[mythtv-users] Raid Performance Tweaking

[Debabrata Banerjee]

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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