On 2020-12-08 04:24:44 +0000, tsunakawa.takay@fujitsu.com wrote: > I'm looking forward to this from the async+direct I/O, since the > throughput of some write-heavy workload decreased by half or more > during checkpointing (due to fsync?)
Depends on why that is. The most common, I think, cause is that your WAL volume increases drastically just after a checkpoint starts, because initially all page modification will trigger full-page writes. There's a significant slowdown even if you prevent the checkpointer from doing *any* writes at that point. I got the WAL AIO stuff to the point that I see a good bit of speedup at high WAL volumes, and I see it helping in this scenario.
There's of course also the issue that checkpoint writes cause other IO (including WAL writes) to slow down and, importantly, cause a lot of jitter leading to unpredictable latencies. I've seen some good and some bad results around this with the patch, but there's a bunch of TODOs to resolve before delving deeper really makes sense (the IO depth control is not good enough right now).
A third issue is that sometimes checkpointer can't really keep up - and that I think I've seen pretty clearly addressed by the patch. I have managed to get to ~80% of my NVMe disks top write speed (> 2.5GB/s) by the checkpointer, and I think I know what to do for the remainder.
Thanks for explaining this. I'm really glad you're looking into it. If I get the chance I'd like to try to apply some wait-analysis and blocking stats tooling to it. I'll report back if I make any progress there.
