Using NFS to extend local email storage

[Marc Roos]

What about moving the mailserver on-premises, buy a big UPS and do some 
batched smtp on the cloud, so if you are down, you will not lose 
incoming mails?


-----Original Message-----
From: Maciej Kokociski [mailto:maciej.kokocinski at hands.pl] 
Sent: Sunday, October 18, 2020 7:51 PM
To: dovecot at dovecot.org
Subject: Using NFS to extend local email storage

We are running our IMAP mail server on a VPS in the cloud 
(dovecot+exim+horde webmail), and we are using our on-premises NAS for 
daily backups through custom rsync scripts (which work well with 
maildir). There is a lot of storage available on the NAS, while not so 
much on the VPS. Since, we often get downtime on-premises due to power 
outages we cannot move the mail server on-premises (though the outages 
do not interfere too much with the backups). Additional storage in the 
cloud is at a premium as we would like to avoid further increasing 
costs.

Now, my idea is to set up an NFS volume on the NAS, and configure the 
cloud-based server as the client. I would like to store only the most 
recent emails on the main server, and keep the whole data set on the 
NAS.

Naturally I have read the dovecot wiki concerning NFS, and I have 
browsed this mailing list's archive for answers, but I still have many 
concerns.

Ideally, I would like to use as much storage as possible from the main 
server, so that only really old emails need to be fetched over the 
network if requested over IMAP. Also I would like the search function to 
work regardless whether the NAS is available or not.

I wonder if anybody tried a similar configuration before. My concerns 
are:
- should I use NFS volume directly and configure FS-Cache, or should I 
configure a cache volume separately using e.g. bcache or some other 
tool?
- how to configure the cache to keep as many files and metadata locally?
- is it possible to fine tune the cache so it always keeps all the newly 
created/modified files locally?
- is it possible to tune the cache so it keeps at least n files for each 
user (each maildir)?
- will it work fine if I put NFS traffic into an ssh tunnel for 
security?
- will dovecot freeze/crash when the NFS volume disconnects or is 
temporarily unavailable, what will actually happen in such 
circumstances?
- will the search function work when the NFS volume is unavailable? (I 
also plan to use solr in the future)
- what happens when the main server crashes and then recovers from the 
NFS share? is the share guaranteed to be consistent?

I hope that I have not asked too many questions for a single message... 
;) I would really appreciate any help, because I am rather a newbie in 
dovecot administration, and it overwhelms me. Perhaps, what I am trying 
to achieve is not the way to go. I am open to suggestions.

Best regards,
Maciej