Re: [PATCH v2 2/2] btrfs-progs: set the proper minimum size for a zoned file system

["hch@infradead.org" <hch@infradead.org>]

On Mon, Jul 10, 2023 at 12:57:52AM +0000, Naohiro Aota wrote:
> It depends on what we consider the "minimal" is.

I think minimal means a file system that can actually be be continuously
used.

> Even with the 5 zones (2
> SBs + 1 per BG type), we can start writing to the file-system.
> 
> If you need to run a relocation, one more block group for it is needed.
> 
> The fsync block group might be optional because if the fsync node
> allocation failed, it should fall back to the full sync. It will kill the
> performance but still works...
> 
> If we say it is the "minimal" that we can infinitely write and delete a
> file without ENOSPC, we need one (or two, depending on the metadata
> profile) more BGs per META/SYSTEM.

Based on my above sentence we then need:

 2 zones for the primary superblock

metadata replication factor * (
  2 zones for the system block group
  2 zone for a metadata block group
  2 zone for the tree log)


data replication factor * (
 1 zone for a data block group
 1 zone for a relocation block group
)

where the two for the non-sb, non-data blocks accounts for beeing
able to continue writing after filling up one bg and allowing
gc.  In fact even just two might lead to deadlocks in that case
depending on the exact algorithm in other zoned storage systems,
but I don't know enough about btrfs metadata placement to understand
how that works on zoned btrfs right now.