> > Hi Andrew,
> >
> > Andrew Klaassen wrote:
> > > During boot, mdadm starts the array before one of
> the two fibre-channel cards has started its disk
> detection. The array comes up, but with only 28 of 56
> drives, and I have to manually re-add the drives and cross
> my fingers that nothing will go wrong during the 10-hour
> rebuild.
> >
> > Have you considered enabling a write-intent bitmap on
> your array? This
> > way, at least your rebuild will take seconds instead
> of 10 hours. Write
> > intent bitmap support for RAID10 was introduced in
> 2005, and hopefully
> > CentOS 5.2 supports it.
> >
> > > Is there any way to tell mdadm to wait longer, or
> to not attempt to start the array if not all devices are
> present, or... (any other solution you can think of)?
> >
> > We have iscsi targets for drives in our array, and we
> make sure that
> > we've logged into all 30 of our drives before we
> continue to enable
> > mdadm (we literally count the number of iscsi sessions
> open). You can
> > try counting the number of block devices present (in
> /dev/block) that
> > match a certain pattern, or perhaps your fiber channel
> driver offers an
> > even more convenient facility in /dev.
> >
> > However, it would be great if there really was a way
> to tell mdadm to
> > wait until the devices are ready. I'm not aware of one
> though.
> >
>
> Time to go back and read the mdadm man page. From top
> to bottom. Twice.
>
> I suspect that --no-degraded is the flag you want.\
>
> It was introduced in mdadm 2.5
>
> There are three scenarios that could be relevant.
>
> 1/ If an array is being assembled explicitly, e.g.
> mdadm --assemble /dev/mdX .....
> then mdadm will refuse to assemble the array if any
> expected devices are
> missing. You need to add "--run" to get it to start
> a partial array.
>
> 2/ If an array is being assembled using auto-assembly,
> e.g.
> mdadm --assemble --scan
> then mdadm will start partial arrays if it cannot find the
> missing parts
> anyway. You can tell it not to with
> --no-degraded. This flag is actually a
> misnomer. It may well assemble a degraded array, but
> only if the array was
> degraded the last time it was active.
This '--no-degraded' option sounds cool. Can you tell it to apply that logic on some arrays but not others? Like I have an OS drive that can happily come up as degraded if need be. But I also have a 7 drive data array that something the cables come adrift on when i am replacing/adding a drive and i'd rather it just not assemble.. so I can go back and check.
(sorry to steal the thread; kinda)
>
> 3/ If an array is being assembled used a sequence of
> --incremental commands,
> e.g.
> mdadm --incremental /dev/first
> mdadm --incremental /dev/second
> etc
>
> then mdadm won't assemble the array until all expected
> devices have been
> found. Using "--run" will override this so the array
> is assembled as soon
> as enough devices are present. Once all possible
> devices have been
> presented to mdadm it "mdadm -incremental device" you can
> tell mdadm to
> start any arrays that haven't been started yet with
> mdadm --incremental --run
>
> Hope that clears it up.
>
> NeilBrown
>
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