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Re: Advanced Startup/Shutdown with Multilayered Block Devices and Related Issues

Petter Reinholdtsen <pere@hungry.com> writes:

> [Christoph Anton Mitterer]
>> Hi folks.
>> IIRC, Jonas already put some of these issues up here some time ago.
>> I was recently investigating, and thanks to the help of many people
>> found out how deep the problems actually are.
> I suspect this problem is one best solved by using an event based
> system for the early boot, to avoid trying to come up with a fixed
> sequence to enable block devices and disks.  Any fixed sequence is
> going to fail for some setup, and the current alternative is for the
> maintainers to decide on some supported setup and create a working
> sequence for that setup, and leave it to the local sysadmin to cope
> with alternative setups on her own.
> Happy hacking,

There is one big problem with an event based startup. Specifically for
raid1/4/5/6 devices. Those you can use just fine with missing devices
but the boot should really wait for all device to be present.

The big question is how long do you wait? How do you detect that a
device is actually broken/missing and its event will never come? You
can't even check if there are any pending events (udev-settle) because
the device might be slow to start (e.g. a disk on a SATA port multiplier
or SCSI with delayed spin up or external enclosures) and no event has
yet been initialized for it.


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