=head2 INBOX OVERVIEW AND DEFINITIONS
-$EPOCH - Integer starting with 0 based on time
-$SCHEMA_VERSION - PublicInbox::Search::SCHEMA_VERSION used by Xapian
-$PART - Integer (0..NPROCESSORS)
-
-foo/ # assuming "foo" is the name of the list
-- inbox.lock # lock file (flock) to protect global state
-- git/$EPOCH.git # normal git repositories
-- all.git # empty git repo, alternates to git/$EPOCH.git
-- xap$SCHEMA_VERSION/$SHARD # per-shard Xapian DB
-- xap$SCHEMA_VERSION/over.sqlite3 # OVER-view DB for NNTP and threading
-- msgmap.sqlite3 # same the v1 msgmap
+ $EPOCH - Integer starting with 0 based on time
+ $SCHEMA_VERSION - DB schema version (for Xapian)
+ $SHARD - Integer starting with 0 based on parallelism
+
+ foo/ # "foo" is the name of the inbox
+ - inbox.lock # lock file to protect global state
+ - git/$EPOCH.git # normal git repositories
+ - all.git # empty, alternates to $EPOCH.git
+ - xap$SCHEMA_VERSION/$SHARD # per-shard Xapian DB
+ - xap$SCHEMA_VERSION/over.sqlite3 # OVER-view DB for NNTP, threading
+ - msgmap.sqlite3 # same the v1 msgmap
For blob lookups, the reader only needs to open the "all.git"
repository with $GIT_DIR/objects/info/alternates which references