[mythtv-users] Realtime commflagging....was: How many simultaneous recordings on one drive?

David Whyte david.whyte at gmail.com
Tue Oct 11 05:26:09 UTC 2011

On Thu, Sep 29, 2011 at 1:38 PM, Brian J. Murrell <brian at interlinx.bc.ca> wrote:
> On 11-09-28 11:29 PM, David Whyte wrote:
>> Is this really how realtime commflagging works?
> Effectively, if commflagging can keep up.
>> If the data is being
>> processed as it is received, I am assuming you mean the data is read
>> off the tuner, buffered somewhere in memory and the commflagger
>> processes it then writes it to disk.
> Not in so many words.  The data is read from the card and written to the
> disk through the VFS.  The VFS includes a page cache though that stores
> frequently and recently read and written pages.
> So, as long as the commflagger can keep up pace enough that the pages
> don't get flushed out of the page cache before it wants to read them
> from disk, when the commflagger reads the pages from disk, they actually
> come from the page cache, eliminating the physical disk I/O.

Hi Brian and others,

Another couple of questions on this if I may.

I record a show (formula 1) in HD, and it is set to real-time
commflag.  When I watch the show at the same as recording it (and
hence comm-flagging it) I get a bit of stutter now and then.  If I
look at the load on the server at this time, it has spiked quite a bit
up to about 5-6 as opposed to 1-2.

Is this extra load being caused by slow access to the disk or the CPU
not being able to keep up?  I note that the commflag is running at a
nice value of 17.  Can you think of anything I can do to enable me to
record, reali-time commflag and watch an HD show and limit the

I have historical data of the loads on the CPU, disks and network for
the last few F1 races if that helps.


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