_mdfd_getseg can be expensive

Hi,

I recently have seen some perf profiles in which _mdfd_getseg() was in
the top #3 when VACUUMing large (~200GB) relations. Called by mdread(),
mdwrite(). Looking at it's implementation, I am not surprised. It
iterates over all segment entries a relations has; for every read or
write. That's not painful for smaller relations, but at a couple of
hundred GB it starts to be annoying. Especially if kernel readahead has
already read in all data from disk.

I don't have a good idea what to do about this yet, but it seems like
something that should be fixed mid-term.

The best I can come up is is caching the last mdvec used, but that's
fairly ugly. Alternatively it might be a good idea to not store MdfdVec
as a linked list, but as a densely allocated array.

Greetings,

Andres Freund

-- Andres Freund                       http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training &
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