I am running rsnapshot, and certain backups are taking up a lot of space. I've run rsnapshot-diff, and it gives results like this:
Between /home/backups/hourly.4/mybackupdir/ and /home/backups/daily.0/mybackupdir/:
1705 were added, taking 15518770836 bytes;
1705 were removed, saving 15518443844 bytes;
...but an rsnapshot du shows that the backup set takes up the advertised 15 GB or so. If I tell it to produce verbose rsnaphot-diff output, it appears to show that the same files were added and removed, so I'm somewhat confused as to why it's using up so much space - is there something wrong with the rsnapshot process that's causing it to use the extra space or something?
Sorry to answer my own question, but I think I've worked it out.
I'm dumping out the mysql databases every night, and these are currently occupying the magic 15GB in terms of disk space. I think that these files are different on a daily basis, and so rsnapshot backs up the difference as it is supposed to.
Because I'm keeping 7 daily and 4 weekly backups of this particular dataset, this unfortunately adds an extra 180GB overall!