dovecot + cephfs - sdbox vs mdbox

[Webert de Souza Lima]

I'm sending this message to both dovecot and ceph-users ML so please don't
mind if something seems too obvious for you.

Hi,

I have a question for both dovecot and ceph lists and below I'll explain
what's going on.

Regarding dbox format (https://wiki2.dovecot.org/MailboxFormat/dbox), when
using sdbox, a new file is stored for each email message.
When using mdbox, multiple messages are appended to a single file until it
reaches/passes the rotate limit.

I would like to understand better how the mdbox format impacts on IO
performance.
I think it's generally expected that fewer larger file translate to less IO
and more troughput when compared to more small files, but how does dovecot
handle that with mdbox?
If dovecot does flush data to storage upon each and every new email is
arrived and appended to the corresponding file, would that mean that it
generate the same ammount of IO as it would do with one file per message?
Also, if using mdbox many messages will be appended to a said file before a
new file is created. That should mean that a file descriptor is kept open
for sometime by dovecot process.
Using cephfs as backend, how would this impact cluster performance
regarding MDS caps and inodes cached when files from thousands of users are
opened and appended all over?

I would like to understand this better.

Why?
We are a small Business Email Hosting provider with bare metal, self hosted
systems, using dovecot for servicing mailboxes and cephfs for email storage.

We are currently working on dovecot and storage redesign to be in
production ASAP. The main objective is to serve more users with better
performance, high availability and scalability.
* high availability and load balancing is extremely important to us *

On our current model, we're using mdbox format with dovecot, having
dovecot's INDEXes stored in a replicated pool of SSDs, and messages stored
in a replicated pool of HDDs (under a Cache Tier with a pool of SSDs).
All using cephfs / filestore backend.

Currently there are 3 clusters running dovecot 2.2.34 and ceph Jewel
(10.2.9-4).
 - ~25K users from a few thousands of domains per cluster
 - ~25TB of email data per cluster
 - ~70GB of dovecot INDEX [meta]data per cluster
 - ~100MB of cephfs metadata per cluster

Our goal is to build a single ceph cluster for storage that could expand in
capacity, be highly available and perform well enough. I know, that's what
everyone wants.

Cephfs is an important choise because:
 - there can be multiple mountpoints, thus multiple dovecot instances on
different hosts
 - the same storage backend is used for all dovecot instances
 - no need of sharding domains
 - dovecot is easily load balanced (with director sticking users to the
same dovecot backend)

On the upcoming upgrade we intent to:
 - upgrade ceph to 12.X (Luminous)
 - drop the SSD Cache Tier (because it's deprecated)
 - use bluestore engine

I was said on freenode/#dovecot that there are many cases where SDBOX would
perform better with NFS sharing.
In case of cephfs, at first, I wouldn't think that would be true because
more files == more generated IO, but thinking about what I said in the
beginning regarding sdbox vs mdbox that could be wrong.

Any thoughts will be highlt appreciated.

Regards,

Webert Lima
DevOps Engineer at MAV Tecnologia
*Belo Horizonte - Brasil*
*IRC NICK - WebertRLZ*
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