Compression: all or nothing?

[Alex King]

Using 2.3.13 (89f716dc2) (debian package), with mdbox storage.

I considering adding the zlib plugin.  I understand that reading mail 
can cope with either compressed or uncompressed mailboxes.

I'm wondering if I can leave the main storage uncompressed, while having 
the alternate storage compressed.  There does not seem to be a way to 
configure this, from 
https://doc.dovecot.org/settings/plugin/zlib-plugin/ it seems 
compression (on saving emails) is an all-or-nothing option?

Can I leave zlib_save unset in my main configuration, and have doveadm 
use some alternate configuration with zlib_save set?  It doesn't look 
hopeful because I don't see the ability to use a different config at 
https://wiki.dovecot.org/Tools/Doveadm/Altmove.

Perhaps I would need multiple copies of the daemon running?

Or is it better just to compress everything?  I plan to store older 
mails (perhaps 3months+) in the alternative storage, and was thinking it 
would be nice to have those compressed, while I don't need to compress 
the main store.

Any feedback welcome.

Thanks,
Alex