Re: Grow array and convert from raid 5 to 6

[Roman Mamedov <rm@romanrm.net>]

On Sun, 1 Mar 2020 19:07:56 +0000
antlists <antlists@youngman.org.uk> wrote:
 
> --add disk2 ..." to give an array with 4 active drives and 4 spares. 
> Then you can upgrade to raid 6 - "--grow --level=6 --raid-devices=8".

This feels risky and unclear, for instance what will be the array state if 2
disks fail during this conversion? Would it depend on which ones have failed?

Instead I'd suggest doing this in two steps. First and foremost, add one disk
and convert to RAID6 in a special way:

  --grow --level=6 --raid-devices=5 --layout=preserve

...due to the last bit (see the man page), this will be really fast and will
not even rewrite existing data on the old 4 disks; But you will get a
full-blown RAID6 redundancy array, albeit with a weird on-disk layout.

Then add 3 remaining disks, and

  --grow --level=6 --raid-devices=8 --layout=normalise

Additionally, before you even begin, consider do you really want to go with
such a setup in the first place. I used to run large RAID6s with a single
filesystem on top, but then found them to be too much of a single point of
failure, and now moved on to merging individual disks on the FS level (using
MergerFS or mhddfs) for convenience, and doing 100% backups of everything
for data loss protection. Backups which you have to do anyway, as anyone will
tell you RAID is not a replacement for backup; but with a RAID6 there's too
much of a temptation to skimp on them, which tends to bite badly in the end.
Also back then it seemed way too expensive to backup everything, now with
todays HDD sizes and prices that's no longer so.

-- 
With respect,
Roman