[mythtv-users] Storage groups to solve I/O wait

[George Mari]

Taco Mel wrote:
> Today I experienced, for the first time, hesitation
> while I was watching a show being recorded in HDTV. 
> My setup for the back end is MythTV 0.21 (r14051),
> pcHDTV 5500, Hauppauge PVR-500, and 3Ware 9500S on an
> old Dell Precision 530.  This is a single PCI bus. 
> The pcHDTV was recording the aforementioned HD show,
> the Hauppauge was recording something else, and all
> recordings access the disk via the 9500S (disks are in
> a true hardware RAID-5 and file system is xfs).  I was
> "chase playing" the HD recording.
> 
Have you chase-played an HD recording successfully before?  Or was this 
the first time, and you saw this hesitation?

> The iowait was around 60-80% for each CPU according to
> "top".  
My suggestion would be, while you are "chase-playing" an HD recording, 
and recording something else on the PVR500, open a console window and 
run iostat -xd 2, and look at your numbers under the columns "svctm" and 
"%util".  (Note: iostat was not part of my standard FC6 install, I had 
to download it separately, I think from the extras repository; check to 
see if it's included in your distro.)


It will list a line for each disk in your system.  If your %util on your 
recording drive(s) is close to 100%, you've saturated the I/O capability 
of that device, meaning you've flooded it with read and/or write 
requests, and it can't handle anymore.  If so, a reconfiguration of your 
RAID array may be in order, or perhaps a parameter is amiss on the 3Ware 
driver?

My guess is that I saturated the PCI bus.
> 
Theoretical bandwidth of the PCI bus is 133MB/s.  This equates to 467 
GB/hr.  A typical HD recording is about 14GB/hr. (or is it 9GB/hr?)  SD 
is about 2 or 3 GB/hr.  Either way, even considering you won't get near 
the theoretical max of 133MB/s on the bus, I don't see the bus being a 
bottleneck here.

> Trying to use the hardware I have now (with an eye to
> possibly upgrading in the future), I was wondering if
> storage groups might help me here.  Currently my IDE
> (dual channel ATA-100) are set up with the system disk
> as a master on one channel and the DVD as the master
> on the second channel.  My idea is this: put another
> IDE drive in there (or maybe 2 with RAID-0), and set
> up MythTV so that recordings are sent there.  Think of
> this as a "staging area" of sorts.  Then, during a
> period of low activity (e.g. middle of the night) a
> process will come along and copy all of the recordings
> from the IDE drive onto the SATA RAID array.  My goal
> is that everything gets protected by RAID, although
> losing one day's worth of recordings would not be the
> end of the world.
> 
> My questions, therefore:
> 
> 1. Is there a better solution (other than purchasing
> faster hardware) - i.e., am I making this too
> complicated?
> 
My personal opinion, yes, your proposal is overly complicated.  I'm not 
saying it wouldn't work, and it is creative, however.

> 2. Is there any misunderstanding in the capabilities
> of storage groups that would make my idea unworkable
> right from the outset?  Is my idea just plain stupid
> for any reason?
> 
> 3. Is it better to configure them as two separate
> drives (mounted in two separate directories with equal
> priority) or as a software RAID-0?
> 
> I've got some old IDE drives sitting around and would
> not mind spending a little money to get drives with a
> larger cache if that might help.  Thanks in advance
> for any advice.
>