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

Eric Sharkey eric at lisaneric.org
Tue Aug 19 02:40:24 UTC 2014


On Mon, Aug 18, 2014 at 8:23 PM, Jean-Yves Avenard <jyavenard at gmail.com> wrote:
> On 19 August 2014 05:56, Eric Sharkey <eric at lisaneric.org> wrote:
>> Yes, it mostly does, thanks.  What you describe sounds pretty good to
>> me.  I still don't like ring buffers in general, but as long as the
>> writes are asynchronous and the sync's don't rely on the ring buffer
>> it really should be ok in practice.
>
> ringbuffer because it's the easiest and most portable approach.
> remember that myth runs on various platforms (BSD, Linux, OS X,
> Windows), not all posix compliant, that the core code is almost a
> decade old and only required updating recently for minor fixes.

A decade isn't that old.  After all, as Mike Perkins said:

"I worked on mainframes back in seventy-<mumble> and the disk and
comms I/O was handled by memory allocation and release just fine. The
procedures for preallocating read-ahead buffers, queuing them and
releasing them back to the pool are techniques well understood."

I wasn't coding anything yet in seventy-<mumble>, but the point being
that read-from-device-write-to-storage is not really some novel
problem never seen before that 10 years would make that much of a
difference.

Then again, given the photo of you that comes up in gmail, I can see
why you might think 10 years is a long time.  It's amazing that
someone of your youth can even type coherent sentences. ;)


> So I'm suspecting that the issue is different, the analysis of it
> being just wrong, and that it's not due to the threaded writer.

You're probably right.

Simon, what were you recording from in your VM?  A network tuner?  Or
were you trying to do USB/PCI pass through?


> 2x1TB does give you higher IOPS, as you can write to two disks at once.
> I/O throughput is rarely a point of contention with DVB/ATSC recordings.

But with the OS and database already segregated off on an SSD, and the
drives only used for storing recordings, I'm not seeing much need for
higher IOPS.

> plus if one disk dies, you only lose 50% of your data (assuming you
> properly use storage group and not some RAID0 setup)

There is that, but in general, drives appear smaller as time goes on
as the standards for what constitutes a large drive change.  What
appears large today eventually becomes so small you don't know what to
do with it, and 1TB drives are already close to that size.

Eric


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