Estimating WAL volume

All this talk of WAL writing lately has me wondering something I haven't
spent enough time looking at the source to figure out myself this
week...any good rules of thumb out there for estimating WAL volume? I'm
used to just measuring it via benchmarking but it strikes me a formula
would be nice to have for pre-planning.

For example, if I have a table where a typical row is X bytes wide, and
I'm updating Y of those per second, what's the expected write rate of WAL
volume? Some % of those writes are going to be full pages; what's
typical? How much does the number and complexity of indexes factor into
things--just add the width of the index in bytes to the size of the
record, or is it worse than that?

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* Greg Smith gsmith(at)gregsmith(dot)com http://www.gregsmith.com Baltimore, MD