Fractions in GUC variables

We have these GUC variables that define a fraction of something:

#autovacuum_vacuum_scale_factor = 0.2 # fraction of rel size before
# vacuum
#autovacuum_analyze_scale_factor = 0.1 # fraction of rel size before
# analyze

#bgwriter_lru_percent = 1.0 # 0-100% of LRU buffers scanned/round
#bgwriter_all_percent = 0.333 # 0-100% of all buffers scanned/round

Autovacuum settings use fractions, and bgwriter settings use a
percentage. Fortunately these settings are not related so there's not
too much potential for confusion, but it seems we should have a common
way to define settings like that.

A nice way would be that the base unit would be a fraction, like in the
autovacuum settings, but you could add a %-sign to give it as a percent,
just like you can use KB/MB etc. I'm not sure if we can do anything for
those without breaking backwards-compatibility, though.

Any ideas? The load distributed checkpoints patch adds one more GUC
variable like. I'm inclined to follow the example of the bgwriter
settings because it's more closely related to them, though I like the
autovacuum style more.

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com