> Do I need a correspondingly large checkpoint timeout then?  Or does
> that matter much?

Yes, you do.

> And does this advice apply if the pg_xlog is on the same RAID partition
> (mine currently is not, but perhaps will be in the future)

Not as much, but it's still a good idea to serialize the load.   With too few 
segments, you get a pattern like:

Fill up segments
Write to database
Recycle segments
Fill up segments
Write to database
Recycle segments 

Compared to doing it in one long run of a single cycle, considerble efficiency 
is lost.   With a proper 2-array setup, the segments become like a write 
buffer for the database, and you want that buffer as large as you can afford 
in order to prevent buffer cycling from interrupting database writes.

BTW, for members of the studio audience, checkpoint_segments of 256 is about 

Josh Berkus
Aglio Database Solutions
San Francisco

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