[linux-lvm] (Why) Does lvmcreate --raid6 need 5 drives?

[t***@tudorholton.com]

This question was originally put on ServerFault here:
https://serverfault.com/questions/931150/why-does-lvmraid6-need-5-drives

I'm investigating using LVMRAID instead of MD+LVM. In my case, I'm
looking at a RAID6. As expected, the docs say you need N+1 devices for
RAID5, and a minimum of 3 drives. The docs say for RAID 6 you need N+2.

However, it then says that the minimum drives required is 5, which means
that N is different.

In an MD RAID6, only 4 drives are required; 2 for striping, 2 for
parity. So if I built RAID6+LVM I'd only need 4 drives whereas,
according to the docs, I'd need 5 drives for LVMRAID6. It was my
understanding that LVMRAID is really just LVM on MD, so I don't
understand the difference.

Thomas on ServerFault pointed out that a minimum of 3 stripes is
hardcoded into the source code[1].

But the commit message[2] doesn't seem to indicate a reasoning behind
requiring a higher number of striped for RAID6 than RAID5.

Why does LVMRAID need 3 stripes for RAID6 where MD only needs 2?

Cheers,
Tudor.

[1]
https://sourceware.org/git/?p=lvm2.git;a=blob;f=tools/lvcreate.c;h=3c22794b6f1c3cbd155df65421de21dac0bfa9fc;hb=HEAD#l528
[2]
https://sourceware.org/git/?p=lvm2.git;a=commit;f=tools/lvcreate.c;h=ea0f604e704dc8418d962e20529fd1bcddf06bb7

[Heinz Mauelshagen]

Post by t***@tudorholton.com
https://serverfault.com/questions/931150/why-does-lvmraid6-need-5-drives
I'm investigating using LVMRAID instead of MD+LVM. In my case, I'm
looking at a RAID6. As expected, the docs say you need N+1 devices for
RAID5, and a minimum of 3 drives. The docs say for RAID 6 you need N+2.
However, it then says that the minimum drives required is 5, which
means that N is different.
In an MD RAID6, only 4 drives are required; 2 for striping, 2 for
parity. So if I built RAID6+LVM I'd only need 4 drives whereas,
according to the docs, I'd need 5 drives for LVMRAID6.  It was my
understanding that LVMRAID is really just LVM on MD, so I don't
understand the difference.

That's correct.

Post by t***@tudorholton.com
Thomas on ServerFault pointed out that a minimum of 3 stripes is
hardcoded into the source code[1].
But the commit message[2] doesn't seem to indicate a reasoning behind
requiring a higher number of striped for RAID6 than RAID5.
Why does LVMRAID need 3 stripes for RAID6 where MD only needs 2?

We do have a constraint in lvm2 to require the raid6 minimum for N to be 3.

Configuring a raid6 LV (an array by MD terms) with 2 data stripes is
suboptimal for performance,
because data striping is minimal in this case.  In addition, the
metadata overhead is maximal
for parity, P- and Q-syndromes being half of the brutto size of the
raid6 LV.

Regards,
Heinz

Post by t***@tudorholton.com
Cheers,
Tudor.
[1]
https://sourceware.org/git/?p=lvm2.git;a=blob;f=tools/lvcreate.c;h=3c22794b6f1c3cbd155df65421de21dac0bfa9fc;hb=HEAD#l528
[2]
https://sourceware.org/git/?p=lvm2.git;a=commit;f=tools/lvcreate.c;h=ea0f604e704dc8418d962e20529fd1bcddf06bb7
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[t***@tudorholton.com]

--------snip------

Post by Heinz Mauelshagen
We do have a constraint in lvm2 to require the raid6 minimum for N to be 3.
Configuring a raid6 LV (an array by MD terms) with 2 data stripes is
suboptimal for performance,
because data striping is minimal in this case.  In addition, the
metadata overhead is maximal
for parity, P- and Q-syndromes being half of the brutto size of the
raid6 LV.

My apologies. I googled "brutto size" but came up with nothing. I do
have a basic understanding of P and Q syndromes so I think I have a
vague understanding of your meaning.

I understand it's suboptimal, but not non-existent. In particular why
is this a restriction and not just a warning?

Performance aside, I could also argue that there's a use case for a
minimal (and not degraded) set before expansion.

Cheers,
Tudor.

[Heinz Mauelshagen]

Post by t***@tudorholton.com
--------snip------

Post by Heinz Mauelshagen
We do have a constraint in lvm2 to require the raid6 minimum for N to be 3.
Configuring a raid6 LV (an array by MD terms) with 2 data stripes is
suboptimal for performance,
because data striping is minimal in this case.  In addition, the
metadata overhead is maximal
for parity, P- and Q-syndromes being half of the brutto size of the
raid6 LV.

My apologies.  I googled "brutto size" but came up with nothing. I do
have a basic understanding of P and Q syndromes so I think I have a
vague understanding of your meaning.

I have to apologize, I meant gross size

Post by t***@tudorholton.com
I understand it's suboptimal, but not non-existent.  In particular why
is this a restriction and not just a warning?

The lvm2 code assumes data stripes to be more than parity/syndrome stripes.

Post by t***@tudorholton.com
Performance aside, I could also argue that there's a use case for a
minimal (and not degraded) set before expansion.

Sure you could,  when expansion is planned one can create the raid6 set
with the intended total stripes though.

That aside, this is a constraint for such use cases and we may
eventually get rid of it.

Regards,
Heinz

Post by t***@tudorholton.com
Cheers,
Tudor.
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