Compaq Smart Array 3200 Controller as a SCSI Controller

Grant Taylor cctalk at
Fri Jul 17 15:19:13 CDT 2020

On 7/17/20 12:58 PM, Paul Koning via cctalk wrote:
> What is "three drive parity"?

A poor choice of words in a complex topic.

How about "three drives worth of redundancy"  Meaning that your data 
will still be accessible if three drives fail.

ZFS has three versions of ZRAID or RAID-Z.

  - RAID-Z1 is analogous to RAID-5.
  - RAID-Z2 is analogous to RAID-6.
  - RAID-Z3 is analogous to ???

I'm not aware of any official definition of a mirror of more than two 
drives.  I've heard of "3-way" / "4-way" / "n-way" mirrors.

I think that the industry has settled on RAID-10 / RAID-01 and possibly 
RAID-11 / maybe even RAID-00.  But that isn't a standard to me.

Further, I see disagreements of what is the strip and what is the mirror 
in RAID-10 / RAID-01.

> "Parity" is the description of RAID-5, and 3 drive RAID-5 is certainly 
> perfectly standard.  RAID-1 is not parity, it's mirroring.

If you think of it as "redundant drives" or "number of drives that can 
fail without destroying data", then yes, RAID-1 does have a numerical 
value quite similar to RAID-3 / RAID-5 / RAID-6 / RAID-Z* / etc.. 
Though nomenclature becomes highly problematic.

> Is the question about triple mirroring, i.e., 3 drives all having 
> the same data on them?

I was stating that I'm not aware of an official RAID level designation 
for ZFS's RAID-Z3.

> That's pretty rare though not unheard of, I've never seen a RAID-x 
> designation for that.

I've known more than a few people to use n-way mirrors (~RAID-1). 
Though I think I've only seen it in software.

> For high availability, RAID-6 is much more economical (and at this 
> point the standard choice); triple mirroring is of that class, with 
> the difference that it performs better for random short writes.

Are you comparing RAID-6 to triple (3-way) mirroring?  Or something else?

I think that things get really weird and deep in minutia when you start 
comparing a 3-way mirror to a 3 drive RAID-6.  Same number of drives 
(3), and same capacity (1 drive worth), and same fault tolerance (2 
drive failures).

One of the other things that I've thus far neglected to mention about 
ZFS is it's abilities to take snapshots and then subsequently send & 
receive said snapshots* between pools / tape / image files.  These are 
some things that I think are nigh impossible to do with typical 
traditional hardware RAID controllers.  Sure, you might be able to do it 
with systems that fall into the broad category of a higher end RAID 
controller, but that is more typically a SAN controller which is 
effectively it's own microcosm.

* You can also send / receive unmounted file systems** in addition to 
snapshots of file systems.

** You can do similar with a zDevice, which is a logical block device 
created by the ZFS pool.

Grant. . . .
unix || die

More information about the cctalk mailing list