Compaq Smart Array 3200 Controller as a SCSI Controller
cctalk at gtaylor.tnetconsulting.net
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
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
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