[Dovecot] Great time savings backing a mdbox versus Maildir

[Timo Sirainen]

On 9.2.2011, at 9.21, Javier Miguel Rodríguez wrote:

>    That is a "tricky" question to ask. It depends on usage, I think the following:
> 
> 
>    - LDA delivery: load is a bit lower (on disk) in Maildir vs mdbox: in both cases the message has to be written, indexes are updated, in Maildir indexes are in ram, so lower "disk" load in this case
> 
>    - POP3 access: the same as the previous post
> 
>    - IMAP access: this is tricky. In mdbox a /"delete message"/ command only lowers the refcount, indexes are updated and in the night a cron job runs doveadm purge. In Maildir, you really delete the message when MUA/webmail /"compacts"/ the folder, and indexes are updated. I think that mdbox has a /"delayed IO" /in this case, and has less load on disk on "production hours".
>    Am I missing anything?

Yes, in theory those are right. I'm interested in finding out some real numbers :)

> The stats in the SAN after the change maildir->mdbox do not help, we have zlib enabled in lda & imap with mdbox, so our # of real IOPs is lower than Maildir (we did not have zlib enabled)


I wonder how large a write can be before it is split to two iops.. With NFS probably smaller I'd guess. Still, I would have thought that even if zlib writes only half as much, the disk iops difference wouldn't be nearly as much.