Re: RAID boot documentation

[NeilBrown <neilb@suse.com>]

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On Wed, Nov 02 2016, Doug Dumitru wrote:

> Does anyone have any documentation on when a non-root RAID device
> mounts and how this is triggered?
>
> I have dug around looking for calls to 'mdadm' and or references to
> mdadm.conf to read the array details, but am not finding the obvious
> things.  Then again, the new boot services still confuses me.
>

This depends on what the distro you are using chooses to implement.

If your distro that uses systemd and uses the udev rules and unit files
that are distrubuted with upstream mdadm, then:

When a device is discovered that contains raid metadata, udev will run
  mdadm --incremental --export /dev/whatever
(or similar)

See /usr/lib/udev/rules.d/64-md-raid-assembly.rules for details.

Once this has happened for all devices in an array, the array will be
started by mdadm.

If mdadm notices that the array *could* be started if needed, but that
it still expects more devices to appear, it reports
    MD_STARTED=*unsafe*
and the udev rules file uses this to trigger a message to systemd to
start "mdadm-last-resort@devicename.timer"
This sets a timer.  When the timer fires, the array is started even
though some devices might be missing.
If the array gets started before that (because the missing devices
appeared) the timer is canceled.

When an md array (or any other device) appear, some other udev handling,
possibly involving udisks, might cause the filesystem to be mounted.
That is completely outside of the control of mdadm.  mdadm just
assembles the device.

Hope that helps.

NeilBrown

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