Redmine

If postgres can replicate data in read-write fashion, you can have one instance of the DB and of redmine on each continent as long as the redmine config (DB, cookie secret, …) is the same everywhere, you'd only have to make sure to replicate the files directory across all instances on your own.

Many readers, one writer unless you've gone through great pains in redmine to use uniqueness properties like sha1 hashes. AND can do all write transactions through a separate connection to the main db.

http://wiki.postgresql.org/wiki/Streaming_Replication

http://www.slony.info/

Tis easy to mirror the files dir. The db, less so.

Dave Täht wrote:

Many readers, one writer unless you've gone through great pains in redmine to use uniqueness properties like sha1 hashes. AND can do all write transactions through a separate connection to the main db.

Nope to the hashes, and nope to the "write through a second connection". That might be doable, maybe there are plugins for that, but Redmine can't do it OOTB. You might have better performance with one instance per continent all connecting to a shared DB, but that's all that I can think of right now.