I have two sites that are Internet connected via standard ADSL, and that want to have a simple backup server in each site that uses the other site to store an off-site redundant copy of the data.
I plan a Linux server at each site, each with a small o/s drive and two pairs of large SATA drives using software RAID (one pair for each site). That combination should give a good idea of the budget available for this.... Forget hot-swap SCSI drives on hardware raid cards etc!
I can easily see how I could rsync the first raid array on site A ("A:md0") across to B:md0, and B:md1 to A:md1.
Where I need advice is picking the best backup approach for the users at each site to use to populate their respective backup storage in the first place. (Most of the users will of-course be using Windows, and should be treated as untrained monkeys for the sake of this project.) Given the bandwidth limited ADSL link between the two sites (but with high off-peak limits) I need a backup method which does not cause rsync to needlessly send unchanged data.
If I used something like Cobian Backup to create incremental backups, although the disk space usage would be relatively small the periodic full backups will cause rsync to send full backups across the link even though 90% of the content of those backups will already have been synchronised previously. I've seen solutions that use hardlinks between identical files to provide rolling backups on Linux, but I need to support Windows.
Any suggestions?