[mythtv-users] Diskless

[Rich West]

Rich West wrote:
> 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
>> 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..
>>
>> 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 2.6.19.2 kernel
> and give that a test this evening...

Got the kernel built and performed a few tests.  I realized that I 
needed to do a little more work in order to get it to work, so I put it 
off until this weekend.

First off, while the Fedora GUI is nice, I just don't like not knowing 
what it is really doing in the backend.  As such, it was next to 
impossible to debug what was going on when I couldn't get things to work 
(disklessrc init not found, etc). :(  I find it ironic that I can 
net-install a bunch of various versions of the OS and net-install 
Windows as well, but can't seem to get a diskless setup to play nice.  
It's one of those things that just seems *so* close..  Ugh.  I tried it 
by hand, and tried the GUI.. I think I got a little farther with the 
GUI, but, still, both turned up as failures..

-Rich