On 01/02/2023 14:07 EET Jose Celestino <japc@co.sapo.pt> wrote:

Hi,

I was wondering if it's possible to lighten the process of converting sdbox mailboxes to zlib by, instead of using dsync (that brings much more io, copying from alt - and copying of unneeded data as we have many mails already compressed from activating zlib a couble of years ago) just compressing each of the u files.

By compression of the u files I mean opening the uncompressed u file for reading, a u.temp file for writting and:

1. copy the dbox message header from u to u.tmp

2. compress the plain message and write it to u.tmp

3. fix the message_size_hex on the u.tmp dbox message header to the new, compressed size

4. copy the dbox metadata from u to u.tmp with Zphysical_size added

5. copy the u stat times to u.tmp

6. replace u with u.tmp

Is the message_size_hex stored anywhere else (indexes) that may cause this to not work?

Thank you.

The header around the u file is about the physical contents only, you can rewrite that header as long as you preserve any other fields.

You can use doveadm dump u.1 to see how it works.

Aki