[mythtv-users] Storing recordings on network share
linux at thehobsons.co.uk
Thu Sep 22 07:11:50 UTC 2011
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 &
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.
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