17 Jul
2020
17 Jul
'20
1:39 p.m.
On Jul 17, 2020, at 4:19 PM, Grant Taylor via cctalk
<cctalk at classiccmp.org> wrote:
On 7/17/20 12:58 PM, Paul Koning via cctalk wrote:
Yes. Marketeers like to use the phrase "erasure coding" as a magic token to
indicate great wonderfulness beyond RAID. Actually, it's a math term that describes
the standard RAID systems plus generalizations. I don't know if there is a clear
terminology for the general thing. I tend to call them "k of n" codes, k data
bits, n message bits. Or in the RAID case, k drives worth of data, n total drives. That
makes RAID-1 a 1 of 2 erasure code, the thing we started talking about 1 of 3 erasure
code, RAID-5 is n of n+1 erasure code, and RAID-6 is n of n+2 erasure code. So in the k
of n notation, n-k drives can fail before you lose your data.
Actually, more realistic with today's drive sizes: n-k-1 drives can fail and one drive
can have a hard read error during reconstruction before you lose data. For sufficiently
large drives, the probability of a hard read error somewhere is nearly 1, which is why
RAID-5 is only a good idea for small disk drives.
People have build storage systems with other erasure codes, for example n of n+3. Not
because it's really necessary but as a research experiment; the "Self-star"
system at CMU comes to mind.
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).