[zfs-discuss] [RFC] Improved versioned pointer algorithms

Akhilesh Mritunjai
Tue, 22 Jul 2008 20:39:02 -0000

> Btrfs does not suffer from this problem as far as I
> can see because it
> uses reference counting rather than a ZFS-style dead
> list.  I was just
> wondering if ZFS devs recognize the problem and are
> working on a
> solution.

Daniel,

Correct me if I''m wrong, but how does reference counting solve this
problem ?

The terminology is as following:

1. Filesystem : A writable "filesystem" with no "references"
or a "parent".
2. Snapshot: Immutable point-in-time view of a filesystem
3. Clone: A writable filesystem whose parent is a given snapsho

Under this terminology, it is easy to see that dead-list is equivalent to
reference counting. The "problem" is rather that to have a clone, you
need to have it''s snapshot around, since by definition it is a child of
a snapshot (with an exception that by using "zfs promote" you can make
a clone a direct child of the filesystem, it''s like turning a
grand-child into a child).

So what is the terminology of brtfs ?


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