du with a cache for read-only dirs

[Brian J. Murrell]

Hi.

I wonder if anyone has considered or has any patches floating around to
speed up du of a tree of (mostly all) read-only dirs by caching the
information from any read-only dirs.

My use case is a tree of dirs where each dir is a backup of a bunch of
machines using hard links across the dirs to synthesize full backups
with what is really incremental information.  I.e. rsync --link-dest as
implemented by a tool called rsnapshot.

Frequently I want to see what the space delta is between any number of
those backups and trawling the whole filesystem for any dirs that have
already been trawled just doesn't seem to make sense.  Why not write
out what is known about that dir into a file that du can then just load
back the next time it is asked to run?

It is clearly a corner case, indeed.  But I wonder if anyone has gone
down this path and has code to share before I try to find the time to
write some myself.

Cheers,
b.

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