Coming back to this now almost a year later.
Thanks for your response Patrick.
On 1/12/22 13:25, Patrick Domack wrote:
It's because it doesn't move email by email to alt storage, but the
whole file. So the emails move as is, be them compressed or uncompressed.
I don't believe the above is correct, going on
https://doc.dovecot.org/admin_manual/mailbox_formats/dbox/#dbox-mbox-format:
"The granularity at which data is moved to alternate storage is
individual messages. This is true even for |mdbox| when multiple
messages are stored in a single |m.*| storage file. If individual
messages from an |m.*| storage file need to be moved to alternate
storage, the message data is written out to a different |m.*| storage
file (either new or existing) in the alternate storage area and the “map
index” updated accordingly."
Also, it says "only the actual message data (stored in files |u.*| under
|sdbox| and |m.*| under |mdbox|) is moved" and (presumably) not the
dovecot.map.index, which maybe suggests that when a message is moved it
can't be compressed/uncompressed, since the dovecot.map.index may not
change, in the sdbox case at least??
But the fact that the map index _does_ get updated when altmove happens
for mdbox according to the paragraph above did give me hope that it
might be possible. And I saw an email on this list from someone who
claimed/implied to be compressing using:
doveadm -o zlib_save=gz -o zlib_save_level=9 altmove
However haven't been able to get it to work, it seems on my testing that
indeed messages can only be compressed on initial delivery and
compressing them with altmove isn't (any longer) possible.
I'm still kind of curious whether it's a limitation in the mdbox format
and the way indexes are handled that precludes compress on altmove, or
whether it's a limitation of the altmove command, or whether altmove
maybe can do it but I just haven't got the options right yet. If I had
more time I'd dig in to the code :)
I think you could do what you want, but passing the correct options,
but I don't think it would be easy to do.
I would just store them compressed all the time, saves disk space, io
bandwidth, and if it goes over the network, actually will give a huge
performance improvement.
I have moved all mine to zstd currently, from gzip I had been using
since it was possible.
I will likely do the same.
Thanks again,
Alex
Quoting 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
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