NFS vs Replication

[Andrea Gabellini]

Thank you all for replies!!!

Some missing infos...

- As load balancer I'm using a pair of keepalived with simple setup and
not the DNS
- Load balancer algorithm is "Weighted Least-Connection"
- About 20 domains and 3000 email
- I'm monitoring my backend servers with poolmon
- The backend servers are virtual machine (vmware) with datastore on
"all flash" storage

based on yours notes, I think the better choice is Replication. Correct?

Thanks,
Andrea

Il 16/07/20 01:43, Gerald Galster ha scritto:
>> 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
>
>
>
>

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TIM San Marino S.p.A.
Andrea Gabellini
Engineering R&D
TIM San Marino S.p.A. - https://www.telecomitalia.sm
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