[mythtv-users] RAID suggestions

[Jules Bean]

Mike wrote:
> After using MythTV for quite some time. I'm starting to think I might 
> want a raid controller to use with my 2 hitachi deskstars (250GB)
>
> I'm sort of looking at a 3ware 7506-4LP but I hear the performance 
> isn't all that and its more for redudancy. I'm going for both size and 
> speed since it seems a lot of the wait is in the drive, plus I want 
> lots of storage. I have little faith in software raid controllers and 
> really want the whole process offloaded from the CPU so it can do 
> things like commflag. (hence the 3ware with good linux support) 
>
> Does anybody have any suggestions to get the most speed out of my 2 
> drives? Also, I am not opposed to buying 2 SATA II drives and a 3ware 
> SATA controller for the backend. Is it really that big of a 
> difference? Things seem slow with mythcommflag running, 3 shows 
> recording and trying to bring up the recorded programs from mythweb. I 
> plan on adding an HD-3000 to the mix which I hear I need almost an 
> entire systems resources by itself just to watch live HDTV since it 
> isn't HW encoded. And I prefer not using XvMC since the only thing 
> I've managed to do with it is make the picture look worse and have the 
> computer crash every couple of hours with it on.
>
> Any suggestions on the raid card and hardware configuration?


I think I'm going to have to challenge your assumptions here.

a) Is it speed or redundancy you're after? With only two drives you can 
have one or the other but not both.

b) Are you sure this is going to help? I'd be really surprised if the 
hard disk was the bottleneck. A typical modern drive can get peak 
throughput of 50-100MB/sec. Compare that to SDTV which is around 
1GB/hour = approx 0.3 MB/sec and HDTV at 6GB/hour = approx 2MB/sec. 
These really aren't significant hard disk loads, even with three or four 
instances going on at once with a mix of reading and writing, I wouldn't 
think. (Incidentally, SATA-II is mostly marketing since it ups the 
supposed maximum throughput from 150MB/sec to 300MB/sec but no hard disk 
hardware can achieve that anyway, as far as I know)

c) Linux software raid is really good, impressively reliable, and 
typically not a significant CPU load. Whereas support for hardware raid 
has always been flaky. Many 'hardware RAID' cards don't actually do the 
work in hardware anyway.

It sounds to me like your system is CPU bound (and it certainly will be 
when you're watching HDTV), and unfortunately spending more money on a 
faster disk subsystem is not going to solve that :-(

As a general engineering principle before you try to 'optimise' it's 
always worth profiling to see what's the bottleneck. (Maybe somebody 
should write a mythtv performance profiling guide). The command "vmstat 
1" will show you a lot of interesting statistics every second, in the 
'bi' and 'bo' columns you can see how hard your disks are being hit in 
KB/sec. It will also show you CPU usage ('us' and 'sy' IIRC) and some 
swapping info.

Without more information I can't be sure, but I wonder if you don't have 
enough memory? That could cause sluggishness...

Jules