Re: Integration of SCST in the mainstream Linux kernel

Linus Torvalds
2008-02-04T23:46:18Z

On Mon, 4 Feb 2008, Jeff Garzik wrote:
>
> Well, speaking as a complete nutter who just finished the bare bones of an
> NFSv4 userland server[1]... it depends on your approach.

You definitely are a complete nutter ;)

> If the userland server is the _only_ one accessing the data[2] -- i.e. the
> database server model where ls(1) shows a couple multi-gigabyte files or a raw
> partition -- then it's easy to get all the semantics right, including file
> handles. You're not racing with local kernel fileserving.

It's not really simple in general even then. The problems come with file
handles, and two big issues in particular:

- handling a reboot (of the server) without impacting the client really
does need a "look up by file handle" operation (which you can do by
logging the pathname to filehandle translation, but it certainly gets
problematic).

- non-Unix-like filesystems don't necessarily have a stable "st_ino"
field (ie it may change over a rename or have no meaning what-so-ever,
things like that), and that makes trying to generate a filehandle
really interesting for them.

I do agree that it's possible - we obviously _did_ have a user-level NFSD
for a long while, after all - but it's quite painful if you want to handle
things well. Only allowing access through the NFSD certainly helps a lot,
but still doesn't make it quite as trivial as you claim ;)

Of course, I think you can make NFSv4 to use volatile filehandles instead
of the traditional long-lived ones, and that really should avoid almost
all of the problems with doing a NFSv4 server in user space. However, I'd
expect there to be clients that don't do the whole volatile thing, or
support the file handle becoming stale only at certain well-defined points
(ie after renames, not at random reboot times).

Linus