Prefaulting for i/o buffers

[Jeremie Le Hen]

Hi,

On Sat, Feb 25, 2012 at 08:46:31PM +0100, Pawel Jakub Dawidek wrote:
> 
> I did not implement rangelocking for ZFS. It came with ZFS when I ported
> it. Until we want to merge changes from upstream (which is now IllumOS)
> we don't want to make huge changes just for the sake of proving that
> this is general purpose mechanism used by more than one file system.
> 
> Attilio, don't get me wrong. In 99% cases it is good to make code more
> general and more universal and reusable, but we can't ignore reality.
> 
> There are reasons why file systems like XFS, ReiserFS and others where
> never fully ported. I'm not saying VFS complexity was the only reason,
> but I'm sure it was one of them.
> 
> Our VFS is very UFS-centric. We make so many assumptions that sounds
> fine only for UFS. I saw plenty of those while working on ZFS, like:
> 
> - "Every file system needs cache. Let's make it general, so that all file
>   systems can use it!" Well, for VFS each file system is a separate
>   entity, which is not the case for ZFS. ZFS can cache one block only
>   once that is used by one file system, 10 clones and 100 snapshots,
>   which all are separate mount points from VFS perspective.
>   The same block would be cached 111 times by the buffer cache.
> 
> - "rmdir(2) on a mountpoint is bad idea, let's deny it at VFS level."
>   It is bad idea, indeed, but in ZFS it is a nice way to remove snapshot
>   by rmdiring .zfs/snapshot/<name> directory.
> 
> - Noone implemented rangelocking in VFS, so no file system can use it.
>   Even if the given file system has all the code to do it.
> 
> etc.
> 
> I'm also sure it will be way easier for Jeff to make VFS MP-safe if it
> was less complex.
> 
> When looking at the big picture, it would be nice to have all this
> general stuff like rangelocking, quota, buffer cache, etc. as some kind
> of libraries for file systems to use and not something that is
> mandatory. If I develop a file system for FreeBSD only and I don't want
> to reinvent the wheel, I can use those libraries. If I port file system
> to FreeBSD or develop a file system that doesn't really need those
> libraries I'm not forced to use them.

I'm a little late on this thread, but I had to mention that there was a
very interesting article on LWN a few years ago about the "midlayer
mistake" design pattern, which really reminds me the subject here.

http://lwn.net/Articles/336262/

Regards,
-- 
Jeremie Le Hen

Men are born free and equal.  Later on, they're on their own.
				Jean Yanne