A few words on how Oak names TAR files. The convention is to always start with adata00000a.tarfile. As data is written to the repository, new TAR files are added with increasing numbers, thus ending up withdata00001a.tar,data00002a.tarand so on.
Each time a compaction cycle ends, there is a cleanup phase in which segments from an old generation are purged. Those tar files that shrink by at least 25% are rewritten to a new tar generation, skipping the reclaimed segments. A shrunk TAR file increases its tail generation character, e.g. fromdata00000a.tartodata00000b.tar.