[mythtv-users] Backend crash. Do I have too few hard drives?

Mike Perkins mikep at randomtraveller.org.uk
Sat Dec 4 11:30:42 UTC 2010

Paul Gardiner wrote:
> I've been running MythTv for several years with no real problems.
> Particularly the backend has been very reliable. Recently I added
> a Nova S2-HD card to the backend so that I could pick up Freesat.
> I also added a second HDD, so that I have one for system and one
> for recordings. I paritioned the recordings disc into two partitions.
> Can't remember why I did that. Perhaps that's making things worse.
> Anyway, since the addition, the backend has been on occasion unreliable. 
> Tonight
> was an example. I had one HD recording on the go, which I was
> simultaneously watching. That was fine, until two SD recordings started
> up. That caused the backend to die. I had an empty log file, which
> happens sometimes. I'll try to get more information next time
> it happens.
> Could this be lack of HDD bandwidth?
> Oh yeah, since the addition, I've upgraded from 0.23.1 to 0.24.
> That seemed to make the backend more reliable, until tonights
> crash. A week ago, I rebuilt the backend from the then latest
> 0.24 fixes, so I'm pretty much up to date.
Not sure about your crashes, but I doubt that splitting your hard disk into two 
partitions is going to help. While you're making one recording, the head is 
going to be reasonably static over the part of the disk it's writing to. Once 
you start up another simultaneous recording the head is going to be thrashing 
about. Two (or more) partitions guarantees that the head is going to have to 
move a considerable distance each time. Not good. Head movement = seek time = delay.

Now modern disks can probably cope with most of this but why make the situation 
worse when it's not necessary? As a rule of thumb I try to arrange one disk (= 
partition) per tuner. That way, although sometimes the storage allocation 
chooses the same directory, you at least stand a chance that each recording will 
write to a separate disk and the heads won't be moving too much.

(Now the fact that all these will be writing to separate seek tables in the 
database, that's another story. Just hope that mysql does the right thing.)


Mike Perkins

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