md devices mounted with async

[Kris Kennaway]

RW wrote:
> 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.  

Well, it doesn't necessarily cause a write to disk for each filesystem 
write, but the synchronization mode of the filesystem to the backing 
store is precisely what the async/noasync/sync mount options control!

>> 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?

It should.

Kris