Re: O_DIRECT use

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