On 05/17/2011 03:00 PM, Robert Klemme wrote:
The main point is that you do not benefit from the larger IO bandwidth if access patterns do not permit parallel access to both disks (e.g. because you first need to read index blocks in order to know the table blocks to read).
This makes me wonder if Pg attempts to pre-fetch blocks of interest for areas where I/O needs can be known in advance, while there's still other works or other I/O to do. For example, pre-fetching for the next iteration of a nested loop while still executing the prior one. Is it even possible?
I'm guessing not, because (AFAIK) Pg uses only synchronous blocking I/O, and with that there isn't really a way to pre-fetch w/o threads or helper processes. Linux (at least) supports buffered async I/O, so it'd be possible to submit such prefetch requests ... on modern Linux kernels. Portably doing so, though - not so much.
-- Craig Ringer -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
