - link

    Encrypted files are not compressible, so you won't get any advantage
    from the hardware write compression.
    Reply
  • 7Enigma - Thursday, February 17, 2011 - link

    Hi Anand,

    Looks like one of the numbers is incorrect in this chart. Right now it shows LOWER performance after TRIM then when the drive was completely full. The 230MB/sec value seems to be incorrect.
    Reply

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