On Tue, Oct 25, 2011 at 08:03:39AM -0200, Adriano dos Santos Fernandes wrote:
> Let consider ext3/4 filesystem with barriers enabled, then:
...
> 2) FW=OFF, and use fsync on COMMIT - pages will not be reordered, and
> when COMMIT happens they will be written to disk in order
No. Barrier is something like a mark saying "everything sent before must
be written to the physical medium before anything that will be sent
after" (if both the controller and the disk support barriers). So what
you have written would be true only if system sent a barrier after every
page.
Filesystem drivers use barriers to guarantee that transaction metadata
(and data if data are journaled which is almost never the case) are
written to journal before the transaction starts being written to the
filesystem itself and that everything is written to the filesystem
before transaction is marked as finished in journal. This is needed to
guarantee that if system is shut down (e.g. by a power outage) at any
moment, info in the journal is sufficient to get the filesystem into a
consistent state.
Michal Kubeček
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