Hard links are not like symbolic links, they are not merely pointers to
other files, but alternative names for the same file. The name of every
file is a hard link, the first so to speak. You can add additional names
which really are the file, they are not just pointers. For the
technically minded, they are not separate inodes, they are additional
directory references to the same inode. When you perform a copy
operation on multiple files, cfengine attempts to preserve hard links
but this is a difficult task.
Because a hard link just looks like an ordinary file (it cannot be
distingiushed from the original, the way a symbolic link can)
there is a danger that any copy operation will copy two hard links to
the same file as two separate copies of the same file. The difference
is that changes a hard-linked file propagate to the links, whereas
two copies of a file are completely independent thereafter.
In order to faithfully reproduce all hardlinks to all files, cfengine
needs to examine every file on the same filesystem and check whether
they have the same inode-number. This would be an enourmous overhead, so it
is not done. Instead what happens is that cfengine keeps track of only
the files which it is asked to examine, for each atomic copy-command,
and makes a note of any repeated inodes within this restricted set.
It does not try to go off, wandering around file systems looking to
other files which might be hardlinks.
To summarize, cfengine preserves hardlinks during copying, only
within the scope of the present search. No backups are made of
hard links, only of the first link or name of the file is backed
up. This is a necessary precaution to avoid dangling references
in the inode table. As a general rule, hard links are to be avoided
because they are difficult to keep track of.
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