hi

as of a previous post here are some thoughts on file_naming and compression

As the documentation says on $Conf{CompressLevel}

Changing compression on or off after backups have already been done will require both compressed and uncompressed pool files to be stored. This will increase the pool storage requirements, at least until all the old backups expire and are deleted.

what about adding some kind of info-file to each pool file

i.e.
<MD5>_<SHA256>_<SHA512>.info

inside that "info" file we have the following:
-) MD5 of compressed
-) SHA256 of compressed
-) SHA512 of compressed
-) Compression-Ration as Number 0-9
-) uncompressed filesize

if we want to change levels afterwards we can read the "info" files and recompress the files if needed

re-compression works as
# checksum of compressed_original (be sure file is ok) + compare to cached 
# decompress
# compress with new level
# save compressed file as TEMP
# create checksum_TEMP of TEMP
# decompress TEMP
# create checksums of decompressed TEMP for rsync compare 
# compare cached checksums with recalculated checksums
# recreate checksum of TEMP + compare to old checksum_TEMP
# create TEMP.info (output of checksum_TEMP + parsing of old file for filesize + changing new compression level)
# change old checksum of old compressed to new checksum of new compressed (-> the cache "for future use" as of the post - "some thoughts of integrity tests for v 4..."
# remove old compressed + info and rename TEMP + info to original filename 

related Posts:
# some thoughts of integrity tests for v 4 ("RsyncCsumCacheVerifyProb")
# BackupPC 4.0 features - attribute file and backup storage

Greetings 
Mike