[mythtv-users] Enough power?

[Matt Mossholder]

Brad,
    Just to clear something up... a 10Mbit client connected to a 100Mbit
network does NOT "10x the bandwidth it is using, because it is time that
matters." The only way to connect a 10Mbit client to a 100Mbit network
is via a switch or bridge, which does rate conversion on each frame.
Hench, a 10Mbit client's traffic will be converted to 100Mbit when the
frame is sent out by the switch. The time in between the frames to/from
the 10Mbit client is free for use by other systems. 

    Now, if we are talking about something like 802.11, that's a
different story.... 802.11b clients definitely have a negative impact on
the 802.11g traffic, when the b and g clients are both on the same
channel. This is because everything is a broadcast, and in this
instance, it really is the time that matters. The 802.11g clients can't
talk or be talked to while the 802.11b systems are...speaking...
so...slowly....

    I agree with everything else though :)

        --Matt


On Tue, 2005-01-18 at 14:03 -0800, Brad Templeton wrote:

> On Tue, Jan 18, 2005 at 09:54:12PM +0100, Jens Peter Vilstrup wrote:
> > This might be slightly OT. Sorry if I offend anyone.
> > I'm in the planning phase of my MythTV project, but unsure if my
> > planned myth-backend has enough oomph.
> > I have:
> > Tyan Tiger MP with dual 1.4Ghz Athlon XP's and 1GB of RAM.
> > Gbit NIC.
> > 2x PVR-250 cards.
> > Some yet-to-be-determined SATA(n) card(s), possibly RAID (that, or
> > software RAID).
> > A whole lot of harddrives.
> > MySQL will be on a different server.
> > 
> > I want to be able to record two shows simultaneously, while watching
> > on two frontends, while copying files at 100Mbit to or from a client
> > PC...
> > In your experience, is this a feasible goal?
> 
> More than feasible, your backend seems to be overpowered.  PVR-250
> effectively capture video with effectively no CPU cost, just a few
> percent on an Athlon 750mhz, for example.   I see no reason why
> a single machine of just about any stripe could not do what you
> describe (except the software RAID, that might require some CPU)
> 
> There is a network limitation.  If your frontends are on the same
> network as the PCs that are saturating the 100mibt network, then
> a saturated net is a saturated net.  I don't know about gbit networking
> but with 100mbit the rule was that a 10mbit client on the network takes
> 10x the bandwidth it is using, because it is time that matters.   Ie.
> if gbit works the same, then a 100mbit client connected to the switch
> uses all of the switch if it is using the full 100mbit, it doesn't
> use 10% of the switch.  
> 
> Recording from pvr-x50 cards and pchdtv cards takes almost no CPU power
> at all.   Things that take CPU power are playback (on that machine)
> and complex SQL queries.  (For example, I have found that by putting
> 100 record-wishlist searches into Myth, the database takes a minute
> to recompute the recording table every time I make a change, but this
> is rather extreme.)
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-- 
Matt Mossholder <matt at mossholder.com>
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