All,
I was looking at these IOZone results for some NAS hardware and thinking
about index scans:
Children see throughput for 6 readers = 72270.04 KB/sec
Parent sees throughput for 6 readers = 72269.06 KB/sec
Min throughput per process = 11686.53 KB/sec
Max throughput per process = 12506.65 KB/sec
Avg throughput per process = 12045.01 KB/sec
Min xfer = 3919344.00 KB
Children see throughput for 6 reverse readers = 17313.57 KB/sec
Parent sees throughput for 6 reverse readers = 17313.52 KB/sec
Min throughput per process = 2569.21 KB/sec
Max throughput per process = 3101.18 KB/sec
Avg throughput per process = 2885.60 KB/sec
Min xfer = 3474840.00 KB
Now, what that says to me is that for this system reverse sequential
reads are 1/4 the speed of forwards reads. And from my testing
elsewhere, that seems fairly typical of disk systems in general.
Now, while index scans (for indexes on disk) aren't 100% sequential
reads, it seems like we should be increasing (substantially) the
estimated cost of reverse index scans if the index is likely to be on
disk. No?
--
Josh Berkus
PostgreSQL Experts Inc.
www.pgexperts.com
