checksum-xattr.diff [CVS update: rsync/patches]

Jamie Lokier jamie at
Sat Jun 30 22:18:14 GMT 2007

Matt McCutchen wrote:
> Second, it is impossible to make xattr-based checksum caching
> foolproof against same-second modification.  Suppose a file is written
> during second 5 and then rsync caches its checksum during second 8;
> now the file has mtime 5 and ctime 8.  Sometime later, rsync notices
> that the file still has mtime 5 and ctime 8.  Does rsync trust the
> cached checksum?  It must; otherwise the benefit of caching checksums
> would be lost.  However, rsync will be fooled if the file was modified
> and then touched back to mtime 5 during second 8, right after the
> checksum was cached.  This concern may not be relevant when the
> content is slowly changing.

There really ought to be a special kind of xattr which automatically
disappears when the file is modified, for this sort of thing.  Or a
modification serial number, perhaps only incremented when somebody
actually has read it.  Alas, I think attempts to get one into Linux
didn't get very far; nobody thought it was that important.

-- Jamie

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