figuring out why ccache misses

When compiling Ceph, ccache may appear to miss more than expected, as shown by the cache miss line of ccache -s

cache directory                     /home/loic/.ccache
cache hit (direct)                     1
cache hit (preprocessed)               0
cache miss                             1
files in cache                         3
cache size                           392 Kbytes
max cache size                      10.0 Gbytes

Compiling Ceph from clones in two different directories does not explain the miss, unless CCACHE_HASHDIR is set. It should be unset with:

unset CCACHE_HASHDIR

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