Re: safety of journal based fs (was: Re: still kworker at 100% cpu…)

[Kai Krakow <hurikhan77@gmail.com>]

Am Mon, 14 Dec 2015 10:10:51 +0100
schrieb Martin Steigerwald <martin@lichtvoll.de>:

> > But the problem is, when recovering journal, there is no journal of
> > journal, to keep journal recovering safe from power loss.  
> 
> But the journal should be safe due to a journal commit being one
> sector? Of course for the last changes without a journal commit its:
> The stuff is gone.

This may not be true for disks having write caching enabled and write
barriers of. Then there's no barrier at a journal checkpoint.

Next thing is: At least for ext4, journal is meta-data only.

But I think what was meant here: The case of powerloss during
log-replay... Tho I think the journal should simply be fully replayed
again if it wasn't marked clean before.

Which turns us back to write barriers and write caching... ;-)

It could be that the checkpointing (or marking the journal clean after
replay) could make it do disk before the actual data made it to disk,
due to write-reordering of the hard disk - which can be effectively
circumvented by disabling write caching and enabling write barriers
(the latter should be default while I would always check the former).

-- 
Regards,
Kai

Replies to list-only preferred.