very strange reaction of the md disks

[Oliver Fromme]

Hello,

I don't know why nobody else has given the correct answer
to this (it's should actually be a FAQ).  So I'll try to
give an answer.

The Ghost <the-ghost at inbox.ru> wrote:
 > Hello,
 > 
 > Maybe I just don't understand something, but anyway, it looks really odd.
 > 
 > I have a machine with 2 Gb RAM with a clean newly-installed
 > FREEBSD-7.0-RELEASE. I make two 300-MB mdconfig disks and populate
 > them, and when the first one was already full and the second one is
 > only half-full, I get a kernel panic "not enough memory".

Use "-t swap" with mdconfig(8), not "-t malloc".

 > [...]
 > I was very surprised to see that one gigabyte of my
 > memory suddenly became used up.

FreeBSD uses all memory for caching, as far as possible.
Free memory is wasted memory.

 > A friend of mine told me that such a panic is really a thing to write
 > a bugreport on it,

No.  It is clearly documented in the mdconfig(8) manpage:
"If the -o reserve option is not set, creating and filling
a large malloc-backed memory disk is a very easy way to
panic a system."

As I said above, you probably don't want a malloc-backed
memory disk (which means it consumes non-pagable kernel
memory), but a swap-backed disk (which is cached in regular
RAM and backed by swap).

If you're absolutely sure you want kernel-malloc for your
memory disk, you need to increase the kmem limit (see
"sysctl vm | grep kmem").

Best regards
   Oliver

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