[mythtv-users] How many simultaneous recordings on one drive?

Simon Hobson linux at thehobsons.co.uk
Wed Sep 28 22:28:10 UTC 2011

Eric Sharkey wrote:

>Four recordings at 2 MB/s is 8MB/s.  The buffers on these drives are
>usually 32 or 64MB.  They can seek at almost 100Hz.  It just won't be
>a problem.  You've got a 1996 mind set here.

They may have large buffers, but then Myth is actively syncing each 
record stream every second. I believe that should have the kernel 
sending the write requests to the disk AND waiting for confirmation 
that they've been written. That's immediately added several 
seeks/second to write each stream plus the filesystem data.

Also, something not mentioned, if you are commflagging then that can 
impose significantly higher I/O loadings than just streaming 
recordings on/off disk.
Now I've got a machine dedicated to the backend, I've configured to 
start commflagging when a recording starts (only one job at the 
moment - need to experiment to see if two works well). The idea is 
that it can process the data as it's received, and so can work from 
what's already held in the OS cache and thus not add to the I/O load 
by reading a recording in after it's finished.

Simon Hobson

Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
author Gladys Hobson. Novels - poetry - short stories - ideal as
Christmas stocking fillers. Some available as e-books.

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