Rich.West at wesmo.com
Sun Feb 11 23:09:39 UTC 2007
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
>> 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
>> 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.
> 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 18.104.22.168 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..
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