Re: [PATCH V2][BTRFS-PROGS] Don't use LVM snapshot device

[Phillip Susi <psusi@ubuntu.com>]

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On 12/4/2014 1:39 PM, Goffredo Baroncelli wrote:
> LVM snapshots are a problem for the btrfs devices management. BTRFS
> assumes that each device have an unique 'device UUID'. A LVM
> snapshot breaks this assumption.
> 
> This causes a lot of problems if some btrfs device are
> snapshotted: - the set of devices for a btrfs multi-volume
> filesystem may be mixed (i.e. some NON snapshotted device with some
> snapshotted devices) - /proc/mount may returns a wrong device.
> 
> In the mailing list some posts reported these incidents.
> 
> This patch allows btrfs to skip LVM snapshot during the device scan
>  phase.
> 
> But if you need to consider a LVM snapshot you can set the 
> environment variable BTRFS_SKIP_LVM_SNAPSHOT to "no". In this case 
> the old behavior is applied.
> 
> To check if a device is a LVM snapshot, it is checked the 'udev' 
> device property 'DM_UDEV_LOW_PRIORITY_FLAG' . If it is set to 1, 
> the device has to be skipped.
> 
> As consequence, btrfs now depends also by the libudev.

Rather than modify btrfs device scan to link to libudev and ignore the
caller when commanded to scan a snapshot, wouldn't it be
simpler/better to just fix the udev rule to not *call* btrfs device
scan on the snapshot?


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