On 8/27/13 3:54 PM, Josh Berkus wrote:
I believe that Greenplum currently uses 128K. There's a definite benefit for the DW use-case.
Since Linux read-ahead can easily give big gains on fast storage, I normally set that to at least 4096 sectors = 2048KB. That's a lot bigger than even this, and definitely necessary for reaching maximum storage speed.
I don't think that the block size change alone will necessarily duplicate the gains on seq scans that Greenplum gets though. They've done a lot more performance optimization on that part of the read path than just the larger block size.
As far as quantifying whether this is worth chasing, the most useful thing to do here is find some fast storage and profile the code with different block sizes at a large read-ahead. I wouldn't spend a minute on trying to come up with a more complicated management scheme until the potential gain is measured.
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