On 09/17/2018 01:36 AM, tudor@xxxxxxxxxxxxxxx wrote:
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.
That's correct.
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
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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