Re: reiser4 plugins

[David Masover <ninja@slaphack.com>]

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Hubert Chan wrote:
> On Mon, 27 Jun 2005 18:27:26 -0500, David Masover <ninja@slaphack.com> said:
> 
> 
>>Kyle Moffett wrote:
>>
>>>I think the '...' is just a bad idea in general, because it breaks
>>>tar and such.  An interface like this might be simpler to design and
>>>use:
>>>
>>># mkdir /attr
>>># mount -t attrfs attrfs /attr
>>>
>>>/attr/fd/$FD_NUM == '...' directory for a filedescriptor
>>>/attr/fs/$DEV_NUM/$INODE_NUM == '...' directory for an inode
> 
> 
> The most obvious (at least obvious for me) complaint about this is that
> the attributes are separate from the file.  (In other words, it's a bit
> ugly. ;-) ) So if you want to backup a file, along with all its
> (extended) attributes (or substreams, or ... etc. ...), you need to
> backup the file, and find the appropriate /attr/fs/$DEV_NUM/$INODE_NUM
> and back that up.  If I want to edit an attribute, I need to find
> $DEV_NUM and $INODE_NUM (which I have no idea how to do), rather than
> just "edit foo/.../extended_attribute".  (Or using your "getattrpath"
> command, it would be a two-step process -- run getattrpath, then run
> editor -- rather than a one-step process.  Of course, this is only
> mildly annoying.)

Especially considering you could add a tar/rsync flag to handle it for
you.  And most likely, each meta dir would have its own serialization
method (read: file) so that you didn't find yourself trying to back up
huge infinitely recursive structures, or accidentally triggering a huge
tar operation, or something...

> It also exposes a difference between attributes and regular files.
> e.g. can I add attributes to an attribute?

Under my scheme, yes, and it works the same way.

> (say, I have a thumbnail
> attribute for the file ~/foo, and I want to add a mime-type attribute to
> that thumbnail attribute.)  If you want to allow it, you have to be
> careful not to run into a big loop generating an infinite number of
> inodes in the attrfs.

I'm not sure if inodes are a problem, but this kind of stuff would be up
to the fs, not the attrfs or metafs.

> That said, I prefer that interface over xattr or openat.

Absolutely.  But I think both should exist, so that programs currently
using xattr will still work.  Maybe you'd have:

/meta/vfs/home/foo/attrs

or something...

Come to think of it, that changes the proposal a bit.  You need a
different way to access the meta-dir for files than for directories, if
we're going to support /meta/vfs.  No biggie:

/meta/vfs/file/home/bob/sue.mpg/acl
/meta/vfs/dir/home/bob/acl

>>>It would be usable from a shell with a simple "getattrpath" command
>>>which returns "fs/$DEV_NUM/$INODE_NUM" by stat-ing any given path.
> 
> 
>>Still pretty annoying, but maybe a good idea, especially if the shell
>>gets extended to support it.  Not sure I like using inode numbers,
>>though -- I like the idea of being able to symlink to stuff inside the
>>meta-file dir.
> 
> 
> The advantage of using inodes rather than pathnames is that it is robust
> with respect to file renaming/moving.  It also allows things like
> adding attributes to symlinks (since the inode number for a symlink is
> different from the inode number for the file that it points to).

I think we need both interfaces, especially because the /meta/vfs
interface makes looping easy.

> You can still symlink.  It just takes a little more effort to figure out
> what $DEV_NUM/$INODE_NUM to use.

Doesn't have the same effect.

>>Actually, I like this.  Give me some time to let it percolate.
> 
> 
>>Hans, thoughts?  Seems to be namespace fragmentation, but seems
>>usable, less breakage, and so on.  And should it be /attr or /meta?
> 
> 
> For the mount point, it doesn't matter; it's up to the user.

No, it's not.  It's up to the user and the application developers.
Sure, I could mount /proc somewhere else, but that would confuse some
things.  Mounting /meta somewhere else would confuse even more things.

> It's the
> attrfs or metafs or ???fs that matters (but which will greatly influence
> whether people user /attr or /meta).

True, but I think they should still hand down a decree of where it is to
be mounted by default.
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