>> Applying classic RAID terms to zfs is just plain
>> wrong and misleading
>> since zfs does not directly implement these classic
>> RAID approaches
>> even though it re-uses some of the algorithms for
>> data recovery.
>> Details do matter.
>
> That's not entirely true, is it?
> * RAIDZ is RAID5 + checksum + COW
> * RAIDZ2 is RAID6 + checksum + COW
> * A stack of mirror vdevs is RAID10 + checksum + COW
Others have noted that RAID-Z isn't really the same as RAID-5 and RAID-Z2 isn't
the same as RAID-6 because RAID-5 and RAID-6 define not just the number of
parity disks (which would have made far more sense in my mind), but instead
also include in the definition a notion of how the data and parity are laid
out. The RAID levels were used to describe groupings of existing
implementations and conflate things like the number of parity devices with,
say, how parity is distributed across devices.
For example, RAID-Z1 lays out data most like RAID-3, that is a single block is
carved up and spread across many disks, but distributes parity as required for
RAID-5 but in a different manner. It's an unfortunate state of affairs which is
why further RAID levels should identify only the most salient aspect (the
number of parity devices) or we should use unambiguous terms like single-parity
and double-parity RAID.
> If we can compare apples and oranges, would you same recommendation ("use
> raidz2 and/or raidz3") be the same when comparing to mirror with the same
> number of drives? In other words, a 2 drive mirror compares to raidz{1} the
> same as a 3 drive mirror compares to raidz2 and a 4 drive mirror compares to
> raidz3? If you were enterprise (in other words card about perf) why would
> you ever use raidz instead of throwing more drives at the problem and doing
> mirroring with identical parity?
You're right that a mirror is a degenerate form of raidz1, for example, but
mirrors allow for specific optimizations. While the redundancy would be the
same, the performance would not.
Adam
--
Adam Leventhal, Fishworks http://blogs.sun.com/ahl
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