This PR does two things, because one lead to the other:
- Move the leveldb specific stuff into a small "backend" package that
defines a backend interface and the leveldb implementation. This allows,
potentially, in the future, switching the db implementation so another
KV store should we wish to do so.
- Add proper error handling all along the way. The db and backend
packages are now errcheck clean. However, I drew the line at modifying
the FileSet API in order to keep this manageable and not continue
refactoring all of the rest of Syncthing. As such, the FileSet methods
still panic on database errors, except for the "database is closed"
error which is instead handled by silently returning as quickly as
possible, with the assumption that we're anyway "on the way out".
This introduces a better set of defaults for large databases. I've
experimentally determined that it results in much better throughput in a
couple of scenarios with large databases, but I can't give any
guarantees the values are always optimal. They're probably no worse than
the defaults though.
This adds a set of magical environment variables that can be used to
tweak the database parameters. It's totally undocumented and not
intended to be a long term or supported thing.
It's ugly, but there is a backstory. I have a couple of large
installations where the database options are inefficient or otherwise
suboptimal (24/7 compaction going on and stuff like that). I don't
*know* the correct database parameters, nor yet the formula or method to
derive them by, so this requires experimentation. Experimentation needs
to happen partly in production, and rolling out new builds for every
tweak isn't practical. This provides override points for all reasonable
values, while not changing anything by default.
Ideally, at the end of such experimentation, we'll know which values are
relevant to change and in what manner, and can provide a more user
friendly knob to do so - or do it automatically based on the database
size.
Flush the batch when exceeding a certain size, instead of when reaching a number
of batched operations.
Move batch to lowlevel to be able to use it in NamespacedKV.
Increase the leveldb memory buffer from 4 to 16 MiB.
This adds a thin type that holds the state associated with the
leveldb.DB, leaving the huge Instance type more or less stateless. Also
moves some keying stuff into the DB package so that other packages need
not know the keying specifics.
(This does not, yet, fix the cmd/stindex program, in order to keep the
diff size down. Hence the keying constants are still exported.)
