how to cleanup old superblock

Hello!
Long time ago I created btrfs on /dev/sda
After some changes btrfs moved to /dev/sda1 (well, to md, and sda1 is part of md).

As result, <btrfs fi show> show me 2 filesystems: new one and old one. Probably I need to do some cleaning. Can someone tell me what to do.

fdisk -u -c -l /dev/sda

Disk /dev/sda: 2000.4 GB, 2000398934016 bytes
81 heads, 63 sectors/track, 765633 cylinders, total 3907029168 sectors Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disk identifier: 0xde07ba46

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1            2048  3907029167  1953513560   fd  Linux raid

btrfs fi show
Label: 'storage' uuid: ea3824ec-2fd5-4813-88da-d96c319029f4
Total devices 1 FS bytes used 50.87GB
devid 1 size 1.82TB used 138.04GB path /dev/md127

Label: 'mainst' uuid: 1bd2b9be-5464-4926-88da-af8e12c21f94
Total devices 3 FS bytes used 45.94GB
devid 1 size 1.82TB used 123.04GB path /dev/sda
*** Some devices missing

Last one is wrong.

quick aid is to run this command:

dd if=/dev/zero of=/dev/sda bs=1k count=4 seek=64

(deletes just the btrfs superblock and will not touch anything else)

david

Updated the wiki:
https://btrfs.wiki.kernel.org/index.php/Problem_FAQ#How_to_clean_up_old_superblock_.3F

Xavier Nicollet

Thanks. I redid the calculations and the statement that it 'will not touch anything else' may not be correct in rare cases.

Depends where the first partition starts. If it's at sector 63, like fdisk created by default in the past, overwriting the offset 64k (relative to /dev/sda) is actually

offset = 64*1024 - 63*512 = 33280

relative to /dev/sda1.

If there are metadata (eg. raid, other fs) at this offset, then it can cause problems. For safety reasons, a btrfs filesystem does not touch first megabyte for allocations, so this does not clash. I'm not sure about md-raid, but if the sda:64k block is used, then it would be overwritten anyway.

Newer fdisks start the first partition at 1M, so it's safe.

Wiki page updated.

For the reference - this feature belongs to progs, discussed here
http://thread.gmane.org/gmane.comp.file-systems.btrfs/17065

david

David Sterba wrote (ao):

What about wipefs?

"wipefs allows to erase filesystem or raid signatures (magic strings) from the device to make the filesystem invisible for libblkid. wipefs does not erase the whole filesystem or any other data from the device. When used without options -a or -o, it lists all visible filesystems and offsets of their signatures."

Sander