On 15/03/2021 06:55, Naohiro Aota wrote:

This commit moves the location of superblock logging zones. The location of
the logging zones are determined based on fixed block addresses instead of
on fixed zone numbers.

By locating the superblock zones using fixed addresses, we can scan a
dumped file system image without the zone information. And, no drawbacks
exist.

We use the following three pairs of zones containing fixed offset
locations, regardless of the device zone size.

  - Primary superblock: zone starting at offset 0 and the following zone
  - First copy: zone containing offset 64GB and the following zone
  - Second copy: zone containing offset 256GB and the following zone

If the location of the zones are outside of disk, we don't record the
superblock copy.

These addresses are arbitrary, but using addresses that are too large
reduces superblock reliability for smaller devices, so we do not want to
exceed 1T to cover all case nicely.

Also, LBAs are generally distributed initially across one head (platter
side) up to one or more zones, then go on the next head backward (the other
side of the same platter), and on to the following head/platter. Thus using
non sequential fixed addresses for superblock logging, such as 0/64G/256G,
likely result in each superblock copy being on a different head/platter
which improves chances of recovery in case of superblock read error.

These zones are reserved for superblock logging and never used for data or
metadata blocks. Zones containing the offsets used to store superblocks in
a regular btrfs volume (no zoned case) are also reserved to avoid
confusion.

Note that we only reserve the 2 zones per primary/copy actually used for
superblock logging. We don't reserve the ranges possibly containing
superblock with the largest supported zone size (0-16GB, 64G-80GB,
256G-272GB).

The first copy position is much larger than for a regular btrfs volume
(64M).  This increase is to avoid overlapping with the log zones for the
primary superblock. This higher location is arbitrary but allows supporting
devices with very large zone size, up to 32GB. But we only allow zone sizes
up to 8GB for now.

Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>

Ping?