Re: RFC - device names and mdadm with some reference to udev.

[Neil Brown <neilb@suse.de>]

On Monday October 27, dledford@redhat.com wrote:
> 
> I've found the udev rules method of starting md devices to be
> problematic (at best).
> 
> Here's the issue (in Fedora at least).  Starting devices via udev means
> starting them as soon as they are capable and not waiting until all
> devices are up and running.  You have to do this in case the device is
> in a degraded state and you aren't going to get all the devices.
> However, we don't create a bitmap on devices by default in the installer
> (a user can add one themselves, but it isn't there by default).  Without
> the bitmap, if the device is written to before all devices are added, it
> triggers a full resync of the device.  As it turns out, for certain
> installations, this happens on *every* single reboot.  It's painful, to
> say the least.   So, I wanted to change the udev rule to work slightly
> differently.  I wanted the invocation of mdadm --incremental that
> happened to be the one that took the array from an unrunable state to a
> runable but degraded state to sleep for say 2 to 5 seconds, and then if
> the array is still not up and running due to subsequent udev rule
> invocations, it would start the array in a degraded state.  This,
> however, breaks udevsettle.  So, the current setup (for the upcoming
> fedora 10) is done such that the udev rule won't start any degraded
> arrays, and instead we have both a specific mdadm invocation in the
> initrd and another in rc.sysinit that will start any degraded arrays
> that are also listed in the mdadm.conf file.  This makes sure that known
> arrays are assembled and started if at all possible, but we only start
> unknown arrays if they are complete.
> 

This is using udev to start md devices, which is not quite the focus
of the previous discussion.  That was more about using udev to create
the entries in /dev when someone else started the arrays.

However this is still a real issue that I would like to handle as best
we can.

I would like to get the md code to always have at least an in-memory
bitmap to allow quite resync after a "re-add".

However even this isn't a perfect solution as there is a window when a
single device failure can kill an array.

Your solution sounds good, but I'd be happy to hear other thoughts on
the issue.

Thanks,
NeilBrown