[mythtv-users] Storing recordings on network share

[Simon Hobson]

Raymond Wagner wrote:

>  > Would a single back end have enough HP to run Myth and be a NAS server
>>  ?   What if there is a high demand for disk services on the NAS from
>>  time to time ?
>
>If MythTV is writing to local disks, and those disks are not being
>accessed separately from MythTV, then you can saturate the network and
>MythTV won't care.  Make sure you OS and database are also not on disks
>that would see high demand from other applications.
>
>>  What if the Myth BE needs to serve 1080 content to 3 to 5 FEs, some of
>>  them requiring transcoding?
>
>Serving content is trivial, as trivial as recording it in the first
>place.  All you're looking at is streaming some 14-18mbps over the
>network.  More importantly, if you briefly saturate the disk, who
>cares.  You get a bit of stuttering on the frontend.  It's not like the
>recording is damaged in any manner.

My experience is that recordings can easily be damaged if there is 
too much disk-I/O. Thus taking a scenario where one or more programs 
are being recorded, one or more are being played back, and then 
someone does some heavy I/O via the NAS service - eg the example of 
generating thumbnails for a directory fill of 20MB photos.

As you say, if the recordings are on completely separate dives it 
shouldn't be a problem. That may not be practical.

Now, I believe the problem is in the way Myth syncs data it writes. 
In an old thread, someone posted a comment to the effect that Myth 
syncs write data very frequently - which in the general case is a 
good thing to do since it means you write "little and often" and 
avoid building a up a big buffer and have the machine pause while it 
all gets written out.

I've asked since but never got a reply - is this the case ?

If it is, then I suspect the issue is that these frequent syncs mean 
that when disks get heavy I/O, instead of buffering the data, the 
process pauses waiting for the sync to return and incoming data gets 
lost. Thus your recording is now corrupted, and you get broken up & 
stuttering playback.

I've certainly been able to force such corruption to occur on-demand 
on my old system, even though on paper Myth should have had adequate 
memory to buffer the incoming data.

-- 
Simon Hobson

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