Well that's not a great support response -- I'm sorry about that.
Arq 3 is good about this when backing up to S3, but not to Glacier. It uses the most recent backup record as its starting point and backs up new/changed files. (When backing up to S3, it gets an object list from S3 so it knows what's there.)
But if the most recent backup record of your Library/Mail folder has the files in it, then it shouldn't re-upload everything. Arq backs up in 2 steps. First, it estimates the amount to be uploaded by comparing the timestamps and other metadata of files on disk to the most recent backup. Then it does the backup. For each file with a different timestamp, it calculates a checksum of the file's contents. If the contents are the same, it doesn't re-upload.
Somehow the records of my Photos folder got "lost" and completely reuploaded, all the incremental are gone. And this was not even after I adopted an old backup, just happened randomly. It reuploaded 15 GB before I saw what was going on. So now my Glacier is about double the size it needed to be.
Support responded and had me send in the logfiles. I looked over the logs and there was nothing helpful there. Sure enough support could not help me and I am left with no solution.
This kind of stuff really makes me wary about the whole software. Strangely enough the other folders did not see the same thing happen.
Arq 3 is good about this when backing up to S3, but not to Glacier. It uses the most recent backup record as its starting point and backs up new/changed files. (When backing up to S3, it gets an object list from S3 so it knows what's there.)
But if the most recent backup record of your Library/Mail folder has the files in it, then it shouldn't re-upload everything. Arq backs up in 2 steps. First, it estimates the amount to be uploaded by comparing the timestamps and other metadata of files on disk to the most recent backup. Then it does the backup. For each file with a different timestamp, it calculates a checksum of the file's contents. If the contents are the same, it doesn't re-upload.
Was it really re-uploading everything?