Re: triggering udev rules based on the state of udevd

[Scott James Remnant <scott@canonical.com>]

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On Wed, 2008-07-02 at 11:15 -0400, Bill Nottingham wrote:

> When using udev to automatically assemble MD devices (using mdadm --incremental),
> we've come across the following conundrum:
> 
> - If you just pass --incremental to mdadm, devices will only be started when all
>   members are present; you can never start a degraded device)
> 
> - If you pass --run (to solve this), devices will be started when the minimum
>   # of devices is present. This causes the array to always start in degraded
>   mode, causing unnecessary resyncs.
> 
> There doesn't seem to be a good happy medium that allows for degraded assembly
> when needed, but normal assembly in most cases.
> 
Surely only the user knows whether to start degraded or not?

Our approach is to incrementally assemble raid arrays through udev, and
after a timeout, if any raid we're expecting to be able to mount is not
ready, ask the user what they want to do about it.

	"RAID-1 device for /home is missing a volume.
	 Please ensure this volume is connected, or press ENTER to
	 use the device in a degraded mode."

Obvious advantage here is that we haven't given up, if the user realises
the cable is hanging out, they can plug it in and the message will go
away and the boot continues.

If they start in degraded mode, the RAID members remember that and the
next time you use --incremental, it'll start anyway. (fwict)

Scott
-- 
Scott James Remnant
scott@canonical.com

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