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 we'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 allocating 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.

  Crucial M500 960GB Samsung SSD 840 EVO 1TB Samsung SSD 840 EVO 250GB SanDisk Extreme II 480GB Samsung SSD 840 Pro 256GB
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Thanks to the EVO's higher default over provisioning, you actually get better consistency out of the EVO than the 840 Pro out of the box. Granted you can get similar behavior out of the Pro if you simply don't use all of the drive. The big comparison is against Crucial's M500, where the EVO does a bit better. SanDisk's Extreme II however remains the better performer from an IO consistency perspective.

  Crucial M500 960GB Samsung SSD 840 EVO 1TB Samsung SSD 840 EVO 250GB SanDisk Extreme II 480GB Samsung SSD 840 Pro 256GB
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  Crucial M500 960GB Samsung SSD 840 EVO 1TB Samsung SSD 840 EVO 250GB SanDisk Extreme II 480GB Samsung SSD 840 Pro 256GB
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Zooming in we see very controlled and frequent GC patterns on the 1TB drive, something we don't see in the 840 Pro. The 250GB drive looks a bit more like a clustered random distribution of IOs, but minimum performance is still much better than on the standard OP 840 Pro.

TRIM Validation

Our performance consistency test actually replaces our traditional TRIM test in terms of looking at worst case scenario performance, but I wanted to confirm that TRIM was functioning properly on the EVO so I dusted off our old test for another go. The test procedure remains unchanged: fill the drive with sequential data, run a 4KB random write test (QD32, 100% LBA range) for a period of time (30 minutes in this case) and use HDTach to visualize the impact on write performance:

Minimum performance drops down to around 30MB/s, eugh. Although the EVO can be reasonably consistent, you'll still want to leave some free space on the drive to ensure that performance always stays high (I recommend 15 - 25% if possible).

A single TRIM pass (quick format under Windows 7) fully restores performance as expected:

The short period of time at 400MB/s is just TurboWrite doing its thing.

RAPID: PCIe-like Performance from a SATA SSD AnandTech Storage Bench 2013
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  • MrCommunistGen - Thursday, July 25, 2013 - link

    YES! I've been excitedly waiting for this review since the announcement! Reply
  • Byte - Thursday, July 25, 2013 - link

    Writes for the 120GB are still quite slow. Reply
  • chizow - Thursday, July 25, 2013 - link

    That's nearly universal though for all the entry-level capacity SSDs on the market, it's similar to RAID 0, when you can write to symmetrical NAND packages you see a significant increase in write speeds. Reply
  • OUT FOX EM - Monday, July 29, 2013 - link

    Speaking of RAID 0, if you'll notice, all the drives of 250GB and higher perform around the same. You are MUCH better off getting 4x250GB drives instead of the 1TB. With most models the cost will actually be about the same, but the speed of the RAID will be 4x faster as well while maintaining the same capacity.

    Of course there are other drawbacks like space inside your PC and amount of available SATA ports on your motherboard, for instance, but if those aren't a factor, buying multiple SSD's is a much better option in terms of performance. I don't see many reviews mention this fact.
    Reply
  • Jorgisven - Thursday, August 1, 2013 - link

    Much better in terms of performance, but I wouldn't recommend RAID 0 for 4 SSD hard drives. RAID6 is likely a better option, as it is fault tolerant without losing too much space. It's a bit of a personal decision, but the RAID concepts stand true whether it's SSD or not. Additionally, 4x250 is likely a good percentage more expensive than the already expensive 1TB SSD. Reply
  • Democrab - Thursday, August 15, 2013 - link

    I'm not sure about you, but I'm only storing replaceable data on my SSDs...There are game saves but they're automatically put on Google Drive too so I get backups easily, it's easy to set something like that up and then just get the benefits of RAID0 although I'd be using a RAID card as the chipset would likely bottleneck it. Reply
  • yut345 - Thursday, December 12, 2013 - link

    I agree. Due to the volatile nature of SSDs, and the fact that if they go down your data can't really be recovered like it could be on mechanical drive, I do not plan to store anything on the drive that I don't also back up somewhere else. Reply
  • m00dawg - Friday, August 23, 2013 - link

    With only 4 drives, a RAID10 would be much preferable. 1/2 the available space (same as a 4 drive RAID6 in this case), but without the need to calculate parity, worry (as much) about partitioning alignment, and you can still handle up to 2 drive failures (though only if they are on different stripes). Reply
  • fallaha56 - Friday, September 19, 2014 - link

    sorry but disagree this will defeat the point unless you're on a top-end raid controller -and then you get no TRIM

    when there's no moving parts reliability becomes much less of an issue, esp for an OS drive with cloud and local backup like most of us high-end users do
    Reply
  • Stas - Tuesday, September 24, 2013 - link

    That's what I did for the recent laser data processing builds. 4x250GB 840s and a 1TB HDD for nightly backup. Only data is stored on the array. Speeds are up to 1600MB/sec. Needless to say, the client is very happy :) Reply

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