On Thu, 05 Mar 2015 15:00:18 -0500 Phil Turmel <philip@xxxxxxxxxx> 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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