Re: [linux-lvm] libdm cannot swap names between two child volumes

[Zdenek Kabelac <zkabelac@redhat.com>]

Dne 11.6.2015 v 04:54 M.H. Tsai napsal(a):
> 2015-06-08 15:17 GMT+08:00 Zdenek Kabelac <zkabelac@redhat.com>:
>> It doesn't really matter here what you could confirm here - there is a race
>> you can't avoid - i.e. udev is completely 'independent' and may execute
>> trigger udev rules at any random point in time or some other command may try
>> to open device in parallel (i.e. 'dd')
>> So the only way how to ensure there is no such race - is to deactivate such
>> device (which should be possible - since as you said - noone has it open)
>> Also remember - activation routine is 'separate' from command code - as it
>> could run on a completely different node - so you cannot 'validate' from
>> command code there is no user of a device on 'activation' node unless device
>> is locally active.
>> Thus going through the proper sequence of steps and allowing udev to
>> properly synchronize (i.e. you should not 'mix'  activation & deactivation
>> under same cookie) is clearly the best way how to achieve your desired goal.
>
> Hi,
>
> Sorry for opening this question again. I could image the potential problem in
> name swapping. But does that mean, It's even unrecommended to rename
> an active device, although the target name are not used? The renaming path
> in LVM is not suggested ?
>
> ## this is not recommended
> # lvchange -ay vg1/lv1
> # lvrename vg1/lv1 vg1/lv2
>
> ## do this instead
> # lvchange -an vg1/lv1
> # lvrename vg1/lv1 vg1/lv2

Rename is fine as long as you are not reusing names while devices are still 
active.

Check which devices names are reported by i.e. mount after you rename a device 
- keep devices mounted and do rename like this:

lvcreate lv1
mkfs,mount lv1
lvrename lv1 -> lv2
lvcreate lv3
mkfs,mount lv3
lvrename lv3 -> lv1

Check 'mount' result or /proc/self/mountinfo

So doing such 'live' rename may lead to major confusion of system admin - 
since those device names might be completely different from reality in your 
/dev dir.

Zdenek