[mythtv-users] Backend Hardware Questions

[Mark Wedel]

On 01/15/15 06:28 PM, Brian S wrote:
> Greetings all,
>
> Very Short Version: Do we think a Phenom II X4 B50 CPU stuck in a M4A87TD
> motherboard will have the ability to run well as a MBE & light-use desktop using
> an HVR-2250, 2 HDHR's, and 3 HDD's (OS on SSD)?
>
> Long Version: After running a rather computery mythtv setup for a few years, I'm
> attempting to downsize a little bit, and am wondering if converting my desktop
> into the only backend is feasible - it seems like it would be, but I have a
> knack for overlooking the obvious sometimes.
> Present setup is MBE w/HVR-2250 recording OTA + HDHR prime, SBE w/HVR-2250
> recording OTA (two antennas - I live halfway between Chicago & Milwaukee) and
> then a FE only.
>
> Was thinking I could move all BE's to an always-on desktop machine. Pertinent
> specs on the box in question:
> CPU Phenom II X4 B50 (4 core, 3.1Ghz)
> mobo Asus M4A87TD/USB3
> 8gb RAM (can bump to 16)
>
> So the general idea here is to pull the HDD's from the respective BE's and add
> them to the desktop computer, add one of the HVR-2250's to the desktop, and then
> replace the other BE recorder with another HDHR (due mostly to cabling issues).
> The OS runs on an SSD. The mobo has 6 sata slots so I should be ok with all the
> HDD's. (I realize I will also need to do other steps with fstab & database and
> what-have-you)
> The Big Question here is mostly just does the above machine have enough oomph to
> (theoretically but rather unlikely) record ~6 streams at once/run jobs/serve
> content to FE's while also being a desktop for mostly light-duty stuff? The only
> real CPU-intensive thing I can think I do is run virtualized windows XP for a
> few minutes at a time to scan documents to dropbox.
> And I suppose Big Question #2 is there anything I might be overlooking here that
> means this won't work or is a Bad idea?

  From a CPU perspectively, almost certainly.  I believe (not 100% familiar) 
that all those recording devices either just provide the OTA mpeg data directly 
to the system, or do actual mpeg encoding on the board.  So all the cpu has to 
do in those cases is move the data from the encoding device to the hard drive.

  Likewise, when serving that data to the other front ends, it is just reading 
the data off the disk and sending it over the network, which also requires very 
little cpu.

  What will require cpu is if you do transcoding and commercial detection.  In 
both cases, I'm pretty sure myth is good about not overloading the cpu, so it 
may take a long time to perform those actions, but shouldn't affect normal 
operation.

  I'm not sure the IO requirements for handling 6 potential streams at once. 
OTA streaming can vary, but ~8 GB seems to be about worse case, which 
corresponds to about 2 MB/sec - actual performance of hard drives can vary, but 
the IO pattern of myth is pretty friendly (sequential reads/writes), so that 
shouldn't be much a problem.

  In the distant past, I used a system as both the backend, frontend, and a 
linux desktop for my personal use (2 video cards - one for the FE, one for my 
desktop).  That was with much older hardware (2005 timeframe), never had an 
issue - I think I only had a single hard drive, but I also only had a single 
tuner card.

  As a more recent comparison, my backend is pretty similar - think it may even 
by the same motherboard, amd B95 quad core cpu, 4 GB memory, 3 OTA tuners, 9 
hard drives.  In addition to the mythbackend task, I'm also running zoneminder 
on it (7 capture devices, ranging from the composite on the ATI HDTV wonder, 
pinnacle nstc PCI card I picked up cheap, 2 pinnacle usb adapters and 2 
hauppauage hdpvr's).  And the zoneminder stuff by large takes up more of the 
system resources, as the motion detection means the cpu actually has to do some 
analysis.  And while the HDPVR's are nice devices (were $15 at local computer 
surplus store), the fact that they provide an encoded mpeg stream means that the 
system actually has to do more work to decode them for the motion analysis. 
But with all that, the backend works just fine, and as I type this, 3 of the 
cores have slowed down to 800 mhz (power saving) because there isn't anything 
for them to do.

  I don't run an SSD in my system - if you already have one, great, but for a 
system that is going to be on all the time, and ideally not rebooted very often, 
I don't care very much if it takes an extra few minutes to boot up.