[Asterisk-Users] MySQL replication for voicemail

[Josh McAllister]

> 
> Hi Josh -
> 
> > Another approach you may want to consider for data redundancy that
> > does not rely on MySQL's finicky replication stuff is DRBD. Think of
it
> > as RAID-1 across Ethernet. I have used it in production on some VERY
> > busy (> 1200 qps) MySQL servers for a couple years with no problems.
> 
> I would very much welcome not having to use a MySQL's replication
> (sorry to all you MySQL geeks)!  I've read up a little on DRBD before,
> but I've avoided it in this particular application because of a few
> questions I had about it:
> 
> 1) Does it work over slow-ish links? In this case, I'm going over WAN
> links to offices around the country.


This would be a function of the amount of data being changed, and the
speed of your link. DRBD does support ASYNC mode, and there is a good
chance this could work for you.

> 2) Can it do two-way replication?
No. This is primarily because of the limitations of the overlying
filesystem. (DRBD sits below the FS). With newer fs's like GFS, this is
now feasible, but DRBD is a little behind. Keep your eyes open for 0.8x
to be released as it will support 2-way with GFS. If you want to
maximize hardware resources, you can of course setup a partition
replicating A -> B, and another replicating B -> A.

> 3) Can it do N-way replication (i.e. multiple slaves)?  We have
> several offices offices that I'd like to have the replicated info.


I know this was something that was in the works a while back. I haven't
followed this issue that closely though, so I'm not sure if this has
been worked out or not. I believe the proposed approach was to simply
get rid of some arbitrary limitations DRBD imposed on letting you layer
drbd on a drbd device.

> 
> Thanks!
> Noah
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To summarize, if you are currently doing MySQL replication with multiple
masters, and multiple+N slaves, and you're trying to duplicate that...
DRBD is probably not going to work for you. But I can say that what it
does it does very well, and with little overhead. I've seen < 5%
reduction in write performance with 15K SCSI drives, IE ~ 70MB/sec
writes with DRBD vs ~72MB/sec without.