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