Re: [PATCH v2] btrfs: handle dynamically reappearing missing device

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

On 11/18/2017 09:52 PM, Goffredo Baroncelli wrote:
> On 11/17/2017 01:36 PM, Anand Jain wrote:
>> If the device is not present at the time of (-o degrade) mount,
>> the mount context will create a dummy missing struct btrfs_device.
>> Later this device may reappear after the FS is mounted and
>> then device is included in the device list but it missed the
>> open_device part. So this patch handles that case by going
>> through the open_device steps which this device missed and finally
>> adds to the device alloc list.
> 
> What happens if the first devices got writes before the "last device" is joined ?
> 
> Supposing to have a raid1 pair of devices: sda, sdb
> 
> - sda is dissappeared (usb detached ?)
> - the filesystem is mounted as
> 	mount -o degraded /dev/sdb
> - some writes happens on /dev/sdb
> - the user reattach /dev/sda
> - udev run "btrfs dev scan"
> - the system joins /dev/sda to the filesystem disks pool
> 
> Because the filesystem is a raid1, btrfs may read from /dev/sdb (updated data) or /dev/sda (old data), or worse read something from the former and something from the later (metadata from sda and data from sdb ) ???

  Thanks for the test scenario, this case is fine as its read from
  the disk having highest generation number, so we pick sdb in the
  case above.

Thanks, Anand


> Am I missing something ?
> 
> BR
> G.Baroncelli
>