It's important to realize that a mixed V3/V4 installation will use more inodes than just V3 or V4 alone. If you migrate an existing V3 installation to V4 (using BackupPC_migrateV3toV4), the inode usage to store the backup trees will double, while the inode usage to store the pool files will be the same.
While V4 doesn't use hardlinks (except transiently for atomic renames etc), there are cases where a pure V4 installation will use more inodes than V3.
Here's an explanation. In V4, each directory in a backup tree consumes 2 inodes, one for the directory and the other for the (empty) attrib file. In V3, each directory in a backup tree consumes 1 inode for the directory, and everything else is hardlinked, including the attrib file. In both V3 and V4, each pool file consumes one inode.
So when you migrate a V3 backup, the number of inodes to store the backup trees will double. The pool inode usage shouldn't change much, but with lots of backups the former number dominates.
In a new V4 installation the inode usage will be somewhat lower, since in V4 incrementals don't store the entire backup tree (just the directories that have changes get created). In a series of backups where the directory contents change every backup, including the pool file, V4 will use 3 inodes per backup directory (directory, attrib file, pool file), while V3 will use 2 (directory, {attrib, pool} linked). So the inode usage is 1.5 - 2x.
Craig
