After running Ubuntu 19.10 with ZFS 0.8.1 for two weeks now, I have the impression that the cache hit rates have improved considerably, since 0.7.x. I do display those hit rates with conky, so they are constantly visible.
I run a 16GB Ryzen 3 and the L1ARC=4 GB and the L2ARC= 35 GB (VMs)+ 10 GB (Data). Everything is LZ4 compressed. I run Ubuntu 19.10 as host with Firefox and I always run a Xubuntu 18.04 Virtual Machine (2GB) with Evolution (Email), Firefox (WhatsApp) and Transmission (Torrents). A couple of times/day I run other VMs for an hour or so, like e.g. Windows 10 (News, Dutch TV player), Windows XP (WMP 11) or one of the 20.04 test versions (ext4).
The Host runs directly from SSD, the VMs run from 3 striped HDD partitions with a 35 GB L2ARC and a 5 GB LOG. The L1ARC is full 3.99 GB (uncompressed 5.58 GB) and the L2ARC is used for 50% and using 21.2 GB (uncompressed 35.7 GB). Those caches were already compressed in ZFS 0.7.x too.
I remember cache hit rates of ~93% for L1ARC and I now often see ~99%. The same happened with L2ARC I remember hit rates between 3% - 8% and now I do see hit rates of 28%-30%. I’m very happy with this improvement, because my HDDs with >6 power-on years are retired, serving and sharing between the three only 0.7% of all disk IO
Did we had some improvements or bug-corrections in this area?