On Fri, 24 Jul 2009, Frank Middleton wrote:
On 07/24/09 04:35 PM, Bob Friesenhahn wrote:
Regardless, it [VirtualBox] has committed a crime.
But ZFS is a journalled file system! Any hardware can lose a flush;
From my understanding, ZFS is not a journalled file system. ZFS
relies on ordered writes followed by a cache sync (to make sure that
the bits are on disk) and so it does not use a journaled transaction
rollback mechanism.
Here is a description of what a journaling file system is:
http://en.wikipedia.org/wiki/Journaling_file_system
Notice that the second sentence introduces the notion of a "race
condition", but since ZFS uses ordered writes using freshly allocated
space, there is no possibility of a race condition. A journaling
filesystem uses a journal (transaction log) to roll back (replace with
previous data) the unordered writes in an incomplete transaction. In
the case of ZFS, it is only necessary to go back to the most recent
checkpoint and any subsequent writes after that checkpoint are simply
forgotten.
Bob
--
Bob Friesenhahn
bfrie...@simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer, http://www.GraphicsMagick.org/
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