[mythtv-users] Commflagging X 4 = CPU ?
linux at thehobsons.co.uk
Sat Oct 29 19:19:29 UTC 2011
Craig Treleaven wrote:
>Is the simple rule of thumb that each commflag job should have its
>own core? I want to try to find a middle point between cost and
>What about disk i/o? I certainly intend to put the os and MySQL
>database on one disk and the recordings on another. If there are
>four simultaneous recordings and four simultaneous commflag jobs, is
>that too much for one 3 Gbps SATA disk?
I think CPU load is largely irrelevant - if you run more jobs than
you have CPU for then they'll just run slower. Disk I/O is probably
the biggest issue - it is in my experience and discussions here don't
contradict that. It's rarely raw I/O rates, but the sustainable I/O
rate while reading/writing multiple streams and thus doing a lot of
performance sapping seeks.
In that respect, having sufficient memory to run commflagging during
recording will help enormously. If you have enough memory and CPU,
then the commflag processes can use the written data while it is
still in the OS cache - and that avoids the need to read it from disk
with the additional seeking that would be entailed.
For some time I ran my backend virtualised as a Xen guest on my home
server which was a single core AMD64 3200. I now have an HP
Microserver running a dual core AMD64 running at only 1.3G dedicated
to being the backend. I haven't done any tests yet (I have two single
tuner DVB-T cards for UK Freeview), but I do know it can handle far
more than my old system.
My old system (also on a single disk) could not record two programs
while I watched another recording. I have so far had my current
system up to four recordings while watching another and commflagging
one of the recordings in real-time. The OS & DB are on a mirrored
setup across two disks. Recordings are stored on the same two disks
(in different partitions), but not raided.
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