Apart from what has already been covered in the Nbu Admin guide vol1, this following is also a good read:
http://www.symantec.com/connect/blogs/frequently-asked-questions-netbackup-accelerator
one bit of information that is missed out is exactly what data about the filesystem is stored in the track log
From the article you linked to, it says this:
The NetBackup client sends to the media server a tar backup stream that consists of the following: The client's changed blocks, and the previous backup ID and data extents (block offset and size) of the unchanged blocks.
So the tracklog tracks block level changes (and "unchanges") on the volume since the last backup.
and how the track log is compared against the filesystem so quickly
In a traditional incremental backup without the use of Accelerator, the Nbu client would have to check evey single file on the client's volume to see which ones have been changed since the last backup, via the archive-bit or time stamp. Say the client has a total of 100 files, then during an incremental backup, all 100 files will have to be enumerated and checked. This takes alot of time.
With Accelerator, the changed blocks since the last backup are tracked. So, in the next incremental backup, only these changed blocks will be sent. On the file system level (one level up), only the files associated with the changed blocks will be checked by Netbackup for cataloging (To record filenames, locations, etc.). Using the same 100 files example, effectively, less than 100 files will have to be checked, which makes the process very fast.
This works with full backups too, only this time, all 100 files will be cataloged. The rest is the same (only changed blocks will be sent, etc.)
Accelerator uses storage-server-based "Optimized" synthetic backups to create fully cataloged backup images out of existing deduplicated data. More from this discussion here:
https://www.symantec.com/connect/forums/difference-between-synthetic-backup-optimised-synthetic-backup-and-virtual-synthetic-backup
Does it use the same techniques on ext3 and NTFS for example
Yes, apart from when you enable the use of the NTFS change journal (USN journal), which makes Accelerator even faster. Essentially, it is NTFS's own change log tracking.
With the NTFS change journal, during the step where Accelerator tries to associate the tracked changed blocks with actual files, instead of doing the investigation and associations itself, the file list will be readily available from the NTFS change journal.
There are special considerations for using the NTFS change journal. Please refer to the Nbu admin guide vol1 for full details.