Does Btrfs use the first 64KB of the partition for data? If not, then what is its significance? I have ran various experiments with it and found that it remains unused even when partition is filled to capacity.
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No, Btrfs does not use this area for data. This is a bootloader area. Chris Mason, the principal Btrfs author, explained it in his answer to the following question (I added [few remarks] and code formatting for readability):
When creating a btrfs volume with
mkfs.btrfs, I'm noticing that the first 64KB are completely blank. Is this gap expressly intended for installing a boot manager/loader? e.g. GRUB 2 allows installation ofboot.img+core.imginto a btrfs formatted partition, without using block lists (the--forceflag). It appears to produce a bootable system.Right, I wanted to leave room for bootloaders. I actually would have used a larger offset, but the other filesystem
mkfscommands don't zero very far into the drive. I had to make sure thatmkfs.foowould overwrite the btrfs super[block], otherwise you might have a stale btrfs filesystem recognized inside the new filesystem.[…]
-chris [Mason]
answered May 12, 2017 at 21:46
