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.a...@wdc.com>
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