[mythtv-users] Raid Performance Tweaking

[Blammo]

lol...

running a server class motherboard with dual CPU's and HT serves
overcome lots of IO issues guys with less horsepower are going to
experience.

however, even with all that horsepower:

1. Once commerical flagging is through the "logo" portion disk usage
is much lower and manageable. Try watching HD while in the logo
portion, you will likely see/feel the impact. I have a very fast
desktop machine, than when I use it for commerical flagging, will pull
>300Mbit during the logo phase.

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.


On 7/7/06, Debabrata Banerjee <davatar at comcast.net> wrote:
> I said 4, not 40.
>
> I watched grey's anatomy in HD last night at 2X speed from my frontend while
> it was still ingesting and commercial flagging on the fly to a 5400 rpm disk
> while the backend was software encoding another program. No issues. Just for
> you I even scanned the backend log, clean. And the backend is a dual 2.8
> xeon with HT, so it's doing commercial detect pretty damn fast. "Serious"
> mythtv setup eh? It's pretty damn serious.
>
> Even with HD content each thread needs a maximum of 2.4 MB/s to work
> properly scrubbing is only slightly higher unless you're using a dumb block
> size. For ext3 this isn't an issue because they are really small. fw and rw
> reading a few frames every 5-10 seconds of video isn't all that much.
> Playing back HD at 2X speed is more tasking by my guesstimate.
>
> If you went through all those software raids and they weren't good enough
> for you, you have _other issues_. Even *gasp* sector relocations in hardware
> or software, hardware relocations which do not get reported, but make disk
> response very, very slow. At a minimum something like dd if=/dev/mythdisk
> of=/dev/null should be run to let the drive firmware exercise the disk for
> force relocations.
>
> ----- Original Message -----
> From: "Blammo" <blammo.doh at gmail.com>
> To: "Discussion about mythtv" <mythtv-users at mythtv.org>
> Sent: Friday, July 07, 2006 2:04 PM
> Subject: Re: [mythtv-users] Raid Performance Tweaking
>
>
> On 7/6/06, Debabrata Banerjee <davatar at comcast.net> wrote:
> > Whatever you do it's going to be irrelevant for mythtv unless you add a
> > massive amount of tuners. Unless you decide to make a RAID array out of
> > 4GB
> > disks from 1996.. Even one disk should be good for many streams of video.
> > Let's not confuse Mbit and MBytes, as people often do.
>
> Sadly, Debabrata, it's very easily to invalidate that statement.
> Here's a couple of examples:
>
> 1. Commericial flagging (especially during "Detecting Logo" phase)
> 2. more than 1 frontend (FFW/REW burn bandwidth)
> 3. HD Content (alone not bad, but combine with #1 and #2 for single
> spindle death)
>
> While your average current disk drive, on sustained SINGLE threaded
> reads, sits about 40MB a second, flipping to reads/writes, or 2/3/4
> simult reads/writes from different parts of the disk, and now your
> throughput on that single spindle is going to be WAY down below that
> magical 40MB number, and you will start having performance issues.
>
> As someone who has been through the single spindle, software
> raid0/1/5/6 and finally settled on NxHW-Raid5, a single disk is
> insufficient for most serious myth setups.
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