On Fri, Apr 09, 2021 at 01:07:19PM +0200, David Sterba wrote:
> On Thu, Apr 08, 2021 at 05:25:28PM +0900, 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.
> Yeah, so how much should we adjust the offsets in favor of smaller
> devices instead of larger ones? I think this is going the wrong
> direction, the capacity will grow, the zones are supposed to be larger.

For the record, we had a group discussion about that and the consensus
is to do sb offsets at 0, 512G and 4T.

The rationale is to prefer large devices slightly more than smaller, but
are 2 superblocks available under 1T to cover small devices. Physical
devices' capacity grows and the 4T copy should be available on all of
the HDD type, while flash storage growth is a bit slower but still
projected to be up to 10T within the time we care about now.

The small devices are for usecases with not necessarily a physical
device, eg. emulated or on top of device mapper targets.

The superblock copy itself is not a frequently utilized feature but we
still want to keep it, for parity with non-zoned mode and "just in

I'll update the patch changelog and code to reflect the changes and
resend to all people involved. The pull request is scheduled for

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