[mythtvnz] Recording dropouts and disk performance...

[Steve Hodge]

On Sun, Feb 17, 2013 at 10:19 AM, Rob Connolly <rob at webworxshop.com> wrote:

> So the general consensus is that LVM == bad in this situation.


It adds complexity and does cost some performance. I'm not sure I'd say
"bad", but my opinion is that it's unnecessary.


> The RAID
> setup is also softraid through mdadm if that makes a difference. I think
> the motherboard RAID controller only does fakeraid anyway.
>

Almost certainly. Software RAID will give you much more flexibility and
performance is just as good. It's also a lot easier to migrate a software
RAID array to another machine (though with RAID1 that'd not a problem
anyway). As usual opinions vary, mine is that hardware RAID is not
worthwhile unless you're looking at really high end setups.


> I had been using LVM to separate partitions for different purposes, e.g.
> root, home, recordings, other media, etc. and to allow expansion of
> space for each as necessary. Now that I think about this, it's kinda
> pointless as creating one partition on the whole disk would achieve the
> same thing!
>

Right. Multiple partitions really just means that there are multiple
chances for something to run out of room. With modern drive sizes it's
better just to give the OS more space than it can possibly use and then
forget about it - there's no need to carefully tune partition sizes for
different parts of the system. There is some value in separating recordings
(which can run out of room quite easily if you don't use autoexpire) from
everything else, but using storage groups to add space is going to be
easier and more flexible than using LVM. For MythTV my layout is this:
1. boot - 40MB or so, which is more than enough
2. swap - a couple of GB
3. OS (ext3) - 30GB which is plenty though I'd probably make it 100 or
200GB these days
4. recordings (jfs) - the rest of the drive, about 1.5TB


> Of course, this doesn't tell me why this is just happening now, when the
> system has been running fine for over a year in this configuration.
>

Yeah, it's frustrating when something that was fine suddenly has problems.
I saw your hdparm speed tests - they look fine and they're definitely using
DMA - you can't hit 100MB/s without DMA. One thing I'd check is the SMART
data (e.g. using smartctl). If one of the drives is starting to fail that
might explain it.

Cheers,
Steve
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