How to get the best disk performance with Linux on Hyper-V

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I was recently reading our documented Best Practices for running Linux on Hyper-V and noticed this section:

Use I/O scheduler NOOP for better disk I/O performance.

The Linux kernel has four different I/O schedulers to reorder requests with different algorithms. NOOP is a first-in first-out queue that passes the schedule decision to be made by the hypervisor. It is recommended to use NOOP as the scheduler when running Linux virtual machine on Hyper-V. To change the scheduler for a specific device, in the boot loader’s configuration (/etc/grub.conf, for example), add elevator=noop to the kernel parameters, and then restart.

This is interesting – as I often get asked by people about what they can do to ensure the best performance when running Linux on Hyper-V.

For a bit of background here – Linux utilizes a number of techniques to try and get the best performance out of your storage (you can read all about this if you do a search on “Linux IO elevator”).  Unfortunately, all of this logic is completely undone inside of a virtual machine – as we are then responsible for mapping virtual storage to physical storage in a way that is hidden from the guest operating system.  Turning effect of turning on the NOOP I/O scheduler is that Linux stops trying to be clever about storage activity – and instead relies on the underlying hardware (virtual hardware in this case) to do the right thing.

In our testing this has always yielded the best results.

Cheers,
Ben

Comments

  • Anonymous
    June 19, 2014
    Thanks for this. Can you be a bit more specific what "best results" mean? I mean, what performance gain should I expect when changing this parameter? 1%? 10%? Thanks.

  • Anonymous
    June 22, 2014
    The comment has been removed

  • Anonymous
    June 22, 2014
    Nejc Škoberne - It really depends on the workload.  Some will see ~30% difference, some will see none. Paul - Anything running kernel 2.6 or later will see this. Cheers, Ben

  • Anonymous
    June 29, 2014
    Does this apply to Linux VMs that have 'direct access' to disks (i.e. have been assigned the physical disk as a device, rather than an VMDX?) - in fact, a discussion of what direct access means in the Hyper-V world would be great (i.e. does it benefit from VT-d support, etc).