Re: Last working drive in RAID1

[NeilBrown <neilb@suse.de>]

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On Thu, 05 Mar 2015 15:00:18 -0500 Phil Turmel <philip@turmel.org> wrote:

> On 03/05/2015 10:55 AM, Wols Lists wrote:
> > Sorry to butt in, but I'm finding this conversation a bit surreal ...
> > take everything I say with a pinch of salt. But the really weird bit
> > was "what does linux do if /dev/sda disappears?"
> > 
> > In the old days, with /dev/hd*, the * had a hard mapping to the
> > hardware. hda was the ide0 primary, hdd was the ide1 secondary, etc
> > etc. I think I ran several systems with just hdb and hdd. Not a good
> > idea, but.
> > 
> > Nowadays, with sd*, the letter is assigned in order of finding the
> > drive. So if sda is removed, linux moves all the other drives and what
> > was sdb becomes sda.
> 
> On reboot, yes.  Not live.  If /dev/sda has anything using it when
> unplugged, it stays there and gives errors.  Existing devices retain
> their names, and new devices are added to the end.  Only if all users
> are completely unhooked will a kernel name get re-used live.
> 
> > Which is why you're advised now to always refer
> > to drives by their BLKDEV or whatever, as linux provides no guarantees
> > whatsoever about sd*. The blockdev may only be a symlink to whatever
> > the sd*n code of the disk is, but it makes sure you get the disk you
> > want when the sd*n changes under you.
> 
> Correct, but once assembled or mounted, the kernel names are locked.
> 
> > Equally surreal is the comment about "what does raid1 do with no
> > working devices?". Surely it will do nothing, if there's no spinning
> > rust or whatever underneath it? You can't corrupt it if there's
> > nothing there to corrupt?
> 
> It has to stay there to give errors to the upper layers that are still
> hooked to it.  When they are administratively "unhooked", aka unmounted
> or disassociated with mdadm --remove.
> 
> Or, quite possibly, the device is plugged back in, at which point the
> device name is there for it (as long as you use the same port, of
> course).  In which case the filesystem may very well resume successfully.

I was with you right up to this last point.
When a device is unplugged and then plugged back in, it will always get a new
name.  Detecting "is the same device" is far from fool-proof, particularly as
the device could have been plugged into some other machine and has 'fsck' etc
run.
Once a mounted device is unplugged, that mount is permanently unusable.

NeilBrown


> 
> > Sorry again if this is inappropriate, but you're coming over as so
> > buried in the trees that you can't see the wood.
> 
> Sometimes the expanded view is appropriate :-)
> 
> Regards,
> 
> Phil Turmel


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