Re: [PATCH] cowlinks v2

[Jamie Lokier]

Eric W. Biederman wrote:
> So I see a problem with Scenario C.   Perhaps you can refute it.

Ick.  You're right.  I cannot refute it.

Fwiw, I would have broken the directory cows on the first write, not
the open.

Thus, snapshots using lazy directory copies requires either that there
are no hard links of the type you described (e.g. when snapshotting
the root of the filesystem), or rather complex metadata to track the
hard links, not dissimilar to the metadata needed to preserve hard
links _within_ the snapshot.  They both seem far too complex to be worth it.

Hard links just don't play well with lazy cowlinked directories.

They are fine with non-lazy directory cowlinking, where the whole
directory tree is duplicated and only files are cow'd.  Note that this
doesn't apply to the original implementation which used hard links
with a special flag: maintaining hard links in conjunction with
cowlinks requires the inode duplication we've been talking about.

Btw, if we have cowlinks implemented using inode duplication, then it
isn't necessary for both inodes to have the same metadata such as
mtime, ctime, mode etc.  Only the data is shared.  That means the
sendfile() system call could conceivably be overloaded, meaning to
copy the data, and let "cp --cow" decide whether it wants to copy the
metadata (same as as "cp -rp" or "cp -rpd"), or not (same as "cp -r").

-- Jamie
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