[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 2.6.19.2 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).
-Rich
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