/etc/modprobe.d/mdadm.conf is in mdadm 3.3.2-5+deb8u2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 | # mdadm module configuration file
# set start_ro=1 to make newly assembled arrays read-only initially,
# to prevent metadata writes. This is needed in order to allow
# resume-from-disk to work - new boot should not perform writes
# because it will be done behind the back of the system being
# resumed. See http://bugs.debian.org/415441 for details.
options md_mod start_ro=1
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