DragonFlyBSD

This RFC patch adds a new hammer command "volume-erase". It erases the hammer volume signature (0xC8414D4DC5523031) or the entire header without using dd. It aims to do the following.

Currently hammer doesn't allow you to volume-add a volume with the signature, even if the volume is not currently used (mounted). When adding a volume, ioctl reads the volume header sector and see if the signature exists. If it does exist then the ioctl tells you to erase it using dd on dmesg. This is right behavior as it protects users from possible operation error, however forcing users to manually do dd is too low level IMO. Filesystem should provide a way to do that with its userspace utility and it's not difficult to implement it with some safety measures.

There isn't anything new in terms of implementation because hammer's userspace already has all the functionalities to do this. It simply finds existing hammer filesystems using getmntinfo(3) and then check each volume to make sure it's currently not used. If it's ok to erase it overwrites the signature or the entire header with 0 after 5 seconds of grace periods. Specifying "header" keyword makes this command erase whole volume header instead of just signature.

Note that hammer volume-del command also erases the signature when removing a device from filesystem and it does that in kernel space. volume-erase command erases the signature by simply memset(3)ing 0 to ondisk structure of the volume header and then write(2) it because it's possible to do it that way with less code change (no need to add another ioctl like HAMMERIOC_ERASE_VOLUME only to expose hammer_clear_volume_header() functionality to userspace).

Also note that this patch moves some code from sbin/hammer/cmd_pseudofs.c to sbin/hammer/misc.c as independent functions that are available from hammer userspace, instead of being static functions, as they are general enough for other hammer commands to use. volume-erase command uses them to do user interaction before erasing.

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some examples

// newfs some devices independently and mount /dev/ad1

1. newfs_hammer -L TEST /dev/ad1 > /dev/null
2. newfs_hammer -L TEST /dev/ad2 > /dev/null
3. newfs_hammer -L TEST /dev/ad3 > /dev/null
4. mount_hammer /dev/ad1 /HAMMER

// erase signature

1. hammer volume-add /dev/ad2 /HAMMER
   hammer volume-add ioctl: Inappropriate file type or format
2. dmesg
   ...
   hammer_volume_add: Formatting of valid HAMMER volume TEST denied. Erase with dd or hammer volume-erase!
   An error occurred: 79
3. od -N 16 -tx1 -Ax /dev/ad2
   0000000 31 30 52 c5 4d 4d 41 c8 00 00 04 00 00 00 00 00
   0000010
4. hammer volume-erase /dev/ad2
   You have requested that volume signature of /dev/ad2 be erased
   Do you really want to do this? y
   Erasing volume signature of /dev/ad2 5 4 3 2 1
   Erased
5. od -N 16 -tx1 -Ax /dev/ad2
   0000000 00 00 00 00 00 00 00 00 00 00 04 00 00 00 00 00
   0000010
6. hammer volume-add /dev/ad2 /HAMMER
7. hammer volume-list /HAMMER
   /dev/ad1
   /dev/ad2

// erase the entire header

1. hammer volume-add /dev/ad3 /HAMMER
   hammer volume-add ioctl: Inappropriate file type or format
2. dmesg
   ...
   hammer_volume_add: Formatting of valid HAMMER volume TEST denied. Erase with dd or hammer volume-erase!
   An error occurred: 79
   hammer_volume_add: Formatting of valid HAMMER volume TEST denied. Erase with dd or hammer volume-erase!
   An error occurred: 79
3. od -N 1928 -tx1 -Ax /dev/ad3
   0000000 31 30 52 c5 4d 4d 41 c8 00 00 04 00 00 00 00 00
   0000010 00 00 04 04 00 00 00 00 00 00 04 14 00 00 00 00
   0000020 00 00 00 00 19 00 00 00 00 00 00 00 00 00 00 00
   0000030 56 1d 07 07 21 a3 e4 11 8f a0 09 00 27 00 47 c3
   0000040 ac 63 dc 61 38 6e dc 11 85 13 01 30 1b b8 a9 f5
   0000050 54 45 53 54 00 00 00 00 00 00 00 00 00 00 00 00
   0000060 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 *
   ...
4. hammer volume-erase /dev/ad3 header
   You have requested that volume header of /dev/ad3 be erased
   Do you really want to do this? y
   Erasing volume header of /dev/ad3 5 4 3 2 1
   Erased
5. od -N 1928 -tx1 -Ax /dev/ad3
   0000000 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 *
   0000780
6. hammer volume-add /dev/ad3 /HAMMER
7. hammer volume-list /HAMMER
   /dev/ad1
   /dev/ad2
   /dev/ad3

// try to erase an already erased volume

1. umount /HAMMER
2. hammer volume-erase /dev/ad3 header
   You have requested that volume header of /dev/ad3 be erased
   Do you really want to do this? y
   Erasing volume header of /dev/ad3 5 4 3 2 1
   Erased
3. hammer volume-erase /dev/ad3 signature
   hammer volume-erase: /dev/ad3 is already erased
4. hammer volume-erase /dev/ad3 header
   hammer volume-erase: /dev/ad3 is already erased

// try to erase a volume used by mounted HAMMER filesystem

1. mount | grep ROOT
   ROOT on / (hammer, local)
2. hammer volume-list /
   /dev/serno/VB1679fd54-1faff0a7.s1d
3. hammer volume-erase /dev/serno/VB1679fd54-1faff0a7.s1d
   hammer volume-erase: /dev/serno/VB1679fd54-1faff0a7.s1d is used by /
4. echo $?
   1