borg 1.x encouraged users to put everything into the archive name:
- name of the dataset
- timestamp (usually used to make the archive name unique)
- maybe also hostname (when backing up to same repo from multiple hosts)
- maybe also username (when backing up to same repo from multiple users)
borg2 now discourages users from putting the timestamp into the name,
because we rather want same name within a series of archives - thus,
the field width for the name can be narrower.
the ID of the archive is now the only unique identifier, thus it is
moved to the leftmost place.
256bits (64 hex digits) was a bit much and as borg can also deal with
abbreviated IDs, we only show 32bits (8 hex digits) by default.
the ID is followed by the timestamp (also quite "interesting", because
it usually differs for different archives).
then following are: archive name, user name, host name - these might be
always the same if there is only one series of archives in a repo.
use 2 blanks separating the fields for better readability.
- changes to locally stored files cache:
- store as files.<H(archive_name)>
- user can manually control suffix via env var
- if local files cache is not found, build from previous archive.
- enable rebuilding the files cache via loading the previous
archive's metadata from the repo (better than starting with
empty files cache and needing to read/chunk/hash all files).
previous archive == same archive name, latest timestamp in repo.
- remove AdHocCache (not needed any more, slow)
- remove BORG_CACHE_IMPL, we only have one
- remove cache lock (this was blocking parallel backups to same
repo from same machine/user).
Cache entries now have ctime AND mtime.
Note: TTL and age still needed for discarding removed files.
But due to the separate files caches per series, the TTL
was lowered to 2 (from 20).
in borg 1.x, we used to put a timestamp into the archive name to make
it unique, because borg1 required that.
borg2 does not require unique archive names, but it encourages you
to even use an identical archive name within the same SERIES of archives.
that makes matching (e.g. for prune, but also at other places) much
simpler and borg KNOWS which archives belong to the same series.
borg1 needed this due to its transactional / rollback behaviour:
if there was uncommitted stuff in the repo, next repo opening automatically
rolled back to last commit. thus we needed checkpoint archives to reference
chunks and commit the repo.
borg2 does not do that anymore, unused chunks are only removed when the
user invokes borg compact.
thus, if a borg create gets interrupted, the user can just run borg create
again and it will find some chunks are already in the repo, making progress
even if borg create gets frequently interrupted.
Note: this is the default cache implementation in borg 1.x,
it worked well, but there were some issues:
- if the local chunks cache got out of sync with the repository,
it needed an expensive rebuild from the infos in all archives.
- to optimize that, a local chunks.archive.d cache was used to
speed that up, but at the price of quite significant space needs.
AdhocCacheWithFiles replaced this with a non-persistent chunks cache,
requesting all chunkids from the repository to initialize a simplified
non-persistent chunks index, that does not do real refcounting and also
initially does not have size information for pre-existing chunks.
We want to move away from precise refcounting, LocalCache needs to die.
