Performance Consistency

In our Intel SSD DC S3700 review I introduced a new method of characterizing performance: looking at the latency of individual operations over time. The S3700 promised a level of performance consistency that was unmatched in the industry, and as a result needed some additional testing to show that. The reason we don't have consistent IO latency with SSDs is because inevitably all controllers have to do some amount of defragmentation or garbage collection in order to continue operating at high speeds. When and how an SSD decides to run its defrag and cleanup routines directly impacts the user experience. Frequent (borderline aggressive) cleanup generally results in more stable performance, while delaying that can result in higher peak performance at the expense of much lower worst case performance. The graphs below tell us a lot about the architecture of these SSDs and how they handle internal defragmentation.

To generate the data below I took a freshly secure erased SSD and filled it with sequential data. This ensures that all user accessible LBAs have data associated with them. Next I kicked off a 4KB random write workload across all LBAs at a queue depth of 32 using incompressible data. I ran the test for just over half an hour, no where near what we run our steady state tests for but enough to give me a good look at drive behavior once all spare area filled up.

I recorded instantaneous IOPS every second for the duration of the test. I then plotted IOPS vs. time and generated the scatter plots below. Each set of graphs features the same scale. The first two sets use a log scale for easy comparison, while the last set of graphs uses a linear scale that tops out at 40K IOPS for better visualization of differences between drives.

The high level testing methodology remains unchanged from our S3700 review. Unlike in previous reviews however, I did vary the percentage of the drive that I filled/tested depending on the amount of spare area I was trying to simulate. The buttons are labeled with the advertised user capacity had the SSD vendor decided to use that specific amount of spare area.  If you want to replicate this on your own all you need to do is create a partition smaller than the total capacity of the drive and leave the remaining space unused to simulate a larger amount of spare area. The partitioning step isn't absolutely necessary in every case but it's an easy way to make sure you never exceed your allocated spare area. It's a good idea to do this from the start (e.g. secure erase, partition, then install Windows), but if you are working backwards you can always create the spare area partition, format it to TRIM it, then delete the partition. Finally, this method of creating spare area works on the drives I've tested here but not all controllers may behave the same way.

The first set of graphs shows the performance data over the entire 2000 second test period. In these charts you'll notice an early period of very high performance followed by a sharp dropoff. What you're seeing in that case is the drive alllocating new blocks from its spare area, then eventually using up all free blocks and having to perform a read-modify-write for all subsequent writes (write amplification goes up, performance goes down).

The second set of graphs zooms in to the beginning of steady state operation for the drive (t=1400s). The third set also looks at the beginning of steady state operation but on a linear performance scale. Click the buttons below each graph to switch source data.

Impact of Spare Area
  Intel SSD DC S3700 200GB Corsair Neutron 240GB OCZ Vector 256GB Samsung SSD 840 Pro 256GB SanDisk Ultra Plus 256GB
Default
25% Spare Area -

The Ultra Plus' performance consistency, at least in the default configuration, looks a bit better than Samsung's SSD 840 Pro. The 840 Pro is by no means the gold standard here so that's not saying too much. What is interesting however is that the 840 Pro does much better with an additional 25% spare area compared to the Ultra Plus. SanDisk definitely benefits from more spare area, just not as much as we're used to seeing.

The next set of charts look at the steady state (for most drives) portion of the curve. Here we'll get some better visibility into how everyone will perform over the long run.

Impact of Spare Area
  Intel SSD DC S3700 200GB Corsair Neutron 240GB OCZ Vector 256GB Samsung SSD 840 Pro 256GB SanDisk Ultra Plus 256GB
Default
25% Spare Area -

The final set of graphs abandons the log scale entirely and just looks at a linear scale that tops out at 50K IOPS. We're also only looking at steady state (or close to it) performance here:

Impact of Spare Area
  Intel SSD DC S3700 200GB Corsair Neutron 240GB OCZ Vector 256GB Samsung SSD 840 Pro 256GB SanDisk Ultra Plus 256GB
Default
25% Spare Area -

Here we see just how far performance degrades. Performance clusters around 5K IOPS, which is hardly good for a modern SSD. Increasing spare area helps considerably, but generally speaking this isn't a drive that you're going to want to fill.

