A per-node cache of message blobs keyed by content hash, 64 MiB by
default (INBUXA_BLOB_CACHE sets it; 0 turns it off), fed on write and
on read. Upstream fetched a message's blob from the blob store every
time: the spool blob straight back for delivery, the same message for
BODYSTRUCTURE and then BODY[], every outbound retry; on the cluster
each is a round trip to the object store.
Server::get_message_blob serves the message read paths (IMAP FETCH,
POP3, JMAP Email/get and blob sections, the index task, outbound
delivery, DSNs, push, holds and the journal). Named keys that are
rewritten in place keep using the blob store directly. A blob above a
sixteenth of the capacity is not cached.
jcoffey-dev