Re: raid is dangerous but that's secret (was Re: [patch] ext2/3:

[david]

On Mon, 31 Aug 2009, Pavel Machek wrote:

> > Actually, there is something the file system can do to make journaling
> > safe on degraded RAIDs: make the (checksummed) journal blocks equal to
> > the RAID stripe size.  Or, equivalently, pad out to the RAID stripe
> > size each commit.
> >
> > This sometimes leads to awkward block sizes, but while writing
> > to any *one* stripe on a degraded RAID-5 endangers the others, you
> > can write to *all* of them with the usual semantics.
>
> Well, that would work... but you'd also have to journal data, with the
> same block size. Not exactly fast, but at least safe...
>
> > That's one thing I really like about ZFS: its policy of "don't trust
> > the disks."  If nothing else, simply telling you "your disks f*ed up,
> > and I caught them doing it", instead of the usual mysterious corruption
> > detectec three months later, is tremendoudly useful information.
>
> The more I learn about storage, the more I like idea of zfs. Given the
> subtle issues between filesystem and raid layer, integrating them just
> makes sense.

note that all that zfs does is tell you that you already lost data (and 
then only if the checksumming algorithm would be invalid on a blank block 
being returned), it doesn't protect your data.

David Lang
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