How to solve very slow IO? - throttling on El Capitan

All your general support questions for OpenZFS on OS X.

How to solve very slow IO? - throttling on El Capitan

Postby lerebel » Thu Feb 18, 2016 5:00 pm

Hi there,

I was hoping to share my findings and see if there might be a better approach in preventing OSX from throttling ZFS IO. My small external array has worked great for some time (>200MB/s), however it recently started to severely underperform (<10MB/s !). In trying to understand what was happening, I came to realise that OSX was throttling IO, as shown by
Code: Select all
fs_usage


Disabling throttling by setting:
Code: Select all
sysctl -w debug.lowpri_throttle_enabled=0
appears to turn off throttling and restores IO performance immediately.

Although this seems to work, it is perhaps not the best solution in my view and I was wondering if you might have a better approach. Can Process/Thread IO priorities by altered via configuration in OpenZFS instead?

Thank you!
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Re: How to solve very slow IO? - throttling on El Capitan

Postby lundman » Wed Mar 09, 2016 10:49 pm

I've tried to find some test case where I can measure this, but I have been unable to, so far. Could be the IO is a bit too fast on the developer VM (7 SAS 15K rpm disks in raid).

Is this something you can measure to be faster if you disable the throttle? And if so, are you able to build from source to see if we can trick the scheduler into giving us priority?
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