Re: [PATCH 2/2] btrfs: btrfs_rm_device() should zero mirror SB as well

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btrfs fi show
Label: none  uuid: e7aae9f0-1aa8-41f5-8fb6-d4d8f80cdb2c
         Total devices 1 FS bytes used 28.00KiB
         devid    2 size 2.00GiB used 0.00 path /dev/sdc   <-- WRONG
         devid    1 size 2.00GiB used 20.00MiB path /dev/sdb

Ok, now it's findable.  Isn't that exactly how this should behave?
What is wrong about this?

 Total devices is still 1.

mount /dev/sdc /btrfs
btrfs fi show --kernel
Label: none  uuid: e7aae9f0-1aa8-41f5-8fb6-d4d8f80cdb2c mounted: /btrfs
         Group profile: metadata: single  data: single
         Total devices 1 FS bytes used 28.00KiB
         devid    1 size 2.00GiB used 20.00MiB path /dev/sdb

Oh good, you could bring it back after a potential administrative error,
using a recovery tool (btrfs-select-super)!  Isn't that a good thing?

 Note, here btrfs fi show used the new option --kernel
 this does not show /dev/sdc though you use it mount.
 Its all messed up.

 If user wants to bring back the intentionally deleted
 disk, then they should rather call btrfs dev add, so
 that it will take care of integrating the disk back
 to the FS.

 recovery tools are for possible recovery from the
 corruption, delete is not a corruption. Thats an
 intentional step that user decided to take and the
 undo for it is 'dev add'.

IOWS: what does this change actually fix?

 Writes zeros to all copies of SB when disk is deleted
 (before we used to just zero only the first copy).
 In that way corruption is distinguished from the
 deleted disk in a fair calculations.

 Otherwise allowing these things would cost us in terms
 of support for the administrative error. Which we don't
 have to encourage.

Thanks, Anand
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