 

Performance vs. Transfer Size AnandTech Storage Bench 2011
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  • Kristian Vättö - Monday, January 07, 2013 - link

    Performance vs Transfer Size and TRIM graphs are missing, I know. Already pinged Anand about those so expect to see them soon (the graphs weren't in our admin engine so I couldn't add them, Anand needs to upload them). Reply
  • vol7ron - Monday, January 07, 2013 - link

    The impact of spare area graphs are interesting. OWC has claimed that the spare area doesn't have much of an influence on drives using SF controller, thus defending their non-TRIM support.

    Perhaps Anand could include an OWC drive in there for comparison.
    Reply
  • dave_the_nerd - Monday, January 07, 2013 - link

    OWC has to run down TRIM because they sell third-party SSDs to Mac OS X users. Apple forces OS X to disable TRIM on SSDs they don't supply, because they're jerks sometimes.

    Apple ships its own machines with TRIM enabled, just like everybody else,

    There are hacks. But nobody, OWC or otherwise, is going to say, "oh, yeah, our product supports TRIM, but you need to download this sketchy looking program from this guys blog to make it work. Good luck."
    Reply
  • Darnell021 - Monday, January 07, 2013 - link

    haha yess I just went through that process myself and it's worth adding that every incremental OSX update resets that sketchy little program hack to turn TRIM back off.

    Still worth it though if you know what you're doing ;)
    Reply
  • Samus - Monday, January 07, 2013 - link

    "but you need to download this sketchy looking program from this guys blog to make it work."

    word. true dat, had to do this when I put an Intel X25-M 160GB in my wife's Macbook a few years ago. after about a year it started running crazy slow and fortunately that was just around the time the 3rd party TRIM tool surfaced. works like a charm, but definitely a sketchy solution.

    no more sketchy than jail-breaking an iPhone though. pretty much the only reason I jailbroke my iPod Touch was to disable wifi while its sleeping, because, for some reason, Apple DOESN'T allow that.
    Reply
  • NCM - Monday, January 07, 2013 - link

    I don't see anything "sketchy" about it. A quick trip to the Unix command line enables/disables TRIM in OSX. All the TRIM Enabler utility does is to offer a convenient GUI for that process. It can hardly be said to rise to the level of a hack. The System Profiler correctly shows whether TRIM is enabled or not for either an original or aftermarket SSD.

    It's always dangerous to impute motives to others, and especially so in the case of the typically secretive Apple. Nonetheless I'd guess that Apple, whose customer support is consistently rated well above that of other PC manufacturers, isn't going to endorse something it hasn't tested, and isn't going to test something it doesn't sell. Therefore OSX neither enables TRIM automatically for third party SSDs, nor prevents users from doing so themselves. Sound pretty neutral to me.
    Reply
  • PJCarmody - Tuesday, January 08, 2013 - link

    NCM,

    To your point "isn't going to endorse something it hasn't tested" - Apple is not being asked to endorse the third party options. Not for a moment. So your point is invalid.

    I like your open mind but let's face facts: Apple has a history of crippling third party competitors e.g. for storage.
    Reply
  • gw019 - Monday, September 16, 2013 - link

    "But nobody, OWC or otherwise, is going to say, "oh, yeah, our product supports TRIM, but you need to download this sketchy looking program from this guys blog to make it work. Good luck."

    Well, I am surprised but in fact Plextor did say something similar: http://www.plextoramericas.com/index.php/faq/22-ma...
    Reply
  • Kristian Vättö - Monday, January 07, 2013 - link

    Yeah, that's true. SandForce drives perform well when it comes to consistency and there is no big benefit from more OP (I've tested this with 240GB Intel SSD 335). Reply
  • Samus - Monday, January 07, 2013 - link

    I'm just dying for a mainstream Intel S3700 to hit the consumer corner... Reply

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