If you want to understand the db
directory, you should probably read through the source code which SVN uses to implement FSFS, which has comments that go into extensive detail of how the pieces fit together. In brief, here is what the files are
txn-protorevs/ Subdirectory containing transaction proto-revision files
<txnid>.rev Proto-revision file for transaction <txnid>
<txnid>.rev-lock Write lock for proto-rev file
And here is what a proto-revision file does
The prototype rev file is used to store the text representations as
they are received from the client. To ensure that only one client is
writing to the file at a given time, the "rev-lock" file is locked for
the duration of each write.