Measured on staging: 46 per-variant build trees held 25 GB - each one a
full copy of the same ESP-IDF component objects - while ccache, the
cache that serves EVERY target and variant, was capped at 8 GB and
permanently full: 99.97% of its limit, 153 evictions, 47% lifetime hit
rate. Objects were being thrown out before they could be reused, so
builds recompiled work they had already done.
Variant cap 12 -> 6 and ccache 8G -> 20G. The asymmetry is deliberate:
an evicted variant dir rebuilds quickly FROM ccache, whereas a ccache
miss recompiles from source. Pruning the cold variants freed 8 GB on
the staging box (88% -> 84% disk) before the larger cache can grow into
it.
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