On Mon, Feb 04, 2008 at 11:44:31AM -0800, Linus Torvalds wrote:
...
quoted text
> Pure user-space solutions work, but tend to eventually be turned into 
> kernel-space if they are simple enough and really do have throughput and 
> latency considerations (eg nfsd), and aren't quite complex and crazy 
> enough to have a large impedance-matching problem even for basic IO stuff 
> (eg samba).
...
quoted text
> So just going by what has happened in the past, I'd assume that iSCSI 
> would eventually turn into "connecting/authentication in user space" with 
> "data transfers in kernel space". But only if it really does end up 
> mattering enough. We had a totally user-space NFS daemon for a long time, 
> and it was perfectly fine until people really started caring.

I'd assumed the move was primarily because of the difficulty of getting
correct semantics on a shared filesystem--if you're content with
NFS-only access to your filesystem, then you can probably do everything
in userspace, but once you start worrying about getting stable
filehandles, consistent file locking, etc., from a real disk filesystem
with local users, then you require much closer cooperation from the
kernel.

And I seem to recall being told that sort of thing was the motivation
more than performance, but I wasn't there (and I haven't seen
performance comparisons).

--b.
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