Re: Preventing a RAID device from starting until all disks are ready

[Jon Hardcastle <jd_hardcastle@yahoo.com>]

> > 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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