[mythtv-users] diskless backend?

[Mark Cooke]

On Mon, 2005-10-10 at 09:20 -0500, Joe Harvell wrote:
> I am considering building a diskless mini-ITX frontend/backend using NFS 
> over switched 100base-T for the storage.  I would be capturing at most 
> two streams at a time and watching at most one.
> 
> Has anyone done this?  Any problems with the setup?

Dear Joe,

I have a diskless backend with a 350 and a 250 [*].  It works well here,
but I did need to tune up the NFS parameters.  2.6 kernels, big buffers,
async, tcp, nfs version 3.

I don't do playback on the backend, but I do use it for transcoding,
which does give 4 streams max (1 to the BE and 3 from the be).  You need
to be sure that your hard disks are fast enough on your storage server
to cope with this (eg raid-stripe).

You need to be sure your network cards and switch have negotiated full-
duplex 100base correctly.

> Also, when watching live TV, does the backend read from the file and 
> stream to the backend?  Or does it take what it is capturing and both 
> write it to the file and stream it to the frontend?  If it reads it from 
> the file, then is there any likelihood of the data being cached?

Not sure, but I believe it reads from the file.  There's shortcut code
to optimise out a network stream if the file is available from disk on
the frontend, but that doesn't help the diskless situation.

Cache would depend on how much ram your backend has and how far behind
live you are (~2GB/hour is my recording rate).  My backend has 1GB ram,
and the storage server 2GB.

I have very occasional trouble with sync loss if I've been plugging in
and out USB devices due to some big pauses in the kernel during that
process.

Final note - one thing I do run into is PCI bus saturation on my storage
server.  Plenty of CPU left, but everything blocks up waiting on IO.
Can really play havoc with recordings.

Cheers,

Mark

[*] Historical accident, as the primary storage server was highly
unstable with early ivtv releases.  I setup the diskless BE as a
workaround.