[mythtv-users] Can't Record Two HD Streams on HDHomerun

Kirk Bocek t004 at kbocek.com
Wed Sep 26 18:17:58 UTC 2007

Taco Mel wrote:
> You mentioned the 3ware controller, which I also have.
> I assume that you have already checked to make sure
> you aren't running a degraded RAID-5 array, for
> example, which would make your system dirt slow...
> It turned out that my 3ware setup was a cause of some
> poor performance I had a while back.  Try measuring
> your throughput, something like this:
> cd /storage  (somewhere on the 3ware array)
> dd if=/dev/zero of=foobar bs=1024 count=1000000
> dd if=foobar of=/dev/null

Thank you Taco Mel (can I call you Taco?), Yan, Tom, Craig, Brian and Mitch
for responding. You all focused on two resources, network and disk bandwidth.

* Network

I have two unmanaged switches in the wiring closet a small 10/100 and a small
gigabit switch. The 10/100 is cascaded off the gigabit. The server has a
gigabit link into the gigabit switch. I *probably* (I'll have to check) have
the HDHomerun plugged into the 10/100 switch since it would be a waste of a
gigabit port to go the other way:

HDHomerun --> 10/100 Switch --> Gigabit Switch --> Gigabit port on server.

* Disk Bandwidth

When I built this server a year ago I did some extensive testing of various
file systems on the 3Ware controller using Bonnie++. My setup is 4x 400GB SATA
in Raid 5. NCQ is disabled -- performance was very poor with it enabled. Reads
were pretty consistent over every file system at 200+ MB/sec.

Writes, however, were all over the map. The best I found was XFS which gave me
 210 to 230 MB/sec writes. Unfortunately, I found Myth crashing when writing
to XFS. I was forced back to Ext3.

My benchmarks of Ext3 shows writes of 80-85 MB/s. Not great but that seems
like enough for MythTV even if I have a few other things running on the server.

Taco, write cache *is* enabled on the array. Here's the results of your test:

$time dd if=/dev/zero of=foobar bs=1024 count=1000000
1000000+0 records in
1000000+0 records out

real    0m6.130s
user    0m0.187s
sys     0m4.793s

$time dd if=foobar of=/dev/null
2000000+0 records in
2000000+0 records out

real    0m1.886s
user    0m0.460s
sys     0m1.427s

Check my math guys, but that's a 977MB file and so gives 159 MB/sec writes,
and 518 MB/sec reads. Given that I've got 4GB of RAM I should probably run
this with a larger file, but still, that seems pretty good to me.

Kirk Bocek

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