Compressing clusters with zstd is the slow part of packing a large
mirror. When a mirror is re-packed after a small change, most clusters
are byte-identical and there is no reason to compress them again.
pack --incremental keeps a content-addressed cache of compressed
clusters in a sidecar next to the output. On the next pack, a cluster
whose uncompressed bytes match the cache is served from it instead of
being recompressed; only new or changed clusters go through zstd. A
cache hit returns exactly what a fresh compression would, so the archive
stays deterministic and byte-identical to a cold pack.
The zim writer gains a settable cluster compressor so the cache can hook
in without changing the format. The cache only writes back the clusters
it touched this run, so clusters that left the mirror drop out and it
cannot grow without bound.
Tam Nguyen Duc
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