[mythtv-users] Diskless

Wed Feb 14 05:37:11 UTC 2007

Hi Rich,

I have a semi-diskless setup for my Linux server, where I boot from a
local harddrive, but use a ramdisk for the root partition. All other
partitions are mounted over NFS. I have set the spin-down time for the
harddrive to 30 seconds so it gets quiet quite quick. I do have
occasional problems with unwanted disk access (not sure why), but those 
are rare.

I've just recently started experimenting with MythTV, and have no tuner 
card yet, but showing pictures and playing movies works just fine!

KR,

/Gunnar

Rich West wrote:
> 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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