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Debian mdadm post boot assemble

After the upgrade to Debian Stretch, I had problems getting my raid volume with external bitmap assembled. After assembling, definition in /etc/mdadm/mdadm.conf, the array was created without the configured eternal bitmap.

Reason for this “Issue” was the fact that the mdadm assemble is done in the¬†initramfs during the boot. In this stage, the volume for the external bitmap doesn’t exist yet.

To solve this, we need to disable the mdadm assemble during boot. The following steps describe how to configure mdadm to assemble an array after booting the kernel and support an external bitmap.

Add an external bitmap to the array.

Disable the initramfs mdadm hook.

Update the initramfs.

To check the current initramfs you can do the following.

After reboot you should find the assembled array with enabled external bitmap.