Geom_label and multiple devices

[Eric Anderson]

On 03/28/07 05:39, Ivan Voras wrote:
> I've just thought of something - the situation where geom_label detects 
> multiple "same" labels is actually very similar to that of 
> geom_multipath. Obviously this doesn't apply 100% to UFS and other 
> filesystems' labels (except if there's a way to identify different file 
> systems by their superblock, for example timestamp of creation?), but in 
> case of "manual" labels, if the unique ID matches and the label string 
> matches, it's probably the same device.
> 
> Currently, geom_label discards "additional" devices with the same label, 
> and I remember there was a discussion about if that should be changed 
> (though I don't remember the conclusions). It would be useful if 
> geom_label would work the same as geom_multipath in this case (i.e. 
> failover to "next" device, add new devices with same label+id to the 
> pool), or even if geom_multipath were merged in geom_label.
> 
> Comments?
> 


I think that the duplicate label case is not really the same as 
multipath.   I see your point, but the duplicate label issue is more of 
a cause from mistake rather than intention.

To me, these are the cases that could arise for geom_label in the 
duplicate label case:

1) Same device, multiple paths.  Can be detected like you mention above 
(checking superblocks). That works fine for UFS but not everything else. 
  This case must be certain that it *is* the same device.  This is hard 
to do.

2) Possibly same device with multiple paths.  Maybe there's no file 
system on the device yet, or geom_label can't determine if it's the same 
device (sizes, etc all match exactly, but it's not 100% certain).

3) Definitely *not* the same device.

For (1), once geom_label is certain it is truly the same path, ignore 
the additional duplicate labels found, but print a warning like:
"geom_label: duplicate /dev/... found: multiple paths for same label."
"geom_label: removing subsequent duplicates for /dev/..."

For (2), we something similar, but rename any duplicates so the 
administrator can still deal with the duplicate via a label.  That helps 
the admin determine what really went on. Something like this:
"geom_label: duplicate /dev/...: possible multiple paths for same label."
"geom_label: renaming duplicate label /dev/... to /dev/...-DUP1"

And for (3), the result is the same as (2), except the error message is 
different:
"geom_label: duplicate /dev/...: SAME LABEL FOR DIFFERENT DEVICES!"
"geom_label: renaming duplicate label /dev/... to /dev/...-DUP1"

Maybe it would be a good idea to allow a priority value in the 
geom_label meta data so that an admin could prioritize his labels.  For 
instance, on my laptop, I have labeled my root partition 
/dev/label/root.  If I have a compact flash plugged in that I'm working 
on for a small embedded device, I also need it named /dev/label/root, 
even though I'm already using that label.  I want to be able to reboot 
my laptop with the compact flash inserted still, without it possibly 
booting off the flash instead.  To protect against that, I could 
prioritize my internal hd's 'root' labeled device to a priority of '1', 
which means 'nothing is higher priority than this device'.  A '0' would 
mean no priority given, and any number greater than 0 is less priority. 
  That way admins could mark their labels and allow certain devices to 
be forced more important than others.  Comments?


Eric