[ceph-users] dovecot + cephfs - sdbox vs mdbox

Wed May 16 22:35:22 EEST 2018

Hello Jack,

yes, I imagine I'll have to do some work on tuning the block size on
cephfs. Thanks for the advise.
I knew that using mdbox, messages are not removed but I though that was
true in sdbox too. Thanks again.

We'll soon do benchmarks of sdbox vs mdbox over cephfs with bluestore
backend.
We'll have to do some some work on how to simulate user traffic, for writes
and readings. That seems troublesome.

Thanks for the plugins recommendations. I'll take the change and ask you
how is the SIS status? We have used it in the past and we've had some
problems with it.

Regards,

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

On Wed, May 16, 2018 at 4:19 PM Jack <ceph at jack.fr.eu.org> wrote:

> Hi,
>
> Many (most ?) filesystems does not store multiple files on the same block
>
> Thus, with sdbox, every single mail (you know, that kind of mail with 10
> lines in it) will eat an inode, and a block (4k here)
> mdbox is more compact on this way
>
> Another difference: sdbox removes the message, mdbox does not : a single
> metadata update is performed, which may be packed with others if many
> files are deleted at once
>
> That said, I do not have experience with dovecot + cephfs, nor have made
> tests for sdbox vs mdbox
>
> However, and this is a bit out of topic, I recommend you look at the
> following dovecot's features (if not already done), as they are awesome
> and will help you a lot:
> - Compression (classic, https://wiki.dovecot.org/Plugins/Zlib)
> - Single-Instance-Storage (aka sis, aka "attachment deduplication" :
> https://www.dovecot.org/list/dovecot/2013-December/094276.html)
>
> Regards,
> On 05/16/2018 08:37 PM, Webert de Souza Lima wrote:
> > 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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> > ceph-users at lists.ceph.com
> > http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com
> >
>
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