Hi, On 2018-06-19 18:41:24 +0200, Tomas Vondra wrote: > I'm confused. I thought you wanted to prefetch directly to shared buffers, > so that it also works with direct I/O in the future. But now you suggest to > use posix_fadvise() to work around the synchronous buffer read limitation. I > don't follow ...
Well, I have multiple goals. For one I think using prefetching without any sort of backpressure and mechanism to see which have completed will result in hard to monitor and random performance. For another I'm concerned with wasting a significant amount of memory for the OS cache of all the read data that's guaranteed to never be needed (as we'll *always* write to the relevant page shortly down the road). For those reasons alone I think prefetching just into the OS cache is a bad idea, and should be rejected. I also would want something that's more compatible with DIO. But people pushed back on that, so... As long as we build something that looks like a request queue (which my proposal does), it's also something that can later with some reduced effort be ported onto asynchronous io. Greetings, Andres Freund
