Block Change Tracking

Hello All,

Clarification question on Block Change Tracking File. From how I read documentation and blogs, I am understanding it as so.

When Oracle has to do a recovery operation it goes to the Block Change Tracking File reads through the file to determine what files have changed since last backup, and the file saves up to the last 8 incremental backups.

So that leads me to my question.

Our instance was recently brought down with a shutdown abort. When it was brought back up it took over 30 min to come back up. After digging it lead me to the CTWR. When I checked that file it was about ~86G (4 PDB's) our other CDB was only 100MB (6 PDB's).