Re: Question regarding to store file system metadata in database

[Al Viro <viro@ftp.linux.org.uk>]

On Mon, Mar 20, 2006 at 05:53:08PM -0500, Xin Zhao wrote:
> Apparently this comparison is not 100% fair. In my experiment, I
> randomly pick pathname from 1.2 million path names to resolve the
> inode number. But in your "cp -rl linux2.6 foo1" experiment, you
> essentially did directory entry lookup sequentially, which maximize
> the possible performance. If you do the same thing in a random
> fashion, you will probably get much worse performance. As I said
> before, I totally agree that 2000/sec is slow. But the point here is
> whether 2000/sec is enough for most scenarios?

It is not; e.g. unpacking a tarball or running make(1) on even a
medium-sized tree is going to hurt that way.  Moreover, the same
goes for a lot of scripts, etc.

And that's aside of the question of CPU load you are inflicting -
it's not just 2000/sec, it's 2000/sec _and_ _nothing_ _else_ _gets_
_done_.

BTW, for real lookup speed, try find(1).  From hot cache.

> I am not saying existing FS implementation is not efficient. I agreed
> that file system has been fully optimized. What I want to say is to
> support complex mapping in the system I described before, we might
> need some extension on existing file systems. Question is what is the
> best extension. Consider how to allow user a, b to share physical copy
> f.1, while allowing user c to use private copy f.2? The virtual
> pathname to physical pathname should be transparent to end users. That
> is, all the users should be able to access right file copies using
> virtual path "f". The file system should be able to tell the different
> identity and return the data from the right physical copy. That's what
> we want to do. But it is hard to achieve without some extension. :)

So what happens upon rename()?