This commit adds zone-aware magics and probing functions for zoned btrfs.
The superblock (and its copies) are the only data structure in btrfs with a
fixed location on a device. Since we cannot do overwrites in a sequential
write required zone, we cannot place the superblock in the zone.
Thus, zoned btrfs uses superblock log writing to update superblocks on
sequential write required zones. It uses two zones as a circular buffer to
write updated superblocks. Once the first zone is filled up, start writing
into the second buffer. When both zones are filled up, and before starting
to write to the first zone again, it reset the first zone.
We can determine the position of the latest superblock by reading the write
pointer information from a device. One corner case is when both zones are
full. For this situation, we read out the last superblock of each zone and
compare them to determine which zone is older.
The magics can detect a superblock magic ("_BHRfs_M") at the beginning of
zone #0 or zone #1 to see if it is zoned btrfs. When both zones are filled
up, zoned btrfs resets the first zone to write a new superblock. If btrfs
crashes at the moment, we do not see a superblock at zone #0. Thus, we need
to check not only zone #0 but also zone #1.
It also supports the temporary magic ("!BHRfS_M") in zone #0. Mkfs.btrfs
first writes the temporary superblock to the zone during the mkfs process.
It will survive there until the zones are filled up and reset. So, we also
need to detect this temporary magic.
Finally, this commit extends probe_btrfs() to load the latest superblock
determined by the write pointers.
Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
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