[mythtv-users] Storage groups to solve I/O wait
george_mythusers at mari1938.org
Tue Aug 21 03:41:49 UTC 2007
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
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
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
> 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
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.
More information about the mythtv-users