Average size of git bookkeeping data (related to Using git as a general backup mechanism)

Hi, This is in connection with the "Using git as a general backup mechanism" thread, but on a different slant more along automatic versioning a subset of the files on the disc (to avoid sucking in things like core files and particularly huge multimedia files). In this case I don't want to discard old "backups" but just grow the chronological database. (Think a more selective version of Plan 9's venti filesystem.)

How big is the "metadata" or "bookeeping data" in git related to a commit? (Eg, "around x bytes per changed file" or "around x bytes per file being tracked (whether changed in the commit or not)" )

[I'm trying to get a feel for, if I switched to git, how much overhead would come from having a cron job automatically doing a snapshot every hour (if anything has changed), plus manual snapshots at points where I want to feel "safeguarded". I'm currently using my own simple, hacked together system for combined versioning/backups that does this. Using naive tools that don't account for wastes space due to disk block size effects the data being tracked is currently just under 9 months of acitvity on 2016 filenames with 17457599 bytes of data (ie, compressed version of their contents at various times) and 7838546 bytes is "metadata", ie, 30 percent of the stored data is metadata. This is in a format using 6 bytes to associate a single blob of contents to a filename (whether changed since last snapshot or not).]

Many thanks for any insight,

cheers, dave tweed