[mythtv-users] Diskless

Rich West Rich.West at wesmo.com
Mon Feb 5 18:15:00 UTC 2007

Richard Stanaford wrote:
> --- Original Message ---
> I came across the diskless how-to on the Wiki and, while I have the
> entire infrastructure in place (I build both windows and Linux boxes via
> PXE booting), I immediately thought "hey, that would definitely be
> However, the question that goes unanswered in the wiki is about
> performance. Since the entire system is loading and running off of the
> network (I've got cat-5 wired) *and* mythtv is also going over that same
> network, doesn't that beat up on the performance? Especially since swap
> would also be on an NFS mounted volume...
> I'd be interested to hear about people's experiences with MythTV and a
> diskless frontend to see if it would be a viable configuration or if it
> is just a pipe dream..
> Thanks!
> -Rich
> --- End Original ---
> Hi Rich,
> If your network is 100 Megabit-switched, you should have nothing to
> worry about, even if you want to stream HDTV.  Consider that under
> most configurations, HDTV encoding will commit up to 9GB per hour of
> video.  Nine gigabytes (or approx 9000MB) per hour is approx 150MB/min
> or 2.5MB/sec of data.  Well, we measure data (disk size) in Bytes (big
> B), but network speed is usually measured in bits (or bits per second,
> little b).  So, in order to smoothly stream HDTV to your frontend, you
> would need to be able to sustain 20Mb/sec ( 2.5 * 8, where there are 8
> bits in a Byte), which is a cinch for 100Mb-switched, assuming
> everything is running at full duplex.  When transferring file between
> my machines, my network routinely sustains 80Mb/s.  And since it's
> full duplex, that means 80Mb/s transmit and 80Mb/s receive at the same
> time.
> So, the bandwidth is there.  Since the worst you can likely do is
> stream an HDTV from each frontend, the only consideration is the
> common network segment, which is your backend to the network switch. 
> Say, for example, you wished to stream three HDTV streams to three
> different frontends.  Each frontend would put 20Mb/s on its own link
> to the switch which would aggregate to about 60Mb/s throughput for
> your server.  You're not likely to ever do that, but I wanted to use
> that as an extreme example.
> With a sufficient amount of RAM, you are not likely to have to NFS
> swap from the frontends, even with 256MB of memory, especially if you
> are doing hardware decoding.  The output from come through the network
> interface, across the bus to the decode hardware of your video card
> (or chipset), so there's really nothing to buffer.  Unless you have
> stuff running on the frontend box other than just MythTV, there should
> be nothing to swap either.  But even if it had to, your network would
> have to be taxed virtually to its "knees" for you to notice it.
> I hope this helps a little.
> -Rich.

Actually, it helps a lot.  :)  I've got a 24 port layer3 10/100 switch
as the central hub and 100Mb FD throughout.  The backend would represent
the only limitation only if each of the 3 frontends were pulling HD
streams at the same time, and the impact of that would have to be tested
before I would worry about it. :)  Since I only have SD streams, I'm
probably fine there. :)

I did some tests over the weekend when I had some time and discovered
that I have to go back and roll my own kernel rather than using the FC5
kernel.  It seems that it is missing a few key components (the ability
to have the root partition over NFS!).  Now, I didn't bother to try the
boot kernel and initrd.img that came with FC5 since I figured that was
based off of a much older kernel.  I'll compile up the kernel
and give that a test this evening...


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