[mythtv-users] Some master/slave backend advise required

Henk D. Schoneveld belcampo at zonnet.nl
Tue Jun 3 03:19:29 UTC 2014


On 02 Jun 2014, at 20:59, Johan Van der Kolk <johan.vanderkolk at gmail.com> wrote:

> Hi,
> 
> I’m running myth 0.27 (v0.27.1-7-g41d04b6) on Ubuntu server 14.04 with an i5(3GHz) and 4GB memory. I’m using 4 DVB-S2 tuners as source.
> Due to relocation of the dish I have to install a slave backend. I can’t (don’t want to) move my server to the garden shed.
> 
> I have found that recording about 8-10 HD channels simultaneously will start killing processes on the backend (not the myth processes though), but the CPU load of the DVB-s2 stuff by itself requires 100% cpu. Mythcommflagging did not help either, although I could limit the simultaneous jobs to get more breathing space. 
> So my thought was to solve both problems at the same time with a new slave backend, where the dvb-s2 stuff is running, by itself. Storage and commflagging still to be done on the master backend (which has 8TB of ZFS storage)
> 
> I can see two issues now:
> Bandwidth: When recording 10 channels (what I want to achieve), I estimate worst case (based on what i recorded so far) 3GB per channel per hour, or 833Mb/s one way traffic, without other overhead. And not watching anything…
10 * 3GB = 30.000MB per hour. 30.000 / (60 * 60)sec = 8,33MB/s
Every single current HD should be able to do that if it doesn’t write randomly, in small blocks. I use xfs, where it is possible to allocate blocksizes of 1M - 500M to solve fragmentation of recordings. see:  http://www.mythtv.org/wiki/Optimizing_Performance 
> How to solve this, and does myth traffic between master and slave benefit from Jumbo Frames. I could use two network cards in each machine, and create a 2 x1 Gb trunk between the switches.
> Second part of the problem might be that the ZFS file storage is not fast enough. (upgrade to SSD maybe)
> 
> Configuration:
> Is it possible to configure mythtv in such a way that it does what I want, or are there better ways to do this? 
> 
> Any help appreciated!
> 
> Johan
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