Joshua Tolley wrote:
It was asserted that reading bigger chunks would help performance; a response suggested that, at least in Linux, setting readahead on a device would essentially do the same thing. Or that's what I got from the thread, anyway. I'm interested to know how similar performance might be between the large block size case and the large readahead case.
Large read-ahead addresses the complaint here (bulk reads are slow) just fine, which is one reason why this whole topic isn't nearly as interesting as claimed. Larger chunk sizes in theory will do the same thing, but then you're guaranteed to be reading larger blocks than necessary in all cases. The nice thing about a good adaptive read-ahead is that it can target small blocks normally, and only kick into heavy read-ahead mode when the I/O pattern justifies it.
This is a problem for the operating system to solve, and such solutions out there are already good enough that PostgreSQL has little reason to try and innovate in this area. I routinely see seq scan throughput double on Linux just by tweaking read-ahead from the tiny defaults to a sane value.
-- Greg Smith, 2ndQuadrant US greg@xxxxxxxxxxxxxxx Baltimore, MD PostgreSQL Training, Services and Support www.2ndQuadrant.us Author, "PostgreSQL 9.0 High Performance" Pre-ordering at: https://www.packtpub.com/postgresql-9-0-high-performance/book -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
