Re: FS-specified FSID for non-device based filesystems?

[Oleg Drokin <green@linuxhacker.ru>]

Hello!

On Fri, Apr 14, 2006 at 02:32:44PM -0700, Bryan Henderson wrote:
> >From my reading of the code it seems that fsid is composed of device's
> >minor/major and export point's inode ino.
> >I propose to replace (if supported by fs) device's minor/major by
> >some unique number returned by fs itself.
> >Should be no worse than what we have now.
> That's for a device-based filesystem.  But isn't your proposal primarily 

Yes. Device-based fs has root->s_dev. For non-device based (and for
device-based too, if they would like) - method to get some sort of
unique id to be used in the same way root->s_dev is used.

> about non-device-based filesystems, and replacing a user-supplied value 
> with this filesystem identifier?  It isn't so clear that that can't make 
> things worse.

No, replacing whole fsid= with fs-supplied value would have implications like
that you mentioned - we might have two different points in filesystem
exported and we do not want those to have same id.

> But I don't actually remember the specific cases in which the fsid was 
> considered insufficient; only that someone (probably Neil) did find that 
> it was.  (Before Linux had export IDs, you simply couldn't export a 
> non-device-based filesystem at all, so we had to make a fake device on 
> which the filesystem could "reside."  The user managed the device number 
> of that device as the filesystem ID).

We still have those "fake devices". In fact non-device based fs can set
REQUIRES_FS_DEV in fs_flags and it can be exported after that without
manually specifying fsid. (yeah, I tried). But distributed filesystems would
like to have same device id on different nodes.

Bye,
    Oleg