[Ocfs2-users] Shared Writeable MMAP

[Joel Becker]

On Wed, Jun 12, 2013 at 10:34:05PM +0200, Robert Schetterer wrote:
> Am 12.06.2013 20:56, schrieb Joel Becker:
> > On Wed, Jun 12, 2013 at 02:27:05PM +0200, Robert Schetterer wrote:
> >> Hi @ll i have a question about feature
> >> Shared Writeable MMAP and its relation to concurent file use.
> >>
> >> Background, over years i am hosting dovecot imap servers behind
> >> loadbalancers on a drbd master/master setup shared formated storage
> >> ocfs2 1.4.3 maildir without problems.
> >>
> >> the dovecot wiki says
> >>
> >> http://wiki2.dovecot.org/Director
> >>
> >> Director can be used by Dovecot's IMAP/POP3/LMTP proxy to keep a
> >> temporary user -> mail server mapping. As long as user has simultaneous
> >> connections, the user is always redirected to the same server. Each
> >> proxy server is running its own director process, and the directors are
> >> communicating the state to each others. Directors are mainly useful for
> >> setups where all of the mail storage is seen by all servers, such as
> >> with NFS or a cluster filesystem
> >>
> >> so the director service is most installed on the loadbalancers mainly
> >> with nfs storage, proved to be a "must have"
> >>
> >> after online investigation and long lasting reality tests , i guess "or
> >> a cluster filesystem " isnt true for ocfs2, cause on drbd it works fine
> >> with concurent files use ( i.e mailbox format maildir ) by feature
> >> Shared Writeable MMAP.
> >>
> >> Can sombody confirm this, or teach in easy words how it works ( or not )
> >> and why ?
> > 
> > 	I'm not sure what you are asking.  I *think* you are saying,
> > "ocfs2 is not a sufficient cluster filesystem for Dovecot
> 
> no my meaning is reverse of that, i am running it fine since years ,
> but others believe using a director service on the loadbalancer is a
> "must have" avoiding concurent use of shared maildir storage and dovecot
> indexes, which is definite true for nfs, but for my understanding its
> not needed using drbd/ocfs2 cause of feature  i.e Shared writeable mmap
> or additional features.
> 
> In using that setup over years without directors, i didnt noticed any
> errors nor i could produce one with file tests or with imap clients

	I believe you are correct.  ocfs2 will make sure your files are
kept in sync between the nodes.  So "directors" are not needed.

> , because it
> > does not have shared writeable mmap(2)."  If that's what you are saying,
> > then you are incorrect.  Shared writeable mmap(2) is a very standard
> > feature of ocfs2; we can't be POSIX without it.
> > 	I also don't understand how you are configured.  Because it
> > sounds like you are using ocfs2+drbd successfully, which conflicts with
> > your statement that ocfs2 is not a sufficient cluster filesystem.
> 
> sorry ,descriptions of what exact "Shared writeable mmap" is, is hard to
> understand for none kernel hackers which are not natural english
> speakers, so i asked for clarification

	mmap(2) is a system call where a program treats an open file as
if it were just an area of memory.  Rather than issuing read(2) or
write(2) calls, the program just accesses the memory with pointers.  The
OS and filesystem handle reading and writing data when this happens.
	On a local filesystem, this mostly Just Works.  Cluster
filesystems have to make sure that modifying memory on node A will
cause node B to see the changes.  That is what is meant by "shared
writable mmap."  ocfs2 supports this.

Joel

> 
> > 
> > Joel
> > 
> >>
> >>
> >> Best Regards
> >> MfG Robert Schetterer
> >>
> >> -- 
> >> [*] sys4 AG
> >>
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> >>
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> > 
> 
> 
> 
> Best Regards
> MfG Robert Schetterer
> 
> -- 
> [*] sys4 AG
> 
> http://sys4.de, +49 (89) 30 90 46 64
> Franziskanerstraße 15, 81669 München
> 
> Sitz der Gesellschaft: München, Amtsgericht München: HRB 199263
> Vorstand: Patrick Ben Koetter, Axel von der Ohe, Marc Schiffbauer
> Aufsichtsratsvorsitzender: Florian Kirstein
> 
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