xfs: define the zoned on-disk format

Zone file systems reuse the basic RT group enabled XFS file system
structure to support a mode where each RT group is always written from
start to end and then reset for reuse (after moving out any remaining
data).  There are few minor but important changes, which are indicated
by a new incompat flag:

1) there are no bitmap and summary inodes, thus the
   /rtgroups/{rgno}.{bitmap,summary} metadir files do not exist and the
   sb_rbmblocks superblock field must be cleared to zero.

2) there is a new superblock field that specifies the start of an
   internal RT section.  This allows supporting SMR HDDs that have random
   writable space at the beginning which is used for the XFS data device
   (which really is the metadata device for this configuration), directly
   followed by a RT device on the same block device.  While something
   similar could be achieved using dm-linear just having a single device
   directly consumed by XFS makes handling the file systems a lot easier.

3) Another superblock field that tracks the amount of reserved space (or
   overprovisioning) that is never used for user capacity, but allows GC
   to run more smoothly.

4) an overlay of the cowextsize field for the rtrmap inode so that we
   can persistently track the total amount of rtblocks currently used in
   a RT group.  There is no data structure other than the rmap that
   tracks used space in an RT group, and this counter is used to decide
   when a RT group has been entirely emptied, and to select one that
   is relatively empty if garbage collection needs to be performed.
   While this counter could be tracked entirely in memory and rebuilt
   from the rmap at mount time, that would lead to very long mount times
   with the large number of RT groups implied by the number of hardware
   zones especially on SMR hard drives with 256MB zone sizes.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>

1 files changed, 2 insertions, 0 deletions

diff --git a/fs/xfs/xfs_rtalloc.c b/fs/xfs/xfs_rtalloc.c
index 98717117bbaa..afe8664804a9 100644
--- a/fs/xfs/xfs_rtalloc.c
+++ b/fs/xfs/xfs_rtalloc.c
@@ -2056,6 +2056,8 @@ xfs_bmap_rtalloc(
 		ap->datatype & XFS_ALLOC_INITIAL_USER_DATA;
 	int			error;
 
+	ASSERT(!xfs_has_zoned(ap->tp->t_mountp));
+
 retry:
 	error = xfs_rtallocate_align(ap, &ralen, &raminlen, &prod, &noalign);
 	if (error)