mdbox: purge only if it's worthwhile

[Sven Strickroth]

Hi,

with the mdbox storage one needs to regularly run purge to really clean 
up deleted mails from the storage.

Running purge can be an IO intensive task as the files are append-only 
and even if in a 10 MB mdbox file only a small mail is deleted all other 
mails need to be copied to a new file.

Therefore, I wonder whether there is the option to apply a threshold 
before a mdbox file is cleaned up, e.g. only purge an old mail if this 
affects > 10% of the mdbox filesize or if at least X bytes will be freed 
in an mdbox file.

Is this possible ATM? Otherwise this can be seen as a feature request.

Best,
  Sven

PS: From the docs it's unclear to me how the mdbox rotating algorithm 
works. Is only to the latest mdbox file appended or it is possible that 
a small mail might be appended to an older mdbox file?