Re: [PATCH 2/3] btrfs: include non-missing as a qualifier for the latest_bdev

[Anand Jain <anand.jain@oracle.com>]

On 30/4/20 9:46 pm, David Sterba wrote:
> On Tue, Apr 28, 2020 at 11:22:26PM +0800, Anand Jain wrote:
>> btrfs_free_extra_devids() reorgs fs_devices::latest_bdev
>> to point to the bdev with greatest device::generation number.
>> For a typical-missing device the generation number is zero so
>> fs_devices::latest_bdev will never point to it.
>>
>> But if the missing device is due to alienation [1], then
>> device::generation is not-zero and if it is >= to rest of
>> device::generation in the list, then fs_devices::latest_bdev
>> ends up pointing to the missing device and reports the error
>> like this [2]
>>
>> [1] We maintain devices of a fsid (as in fs_device::fsid) in the
>> fs_devices::devices list, a device is considered as an alien device
>> if its fsid does not match with the fs_device::fsid
>>
>> $ mkfs.btrfs -fq /dev/sdd && mount /dev/sdd /btrfs
> 
> Please put each command on one line for clarity
> 

yep.

>> $ mkfs.btrfs -fq -draid1 -mraid1 /dev/sdb /dev/sdc
>> $ sleep 3 # avoid racing with udev's useless scans if needed
>> $ btrfs dev add -f /dev/sdb /btrfs
>> $ mount -o degraded /dev/sdc /btrfs1
> 
> So the cause is a second mkfs on some devices, but is the degraded mount
> supposed to work? The example goes:
> 
Yes. It must work. We don't know if the user is mounting B just after
mkfs or if it already contains some data.

> - create first filesystem with device A
> - create second filesystem with device B and C
> - add device B to the first filesystem, effectively making it missing
> - mount first filesystem, degraded because of the missing device
> 
> For a reproducer that's ok, but is this something that we can expect to
> happen in practice? The flag -f should prevent accidental overwrite, but
> yes the kernel code needs to deal with that in any case.
> 

Its a configuration related, so is left the user how they arrive at
their understanding of what configuration is suitable for them. Yes its
better to fix loop holes in its path. I encountered it when testing
something else.