Continuation of the write throttle discussion

[HoneyBadger]

Picking up from here:

https://forums.freenas.org/index.php?threads/iscsi-drops-connection.62272/page-2#post-490153

Flip side: blowing out write caches don't typically freeze all I/O and cause your storage array to drop offline.

Admission: Ye Olde Write Throttle could do that.

Counterpoint: The new one will eventually throttle your pool down to 10 IOPS if you're running right up against the dirty_data_max value, but I've yet to see that happen in real-world use.

Sidebar: If you know that you're going to have a huge burst of write activity, the array should be prepared for it - whether that be a sufficiently-large write cache (or not-a-write-cache SLOG) or being made of sufficiently quantity/quality devices depending on I/O profile.

Conclusion: I made a new thread in Off-Topic so we can quit hijacking this one. ;) (You are here.)

 

[kdragon75]

Where can I find more information about the write throttle knobs? I would love to play with (de)tuning them to see the effects.

 

[HoneyBadger]

Where can I find more information about the write throttle knobs? I would love to play with (de)tuning them to see the effects.

Here's some "light reading" for you:

Fundamentals and discussing the old throttle
http://dtrace.org/blogs/ahl/2013/12/27/zfs-fundamentals-the-write-throttle/

The new one and how it came about
http://dtrace.org/blogs/ahl/2014/02/10/the-openzfs-write-throttle/

The good stuff - tuning the new one
http://dtrace.org/blogs/ahl/2014/08/31/openzfs-tuning/

Notably that last one is what got me started digging into dtrace and all of its goodness.

 

[kdragon75]

Grinning ear to ear. :D