I have an odd issue with xcp-ng and freenas as it pertains to my NFS SR. My SR Latency is north of 200ms, and it is only running 4 non-disk intensive vms. I am using a Dell R420 for the freenas server with intel 520 spf+ for the storage connectivity. There are 8 drives (1tb nlsas LSI 9211-8i IT mode) broken into 4 mirrored vdevs. The only way I am been able to get the Latency down is to disable sync. Which drops it to 13ms, but honestly that is still really high for 4 vms. In comparison my Netgear Ready NAS with 6 WD Reds with 1Gb nics runs @ 28 ms running the same 4 servers. Makes me think there is something wrong with my FreeNas setup.
There is a lot of factors that go into the performance of external storage.
And my video on the topic.
I have seen your video and understand the impact of the sync settings and slog. Though most of these discussions talk specifically about throughput, and don’t talk much about actually write latency. I did found out that the Netgear had async turned on, previous this option was disabled by default, looks like a recent firmware update may have changed that. So my initial comparison was not one to one. After retesting the Netgear the performance differences in regard to latency now make more sense. I have been doing local storage or hyperconverged for so long it taking me a bit to go back to NAS/SAN and be objective.