[mythtv-users] Raid Performance Tweaking

Debabrata Banerjee davatar at comcast.net
Mon Jul 10 02:01:26 UTC 2006


Based on this I've been hammering my system, and I still see no problems, 
but I don't use continues seek modes, why would you want to do 60X FF in 
real-time? This is a ridiculous requirement to put on the system. Between 
commercial skip and single FF/RW buttons there just isn't need at all.

FYI now that I think about I recall that when I originally tested by system 
for stability I ran loadgens on every disk (including a 6-disk software raid 
5 and 2 disk raid 1 also in the box) while simultaneously encoding on the 
WinTV card for more than a day. (It's been like 3 years now) I'm sure this 
was a much more rigorous ordeal than adding 2 more video sources. Since the 
WinTV cards actually end up overwriting important areas of memory if they 
get ignored for too long, this test was quite in danger of hard locks and 
kernel panics in case of bottlenecks. None observed.

I haven't looked at what the read and write threads look like in myth, but 
if you do indeed have a problem with a single disk they could probably be 
solved by optimizing the read and write sizes. Almost everything happens in 
a sequential fashion and simply increasing the size will give you less head 
movement. However again I doubt this since you had to even resort to 
hardware raid, that's just excessive.

----- Original Message ----- 
From: "Blammo" <blammo.doh at gmail.com>

2. The 2.4M number you're listing there for HD playback, is just
that.. playback. During FF/REW you will generate up to more than 10
times that bandwidth, assuming your network can sustain it.  24MB is
more than half the performance of most single spindle modern drives.
You may think it works differently, but watch network util during a
60x FFW and you'll be shocked.

3. Run something like bonnie++ on your single drive, see what your
performance is like.. I'd suggest setttings like this:

bonnie++ -d /myth -u0 -g0 -n0 -s 1024

Not trying to start a "whose system is bigger/better".. but making a
point. Single disk is insufficient for most serious myth setups.
Things like server motherboards and dual cpu's can help offset some of
those disadvantages, but you'll still suffer from it.



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