[Borgbackup] Lots of files that change rarely and predictably

[Thomas Levine]

I have a directory of about 670,683 files in a single borg repository.
Archive creation takes a long time because borg checks all of the files.
I should be able to make the backup go faster because the files change
very predictably, and I seek your advice on how to do this.

The directory contains emails in mail handler format, with one email per
file. The subdirectories look like this, "->" indicating symbolic links.

  old/2006
  old/2007
  old/2008
  old/2009
  old/2010
  old/2011
  old/2012
  old/2013
  old/2014
  old/2015
  old/2016
  old/2017
  drafts
  old/current -> old/2017
  inbox -> old/current/inbox
  sent -> old/current/sent

(It is in fact more complicated, but this gets the point across.)

Usually, only the files in "drafts" and the target of "old/current"
change. I want to configure the backups to take advantage of this
pattern, by having borg ignore the files that I know have not changed.

The only way I know to do this is to separate repositories. For example,
I could use a separate repository for each new "old" directory and
create new borg archives only for that directory in that repository.
Are there other safe ways to instruct borg to consider changes only to
particular files? I think it would work to check the files cache only
for the files in the target of "old/current", treating all other indexed
files as unchanged. But I'm not sure, and it does not seem to be
supported at the moment.

Thank you