Re: mdconfig unable to allocate memory

On Fri, Feb 27, 2004 at 02:08:48PM +0000, Colin Percival wrote:
> At 13:58 27/02/2004, Chris Elsworth wrote:
> >So, in short, is there any rule of thumb to follow when tweaking these
> >settings?
> 
>   Yes, several:
> 1. Don't do it.
> 2. Pray.
> 3. Don't do it.
> 4. If you absolutely must do it, make sure you have a serial console
> connected.
> 5. Don't do it.

Right, got it :) Having the machine crash is no problem. It's sat in
my lounge, so I can tweak and reboot and restore from kernel.generic
to my hearts content. This seems to be a problem that's been gone over
and over and over again, but never with any clear answer according to
Google. Given a FreeBSD machine with 4GB in it, be it 4 or 5, how can
one ensure that all the memory is being used for suitable file caching
if it's running MySQL? I was aiming at raising vfs.maxbufspace to be
2GB or so.  Since that was just resuling in kernel panics pretty much
whatever I tried, I was giving mdconfig a go.


> > It's a bit worrying that an out of the box 5.2.1 managed to
> >crash at all.
> 
>   The fact that running out of kernel address space can cause
> problems is quite well documented.

Well, yes, but why would "vinum start" on a clean boot before any
tweaking (this is an out of the box 5.2.1-R) cause that? Surely the
kernel address space is sufficiently large just by self-tuning on a
machine with 4GB, for a vinum start to succeed?


>   Why do you want to have a GB malloc-backed disk anyway?

This machine is for MySQL, and there's a wild un-backed-up claim on
a mysql list that moving MySQL indexes into a ramdisk (they're all
trashable data and easily recreateable) can give a fourfold
performance increase. So I wanted to try it. In order to get them all
in there, though, I'm going to need at least 2GB.

I think I'm just missing the point of having a swap backed memory
disk, really; I assumed that it would immediately start dumping
anything I put in it, into swap, causing disk access. Does it only put
stuff into swap once it runs out of address space?

-- 
Chris