Hi,
None the two methods that you mean in the user's guide are suitable for my
case. "Image+normal incremental" that you emphasized in your post means
getting full image backups for example every week. For the incremental part,
one file-based full backup is needed which is a nightmare for 20 millions.
OK, if I accept the initial incremental backup time (that might take for
days), what happens in restoration?

Naturally, last image backup should be restored first and it will take A
minutes. Provided that image backups are weekly, the progressive incremental
backups of the week is about 6*20MB=120MB. Now imagine 120MB of 15-20K files
are to be restored in filesystem with an incredibly big file address table
and system should create an inode-like entry for each. If this step takes B
minutes, the total restoration time would be A+B. (A+B/A) ratio is important
and I will try to measure and share it with the group.

Steven, your solution is excellent for ordinary filesystems with a limited
number of files. But I think for millions of files, only backup/restore
method that do not care how many files exist in the volume are feasible.
Somehing like pure image backup (like Acronis image incremental backup)  or
the method that FastBack exploites.

Your points are welcomed.

Regards,
Mehdi Salehi