Writeback cache

[JoakimZiegler]

Describe the feature would like to see added to OpenZFS

I would like to know if there are any plans to implement/integrate the writeback cache functionality Nexenta apparently implemented, which was presented at the OpenZFS developer summit 2015. See link: https://openzfs.org/wiki/Writeback_Cache

How will this feature improve OpenZFS?

This would, according to the presentation by Nexenta, considerably improve write throughput in ZFS, benchmarks as shown in this video: https://www.youtube.com/watch?v=MkdrnG7Gwd

[h1z1]

Invalid video id... Think you meant https://www.youtube.com/watch?v=MkdrnG7GwdE ?

[clefru]

Slides from Nexenta's presentation: https://storagetarget.files.wordpress.com/2015/10/zfs_writeback_cache-alex_aizman.pdf

I couldn't find the code that they demoed. sourcegraph.com doesn't know about any of their keywords used in the demo, like zfs_wrc_data_max or wrc_mode.

Also this issue was raised in the past, see #4879 . Please keep this issue open as feature request, as the performance increases with writeback caching are massive.

[Harliff]

@clefru, please take a look at this file: https://github.com/Nexenta/illumos-nexenta/blob/641841bbe8fbd19e75c4f306db16b28ba6d4520d/usr/src/uts/common/fs/zfs/wrcache.c

Is it related to write caching?

[Harliff]

An Reddit user posted link to commit: Nexenta/illumos-nexenta@641841b

Not sure is it related to write cache or not. Could you please look at the commit too?

[clefru]

@Harliff I am not a OpenZFS contributor. I have no idea whether this is usable code, sorry.

[pondermatic]

@Harliff, that repository does seem to be involved with the writeback cache presentation by Nexenta. The most recent version seems to be in the Nexenta/illumos-nexenta release-5.5-patch branch. It appears to be a set of patches for illumos that have not been merged to OpenZFS.

[JoakimZiegler]

Just chiming back in to say that now that I've been running large ZFS pools for some years (I was not at all as experienced when I first opened this issue), I still think it would be a huge boon for ZFS to add this. Given that the codebase is now the same for both Linux and BSD, it would be of benefit to even more users. I hope there's some will to do this, or maybe there's some way we can donate to ZFS for specific features?