Subversion FSFS - how are revisions stored in the repository?

Question

I'd like to understand how subversion stores revisions in FSFS, and how a view/shapshot is constructed for a given revision number.

What I have gleaned from Googling is that FSFS is a simple directory structure, with sub-directories for each revision like:

..svn/rev/0/
..svn/rev/1/
..svn/rev/2/

Presumably only the changes (deltas) are recorded under each revision directory. So does this mean that when constructing the view/snapshot for revision N, all the deltas from 0 to N have to be looped over?

Any links to resources on this much appreciated.

Thanks

Answer 1

Subversion stores all deltas of each revision in one single (flat) revision file. Each file/folder inside the repository (called a "node") has an internal ID.

A single revision file consists of all compressed deltas for this particular commit, however the deltas are not against the previous revision, but use a scheme called "skipped deltas" avoiding linear growing search time for growing version history.

Important is that FSFS uses forward deltas instead of backward deltas using the BDB-backend. So FSFS is faster on commits, but slower on checkout, Berkeley DB's performance characteristic is other way around.

You can read a lot more inside SVN design note about FSFS.

Answer 2

Here is a link to the FSFS structure reference (including file format description): https://svn.apache.org/repos/asf/subversion/trunk/subversion/libsvn_fs_fs/structure