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New disk IO scheduler used in vSphere 5.5

Duncan Epping · Jul 14, 2014 ·

When 5.1 was released I noticed the mention of “mClock” in the advanced settings of a vSphere host. I tried enabling it but failed miserably. A couple of weeks back I noticed the same advanced setting again, but this time also noticed it was enabled. So what is this mClock thingie? Well mClock is the new disk IO scheduler used in vSphere 5.5. There isn’t much detail on mClock by itself other than an academic paper by Ajay Gulati.

disk io scheduler

The paper describes in-depth why mClock was designed / developed, it primarily was to provide a better IO scheduling mechanism that allows for limits, shares and yes also reservations. The paper also describes some interesting details around how different IO sizes and latency is taken in to account. I recommend anyone who likes reading brain hurting material to take a look at it. I am also digging internally for some more human readable material, If I find out more I will let you guys know!

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Storage 5.5, Storage, VMware, vSphere

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Comments

  1. Sunny Dua says

    14 July, 2014 at 17:58

    Human Readable… LOL

    • dedwards737364 says

      17 September, 2014 at 19:32

      Ajay also has a paper published on purdue’s site dated Aug 2012 that has more visualization in presentation form. It seems to phrase the problem a little nicer and is a little more human readable 🙂
      https://www.cs.purdue.edu/homes/kompella/teaching/cs591dsn/Purdue-tech-talk-24-Aug-2012.pdf

  2. John Nicholson. says

    14 July, 2014 at 20:33

    This explains IOPS throttling showing up randomly as a feature.

    • Sketch says

      15 July, 2014 at 19:29

      it has to be a “feature”, otherwise it would be a “bug” in the system.

      • Duncan Epping says

        15 July, 2014 at 20:26

        It is a feature for sure… Just poorly documented behaviour right now.

  3. Carl Slaughter says

    14 July, 2014 at 23:35

    Duncan, how do I contact you, or can I. I have a question about my current config with 3 ESXi 5.5 hosts (64GB RAM) and one Dell 3200i That I am experiencing some latency issues that I cant figure out.

    • Marius Flage says

      15 July, 2014 at 10:36

      Carl,

      check the option for disabling delayed ACK. That proved to be the night/day solution in our environment (5.5 and IBM StorWize V3700).

      Check the following KB for how to enable this:

      http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1002598

      Make sure you reboot each of the ESXi hosts for the changes to take effect.

      – Marius

  4. Aashish Parikh says

    15 July, 2014 at 01:14

    Brain-hurting 🙂

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About the author

Duncan Epping is a Chief Technologist in the Office of CTO of the Cloud Platform BU at VMware. He is a VCDX (# 007), the author of the "vSAN Deep Dive", the “vSphere Clustering Technical Deep Dive” series, and the host of the "Unexplored Territory" podcast.

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