malloc backed md/mfs filesystem swapped?

[Robert Watson]

On Fri, 13 Feb 2004, Andrew J Caines wrote:

> After Ring the various FMs including, but not limited to, mdmfs(8),
> mdconfig(8) malloc(9), I am unclear whether of not the memory used by md
> of type MD_MALLOC is kernel memory which will not be swapped, or not. 
> 
> On the same subject, does the the MD_SWAP backed device simply use
> swapable userland VM or does it specifically use a piece of the
> (presumably) disk backed swap partition? 
> 
> FYI, the relevant fstab entries for a malloc backed disk having a UFS2
> with softupdates and async would look like: 

Malloc-backed md devices will be backed by unpageable kernel address
space, and doing this with anything but a very small virtual disk will
result in a kernel panic once the pages are allocated and the rest of the
kernel runs out of address space and memory.  Swap-backed md devices will
be backed by pageable memory, but I'm not sure what the practical limits
(if any)  are for address space concerns.  In general, I use malloc-backed
disks only for diskless systems, and then, only in a sparing way.  If you
have swap available, you pretty much always want to use swap-backing for
memory disks -- if there's room in memory they will run as fast as
malloc-backed, but you don't have to be as worried about the "Oh shoot,
I'm out of room" case.  I use a pretty large swap-backed file system for
/tmp on almost all of my production systems, since swap is cheap, and most
of the time so is memory. 

Robert N M Watson             FreeBSD Core Team, TrustedBSD Projects
robert at fledge.watson.org      Senior Research Scientist, McAfee Research

> 
> md	/tmp		mfs	rw,-M,-s128m,async	2	0
> md	/var/run	mfs	rw,-M,-s1m,async	2	0
> 
> 
> -Andrew-
> -- 
>  _______________________________________________________________________
> | -Andrew J. Caines-   Unix Systems Engineer   A.J.Caines at halplant.com  |
> | "They that can give up essential liberty to obtain a little temporary |
> |  safety deserve neither liberty nor safety" - Benjamin Franklin, 1759 |
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