On 4/8/21 4:25 AM, Naohiro Aota wrote:
This commit moves the location of the superblock logging zones. The new
locations of the logging zones are now determined based on fixed block
addresses instead of on fixed zone numbers.
The old placement method based on fixed zone numbers causes problems when
one needs to inspect a file system image without access to the drive zone
information. In such case, the super block locations cannot be reliably
determined as the zone size is unknown. By locating the superblock logging
zones using fixed addresses, we can scan a dumped file system image without
the zone information since a super block copy will always be present at or
after the fixed location.
This commit introduces 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 a logging zone is outside of the disk capacity, we do not record the
superblock copy.
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 sizes, up to 32GB. Such large zone size is
unrealistic and very unlikely to ever be seen in real devices. Currently,
SMR disks have a zone size of 256MB, and we are expecting ZNS drives to be
in the 1-4GB range, so this 32GB limit gives us room to breathe. For now,
we only allow zone sizes up to 8GB, below this hard limit of 32GB.
The fixed location addresses are somewhat arbitrary, but with the intent of
maintaining superblock reliability even for smaller devices. For this
reason, the superblock fixed locations do not exceed 1TB.
The superblock logging zones are reserved for superblock logging and never
used for data or metadata blocks. Note that we only reserve the two zones
per primary/copy actually used for superblock logging. We do not reserve
the ranges of zones possibly containing superblocks with the largest
supported zone size (0-16GB, 64G-80GB, 256G-272GB).
The zones containing the fixed location offsets used to store superblocks
in a regular btrfs volume (no zoned case) are also reserved to avoid
confusion.
Signed-off-by: Naohiro Aota <naohiro.a...@wdc.com>
Thanks Naohiro, this makes it much easier to understand,
Reviewed-by: Josef Bacik <jo...@toxicpanda.com>
Dave, I know you're on vacation, this needs to go to Linus for this cycle so we
don't have two different SB slots for two different kernels. I don't want to
disturb your vacation, so I'll put this in a pull request tomorrow to Linus. If
you already had plans to do this let me know and I'll hold off. Thanks,
Josef
