[mythtv-users] Combined FE/BE using USB for all I/O?

[Stephen Worthington]

On Sat, 16 Aug 2014 08:05:45 +1000, you wrote:

>On Fri, 15 Aug 2014 11:59:02 AM Brian J. Murrell wrote:
>> But transcoding will be another seek/write.  Just not a (physical) read.
>> And commflagging shouldn't be a (physical) read.
>> 
>> Indeed, I would say that enough memory to avoid those additional reads
>> is very well worth it if you are hitting peak disk seek/io throughput
>> during recording without it.
>
>
>This has been a fascinating discussion, thanks all.
>
>Brian, I'm not sure if you're saying now that one disk plus a chunk of ram 
>should be ok now?
>
>Say I'm looking at at:
>
>- Main system+MythTV MySQL DB on a SSD
>- 4GB RAM
>- one 2TB Storage on USB3
>- No Transcoding 
>- No Comm Flagging
>
>Worst case, simultaneous recording of 4 SD channels + 1 SD Playback
>
>Do you see that leading to seek contention?
>
>thanks,

I would be wary of doing more than three recordings at a time on one
hard disk.  The track stepping speeds have not really kept up with the
other speed and capacity increases and it can take quite a while for
the heads to move long distances.  As the drive fills up, the heads
can have to move very long distances, and the worst case is when the
directory or allocation information needs to be updated as the file
grows - then the heads have to move to yet another location as well as
just the files being written to.  All it takes is one moment when the
disk is too busy and you will lose a block of data and seconds of one
or more recording.  I have had this happen before I added more drives,
and it ruins the watchability of a recording.  And it usually
continues for some time so several minutes of recording are
unwatchable.

The only way to be sure is to try it.  It is more likely to work if
you are using a performance type drive (eg WD Black or a Hitachi)
rather than a "green" drive.  Personally, I have recorded 10 things at
once with commercial flagging.  But I have five recording drives, and
my CPU is fast enough to do the flagging in real time out of the RAM
buffers.  My rule of thumb is to have no more than two recordings at
once per drive if possible.  I know that always works.  Whether three
at once or four at once work, I am not sure.  I am pretty sure five at
once would not work.