[mythtv-users] Odd i/o issues

George Mari george_mythusers at mari1938.org
Wed Jan 14 01:30:23 UTC 2009


Michael D. O'Brien wrote:
> In the last week I've upgraded my back and frontends to Ubuntu 8.10 and the
> newest myth-fixes.  Last night, I noticed that the show I was viewing was
> pausing every few seconds, rendering it nearly unwatchable.  I knew the
> backend was recording a few other shows and the frontends hadn't shown this
> behavior while the backend was idle earlier that day, so I did some looking
> around:
> 
> -top on the backend and frontend showed plently of available memory and idle
> cpu
> -it's not a network issue, the maximum usage in iftop on the frontend looked
> to be about 12mbps (out of 100) and on the backend around 30mbps (out of
> 1000), due to recording 2 shows off our hdhomerun while I was watching this
> other show.
> -on the backend, top shows iowait hovering around 20-40%, which isn't bad
> and was common while recording 3 HD shows and watching 1
> -on the backend, iostat -xk 1 shows something interesting: my raid5 array
> participants are all being used, but it seems that two of the drives hover
> around 10% usage, while one is constantly pegged at 100% usage.  Is this
> normal behavior in iostat -xk for a raid5 array?  it seems odd to me.
> 
> My backend is a C2D e6320 with 3GB of ram running Ubuntu 8.10 and the newest
> myth-fixes.  Myth records to a raid5 array of 3x750gb drives with a 1.5tb
> xfs file system on top.  It's connected to the network via 1000bT ethernet.
> 
> My frontends are AMD x2 4000+ with 1GB of ram running Ubuntu 8.10 and the
> newest myth-fixes.  They are connected to the network via 100bTX ethernet.
> 
> Thoughts?  I'm intrigued by the what seems to be strange iostat results on
> the backend...
> 
> -mike
> 

I see similar behavior in my backend's RAID5 array. One drive always 
shows a higher % usage than the other 5, (not 100%, but noticeably 
higher than the others) but this higher usage seems to "rotate" among 
the all the other drives every few seconds.  My totally unscientific 
theory (guess) is that this is the RAID5 parity stripe being written.

Is anything else writing to/reading from the RAID5 array, or is it 
dedicated to mythtv?

Have you benchmarked the array with a tool like bonnie++?


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