[mythtv-users] Help: Disk setup for multiple backends.

[Bruce Markey]

Robert Kulagowski wrote:
> 
>> Should the slave-backend save his files locally and the
>> master will be able to stream them or should the slave save his files
>> to a mounted disk from the master?

If a file is stored on a slave the backend at that slave will
stream the file. Any frontend can watch a file stored on any
backend, master to FE on a slave, slave to FE on the master,
slave to FE on another slave, or a frontend only system can
play files from the master or any slave.

> Personally, my masterbackend hosts all the storage.  This way, even if 
> the slave backend isn't running you can still watch the program.  
> There's a setting called "Masterbackend Override" that will have the 
> master backend stream the file if the slave backend isn't available.

Robert, I see nothing wrong with using your approach when it
is the users best option but in answer to the question, I'd like
to point out some advantages of local disk to consider. I always
write to local disks for reliability and efficiency.

The most obvious thing is that if you watch a local recording in
progress or live TV from a slave, the data is written over the
network to the file then read back across the network for playback
crossing the network twice. If each backend writes locally, the
data only goes over the net once for remote playback and not
at all for local.

I can have as many as four systems recording at the same time.
If I had one NFS mounted dir, not only would the three slaves
each be sending data over the network constantly but the disk(s)
would be thrashing as it oscillated to append to each of the four
files. Writing video files can be very efficient if the disk heads
are positioned and it streams to contiguous blocks and makes
track to track seeks. Disks do not write while they are seeking(!)
so not only would there be four times as much data to write but
more time would be spent seeking and less time writing. Even
if your network and disks can keep up, there is more opportunity
for problems plus this doesn't scale as well as using local disks.

The other reason I like local disks is that there is always
the possibility of network outages or periods of high latency
or any number of network difficulties. If there is a problem
during playback, well, that's annoying but you can still watch
the show after the problem is resolved. If there is a problem
while recording to an NFS file then the file may be damaged and
you may never get to see the show. If you are using wireless
802.11g, have some odd bridge or repeater, have a mixed network,
other apps that may occasionally flood the network, etc. it would
make sense to use local disks for your slaves.

--  bjm