[Gllug] Re: RAID on laptop or xfs?

[Liam Smit]

Hi

I can imagine that scenario.  But I reckon I' d rather have a
journaled FS suffer a loss of power than a non-journaled FS. The
effect of having a journal tends to minimize the impact on the rest of
the FS. I'm basing that statement on past expereince. But again you'
re still potentially stuck until you go for the 'prevent loss of power
to the cache' route.

cheers
Liam  


On Fri, 1 Apr 2005 22:46:55 +0100, Bruce Richardson
<itsbruce at uklinux.net> wrote:
> On Fri, Apr 01, 2005 at 12:00:50PM +0100, Liam wrote:
> > My understanding is that the integrity of the journal itself is not
> > that crucial fro the integrity of the filesystem
> 
> The journal is critical to the integrity of the filesystem.  With most
> fileystems, that's the only thing the journal actually protects; in most
> cases they only do metadata journalling, not full data journalling.
> 
> > because as soon as
> > the journal gets hosed by the drive losing power and the contents of
> > the cache dissapearing into the ether, the journal is no longer in a
> > postition to write its contents to the rest of the file system.
> 
> You are missing the point that with writeback caching enabled, the drive
> may reorder the sequence in which blocks are flushed to disk, to improve
> performance..  This can be very bad for journalled filesystems, as they
> depend on blocks being written to disk in the correct order.  If, as a
> result of writeback caching, the journal records indicating a completed
> filesystem update are written to disk before the inode changes
> themselves (the opposite way round to the way it should be) and there is
> a power failure before or during the second write, this is bad.  When
> the machine powers up again, the incomplete action will not be replayed
> because the journal will have it listed as finished.
> 
> If you think about it, there is even a small risk of this happening when
> you simply run "shutdown -h now", if the box shuts down quickly enough.
> 
> -- 
> Bruce
> 
> The ice-caps are melting, tra-la-la-la.  All the world is drowning,
> tra-la-la-la-la.  -- Tiny Tim.
> 
>
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