NFS vs Replication

[Gerald Galster]

> I built an email system using a proxy / director pair (IMAP, POP3, LMTP)
> and a backend pair.
> 
> To have an HA system, I would like to understand if it is better to use
> an NFS export or replication to save emails and index files
> 
> NFS is provided by a NAS (in HA), while for replication I would use the
> local backend disks
> 
> Which of the two systems is more reliable? Are there any drawbacks for
> one or the other?

This decision is more about how many users you have in total and how you
can partition them.

A) 200 domains with 10 IMAP accounts each

For high availability two dovecot servers with replication are sufficient,
no director/nfs needed. Return both server ips via dns for imap.domain.com
and you get active/active load balancing for free.

There is no shared storage which means no locking problems.
Dovecot can use optimizations like mmap which is not possible with nfs.


B) 200000 IMAP accounts, all within the same domain

You cannot partition by domain and a single server cannot handle the load.

Here imap.domain.com could return e.g. 5 ips via DNS that point to your directors.
The director's job is to send all connections of one particular user to the
same backend, i.e. Outlook at work, Thunderbird at home and K9 Mail on a
mobile phone could be active at the same time, but all are directed to the
same backend server. This way locking issues with nfs are avoided because
only one server is accessing the mailbox at a time.

IIRC you need to monitor your backend servers and add/remove them on failure.

If the nfs mount is not available on the backend, dovecot may create
a new (empty) mailbox, which could break things. You need to set permissions
in a way that cannot happen.


C) like B) but with a static proxy mapping where users are assigned to a 
certain backend server by configuration, that could be replicated like A)
without nfs. 


While A) in principle has a higher performance due to local disks and
optimizations B) can have a higher overall performance as dedicated
storage appliances usually have a lot more disks (ssd caching, ...)
and 10G+ networking.

C) avoids nfs but may introduce more complexity when software like pacemaker
is used to provide failover.

See https://wiki2.dovecot.org/Director and https://wiki2.dovecot.org/NFS


Best regards
Gerald