[mythtv-users] Streaming better than NFS

[Bruce Markey]

Brian Foddy wrote:
> Playing this weekend, I found the AlwaysStreamFiles setting, and even
> tho my FE has the single filesystem NFS always mounted, I decided to
> try it.  Partly, because I felt it might transfer some cpu load to
> the BE, and the files are all local on the BE.
>
> I found skipping, FF to be much more responsive than with NFS.

Correct.

> Overall FE cpu load was also lower.  Very little jitter after a FF
> that used to happen for a few seconds while it cached up the new
> location.

Yep.

> Just thought it was interesting, and people may want to try it for
> themselves.

An unstated assumption is the illusion that NFS looks like
files are local so it must be magic somehow. An application
opening a network connection must be clunky, inefficient and
not magic. However, the speed that data can be sent over an
IP network is the same whether the connection is set up by
NFS code or MythTV code. The difference is that an application
can be aware of that application's needs whereas NFS cannot.
The myth network connection has to be more efficient unless
we are doing something wrong.

The myth protocol reads are aware of this application's needs
and can adjust the read size on the fly and this is important
for mixing FF and normal playback.

http://cvs.mythtv.org/trac/changeset/11834

Starting with small reads after each seek is efficient for FF
so that we don't request excessive data when we only need to
show one frame then seek to a new position in the file to show
another frame. When playback continues, the read size is adjusted
for efficiency whether it is a 160x240 analog recording up to
a 7.2GB/hr HDTV file.

NFS knows nothing of this and always uses the same blocksizes
and read ahead policies. This means that it will send too much
data for each frame while fast forwarding and lots of relatively
small packets (no matter what the video bitrate) during normal
playback.

If someone has enough network bandwidth and average size files,
NFS may work just fine and it may not be noticeable that a  myth
backend connection would be more efficient. However, if you watch
HD over a wireless network (like me) you really want to connect
to the backend and not rely on NFS.

--  bjm