Bruce Momjian <pgman@candle.pha.pa.us> writes:
>> For that matter, I would expect that O_DIRECT also defeats readahead,
>> so I'd fully expect it to be a loser for seqscans too.
> I am told on FreeBSD it does not disable read-ahead, just caching;
> something that needs more research.
Hmm. I always thought of read-ahead as preloading buffer cache entries.
It'd be interesting to get a description of *exactly* what this flag
does, rather than handwavy approximations. Time to start reading the
kernel code, I suppose.
regards, tom lane