[mythtv-users] Diskless

Richard Stanaford rstanaford at gmail.com
Fri Feb 2 14:38:35 UTC 2007

--- 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 QUIET".

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..


--- 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.
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