[mythtv-users] video artifacts on recordings: could building a raid be the cause?

UB40D ub40dd at googlemail.com
Wed Dec 27 09:48:00 UTC 2017

I rebuilt my myth box after several months of downtime and I decided to
bite the bullet and finally build a RAID to store the recordings. This is
on ubuntu 16.04, using mdadm.

Several (all?) recordings made yesterday had video artifacts ranging from
mild to unwatchable: square patches in areas with motion, frame hiccups,
sometimes the bottom 2/3 of the frame without picture.

Could this be due to the fact that the raid array is being built in the
background? I originally didn't think it would, given that
- the machine usually copes with several simultaneous recordings
- top says that the md0_raid5 rebuild process only takes between 2% and 20%
of the cpu

On the other hand, test recordings made the day before yesterday, before
starting to build the raid array, came out without glitches, which seems to
point the finger at the raid and also exclude the hypothesis that I should
take a ladder and clean the satellite dish or LNB (also, yesterday was a
fine day and it wasn't snowing, so that was not the reason either).

Building the raid array seems to take 2-3 days (!) so I'm not super-keen to
stop it just to check if recordings come out clean. I did have to
power-cycle the machine at some point and had to restart building the raid
from zero ;-(

Does it seem plausible to anyone that building the raid could be the cause
for the video artifacts? The machine is a 4-core i5 with 8 GB RAM. Note
that the faulty recordings were made to an independent HDD, not to the raid
that was being built.

I can imagine the problem being disk bandwidth rather than CPU. I have 6
SATA ports on the MB and 4 on a PCI-E card, all connected to drives (and a
700 W PSU). The raid array uses 5 drives and the recording(s) would use a
6th, with the other 4 being idle. There were at times several simultaneous
recordings, all going to the same independent drive.
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