Re: [PATCH 09/19] btrfs: limit super block locations in HMZONED mode

[David Sterba]

On Fri, Jun 07, 2019 at 10:10:15PM +0900, Naohiro Aota wrote:
> When in HMZONED mode, make sure that device super blocks are located in
> randomly writable zones of zoned block devices. That is, do not write super
> blocks in sequential write required zones of host-managed zoned block
> devices as update would not be possible.

This could be explained in more detail. My understanding is that the 1st
and 2nd copy superblocks is skipped at write time but the zone
containing the superblocks is not excluded from allocations. Ie. regular
data can appear in place where the superblocks would exist on
non-hmzoned filesystem. Is that correct?

The other option is to completely exclude the zone that contains the
superblock copies.

primary sb			 64K
1st copy			 64M
2nd copy			256G

Depends on the drives, but I think the size of the random write zone
will very often cover primary and 1st copy. So there's at least some
backup copy.

The 2nd copy will be in the sequential-only zone, so the whole zone
needs to be excluded in exclude_super_stripes. But it's not, so this
means data can go there.  I think the zone should be left empty.