linux-kernel - Will mdadm make a backup if it takes two steps to covert raid5 to raid6?

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Message-ID: <CAJtTb3G_GAepixfgY38e9GVrB6RrXDcTkaEWcfvLU14yjc734Q@mail.gmail.com>
Date:   Wed, 13 Dec 2017 11:38:03 +0800
From:   Amy Chiang <amychiang@...p.com>
To:     linux-kernel@...r.kernel.org
Subject: Will mdadm make a backup if it takes two steps to covert raid5 to raid6?

Hi,

I was trying to grow my raid5 array to raid6 with one more disk. And I
was wondering about when would the backup take place and when would
the backup file is required (or not required).

And I found this thread "Re: Backup file size when migrating from
raid5 to raid6?"
two steps were mentioned in the thread to reduce the time of level changing.

so I followed the steps,

currently there were 3 disks in my array.

$ mdadm --grow /dev/md0 --level=6 --layout=preserve
mdadm: Need 1 spare to avoid degraded array, and only have 0.
       Use --force to over-ride this check.
$ mdadm --grow /dev/md0 --level=6 --layout=preserve --force
mdadm: level of /dev/md119 changed to raid6

the array became raid6 and the state is clean, degraded and nothing
more happened, I thought it would create Q in this step, but there
were no writing from iostat.

so I added one disk
$ mdadm --grow /dev/md0 --raid-devices=4 --layout=normalise

State : clean, degraded, reshaping

but from previous post in this thread, I thought the backup file is a
must under this circumstance, and according to the throughput of those
disks (not slow), there seemed no backup was happening.

my question is,
1. "mdadm --grow /dev/md0 --level=6 --layout=preserve" will this
command create Q even though it degrades the array?
2. Is backup happening in second step (reshaping)? if it's happening,
why don't I need a backup file?

Thanks,