[mythtv-users] Diskless

Rich West Rich.West at wesmo.com
Tue Feb 13 23:05:17 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
>> 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 kernel
> and give that a test this evening...

It looks like I was wrong.. I didn't have to roll my own kernel. :)  I 
followed the Myth Diskless how-to and managed to get the system to 
boot.  But (isn't there always a 'but') the entire system is coming up 
with its filesystems mounted read-only.  I dug pretty far in to it and 
it looks like everything is hinging on the fact that the filesystem is 
read-only, and it then fails to mount the read-write files over top of 
it.  Unfortunately, a lot spews to the screen causing the message to be 
lost before I can actually read it, and, since it is read-only, the 
message gets lost forever

It is just *so* close.  I've managed to get it up if, and only if, I :
o boot the system and watch all of the failures, waiting for it to 
*eventually* error out on starting X.
o log in via the console
o mount -o remount,rw /var/log
o mount -o remount,rw /var/lib/mythtv
o init 3
o init 5

This gets the system up and I can watch TV, get the weather (I use 
mythweather-revamp with some hacks!), etc.  I cannot, however, play any 
DVD ISO images or display any gallery pages because those are mounted 
off of NFS mount points, and, oddly enough, I cannot get nfsd to run 
(but everything is mounted via NFS..?).

I looked at MiniMyth (www.linpvr.com) as an option, and my brain 
screamed in agony.

I'm beginning to agree with the folks who responded with "I tried a 
diskless setup, but it was too much work" and "slapping in a laptop 
drive + adapter was easier".  Diskless looks nice as a concept, but, 
since so much "magic" tends to happen along the way, it seems more like 
a pipe dream than a reality.  It looks like I should just toss in the 
towel and use disk-based setups (diskless would have been excellent 
since I have three identical FE's with only minor config differences).


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