md devices mounted with async

[RW]

On Sun, 15 Jun 2008 01:19:46 +0200
Kris Kennaway <kris at FreeBSD.org> wrote:

> RW wrote:
> > mdmfs(8) has an example of a malloc-backed md device mounted with
> > the async option. Is there any point in doing this with malloc and
> > vnode devices? In neither case does a write to the file-system
> > require a write to a physical disk.
> 
> Well, for vnode devices it does write to the disk, 

I meant that a write to the filesystem doesn't require a corresponding
write to disk, and the change can stay in memory indefinitely.
Presumably, more or less, the same inactive pages get written-out to
swap, with or without async.  

> but that isn't the 
> point; in both cases you are writing to the filesystem that is
> mounted on top of the md, so that will be faster if it is mounted
> async.

In that case, why doesn't /etc/rc.d/tmp default to mounting its
swap-backed /tmp with async?