>>>>> "Thomas" == Thomas Munro <thomas.mu...@enterprisedb.com> writes:
Thomas> * it's also been claimed that readahead heuristics are not Thomas> defeated on Linux or FreeBSD, which isn't too surprising Thomas> because you'd expect it to be about blocks being faulted in, Thomas> not syscalls I don't know about linux, but on FreeBSD, readahead/writebehind is tracked at the level of open files but implemented at the level of read/write clustering. I have patched kernels in the past to improve the performance in mixed read/write cases; pg would benefit on unpatched kernels from using separate file opens for backend reads and writes. (The typical bad scenario is doing a create index, or other seqscan that updates hint bits, on a freshly-restored table; the alternation of reading block N and writing block N-x destroys the readahead/writebehind since they use a common offset.) The code that detects sequential behavior can not distinguish between pread() and lseek+read, it looks only at the actual offset of the current request compared to the previous one for the same fp. Thomas> +1 for adopting pread()/pwrite() in PG12. ditto -- Andrew (irc:RhodiumToad)
