ZFS storage and backup

[Joseph Tam]

On Fri, 19 Nov 2021, James wrote:

> On 15/11/2021 16:18, infoomatic wrote:
>> Regarding
>> storage I tend to use sdbox, from what I have read it seems to be the
>> better option when using a COW filesystem compared to mdbox. One more
>
> https://doc.dovecot.org/admin_manual/mailbox_formats/
> sdbox   single-dbox, one message per file.
> mdbox   multi-dbox, multiple messages per file.
>
> so I guess sdbox is better with ZFS.  I could test each but I think I will 
> find the IO used by dovecot is low for each.  I have one user with 32,164 
> emails in INBOX and IO is not a problem.

It depends on what aspect of performance you're talking about and how it
is implemented, but as I understand it, ZFS snapshots are done at the
block level, and just as long as mdbox leaves message blocks in situ
(by manipulating indices instead?) and doesn't shuffle them around,
unchanged messages won't bloat snapshot storage, unlike MBOx where a
one message insertion/deletion at the beginning will cause the entire
mailbox to end up in snapshot storage.

>> question is: compression at file system level or in dovecot storage?

This relates to my comment -- if the compression is done at the message level
rather than the whole MDBOX, the above is not applicable as any change to
a byte will affect all subsequent bytes.

I think MDBOX is a compromise in data granularity that tries to strike
a balance between various aspects of I/O performance.

Joseph Tam <jtam.home at gmail.com>