Re: Carrying Attributes too Far

[Hans Reiser <reiser@namesys.com>]

lrc1@st-andrews.ac.uk wrote:

>Quoting Hans Reiser <reiser@namesys.com>:
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>>lrc1@st-andrews.ac.uk wrote:
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>>>Sorry for the delay in replying.
>>>
>>>Quoting Hubert Chan <hubert@uhoreg.ca>:
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>>>Yes, it is impossible to hard-link between two files on different volumes
>>>(except at mount points) in the Unix filesystem, but it shouldn't be. (More
>>>generally, with the necessary permissions it should be possible to make any
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>>file
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>>>the child of any directory via a hard link, except where doing so would
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>>create a
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>>>cycle.) 
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>>This implies that two filesystems can share an identification number
>>between them.
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>I would suggest that for every link to a file on a different filesystem, the
>linking directory should store both an identifier for the child file's
>filesystem and the child file's on-disk inumber on that filesystem.* The reverse
>links would be stored in the same way. 
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I can accept a patch that does this.

>That way, filesystems don't have to
>understand other filesystems' identifier schemes, or coordinate their own
>schemes with them. (The obvious alternative approach - maintaining a list of
>unique identifiers for all files on all mounted filesystems - is undesirable for
>a number of reasons. For one thing, it is limiting and unsafe to assume that all
>mounted filesystems are willing and able to provide an accurate list of all
>their files in a reasonable time.)
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>This leaves one significant problem: what happens when a file is moved from one
>filesystem to another? What will prevent the cross-filesystem hard links to the
>file from breaking? Cleaning up cross-filesystem links after explicit file
>deletion is a related but simpler problem. I think a good but not ideal solution
>to this problem is possible. I'll go into detail if anyone is interested.
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More interested in patches....

>* Actually filesystems should hand out file identifiers for external linking
>which may or may not be the same as their on-disk inumbers.
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>Leo.
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-- 
Hans