RAID-Z expansion feature

This feature allows disks to be added one at a time to a RAID-Z group,
expanding its capacity incrementally.  This feature is especially useful
for small pools (typically with only one RAID-Z group), where there
isn't sufficient hardware to add capacity by adding a whole new RAID-Z
group (typically doubling the number of disks).

== Initiating expansion ==

A new device (disk) can be attached to an existing RAIDZ vdev, by
running `zpool attach POOL raidzP-N NEW_DEVICE`, e.g. `zpool attach tank
raidz2-0 sda`.  The new device will become part of the RAIDZ group.  A
"raidz expansion" will be initiated, and the new device will contribute
additional space to the RAIDZ group once the expansion completes.

The `feature@raidz_expansion` on-disk feature flag must be `enabled` to
initiate an expansion, and it remains `active` for the life of the pool.
In other words, pools with expanded RAIDZ vdevs can not be imported by
older releases of the ZFS software.

== During expansion ==

The expansion entails reading all allocated space from existing disks in
the RAIDZ group, and rewriting it to the new disks in the RAIDZ group
(including the newly added device).

The expansion progress can be monitored with `zpool status`.

Data redundancy is maintained during (and after) the expansion.  If a
disk fails while the expansion is in progress, the expansion pauses
until the health of the RAIDZ vdev is restored (e.g. by replacing the
failed disk and waiting for reconstruction to complete).

The pool remains accessible during expansion.  Following a reboot or
export/import, the expansion resumes where it left off.

== After expansion ==

When the expansion completes, the additional space is available for use,
and is reflected in the `available` zfs property (as seen in `zfs list`,
`df`, etc).

Expansion does not change the number of failures that can be tolerated
without data loss (e.g. a RAIDZ2 is still a RAIDZ2 even after
expansion).

A RAIDZ vdev can be expanded multiple times.

After the expansion completes, old blocks remain with their old
data-to-parity ratio (e.g. 5-wide RAIDZ2, has 3 data to 2 parity), but
distributed among the larger set of disks.  New blocks will be written
with the new data-to-parity ratio (e.g. a 5-wide RAIDZ2 which has been
expanded once to 6-wide, has 4 data to 2 parity).  However, the RAIDZ
vdev's "assumed parity ratio" does not change, so slightly less space
than is expected may be reported for newly-written blocks, according to
`zfs list`, `df`, `ls -s`, and similar tools.

Sponsored-by: The FreeBSD Foundation
Sponsored-by: iXsystems, Inc.
Sponsored-by: vStack
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Mark Maybee <mark.maybee@delphix.com>
Authored-by: Matthew Ahrens <mahrens@delphix.com>
Contributions-by: Fedor Uporov <fuporov.vstack@gmail.com>
Contributions-by: Stuart Maybee <stuart.maybee@comcast.net>
Contributions-by: Thorsten Behrens <tbehrens@outlook.com>
Contributions-by: Fmstrat <nospam@nowsci.com>
Contributions-by: Don Brady <dev.fs.zfs@gmail.com>
Signed-off-by: Don Brady <dev.fs.zfs@gmail.com>
Closes #15022

include/sys/vdev_impl.h

@@ -72,7 +72,7 @@ typedef void	vdev_fini_func_t(vdev_t *vd);
 typedef int	vdev_open_func_t(vdev_t *vd, uint64_t *size, uint64_t *max_size,
     uint64_t *ashift, uint64_t *pshift);
 typedef void	vdev_close_func_t(vdev_t *vd);
-typedef uint64_t vdev_asize_func_t(vdev_t *vd, uint64_t psize);
+typedef uint64_t vdev_asize_func_t(vdev_t *vd, uint64_t psize, uint64_t txg);
 typedef uint64_t vdev_min_asize_func_t(vdev_t *vd);
 typedef uint64_t vdev_min_alloc_func_t(vdev_t *vd);
 typedef void	vdev_io_start_func_t(zio_t *zio);
@@ -281,6 +281,7 @@ struct vdev {
 	uint64_t	vdev_noalloc;	/* device is passivated?	*/
 	uint64_t	vdev_removing;	/* device is being removed?	*/
 	uint64_t	vdev_failfast;	/* device failfast setting	*/
+	boolean_t	vdev_rz_expanding; /* raidz is being expanded?	*/
 	boolean_t	vdev_ishole;	/* is a hole in the namespace	*/
 	uint64_t	vdev_top_zap;
 	vdev_alloc_bias_t vdev_alloc_bias; /* metaslab allocation bias	*/
@@ -536,6 +537,7 @@ typedef struct vdev_label {
 /*
  * Size of embedded boot loader region on each label.
  * The total size of the first two labels plus the boot area is 4MB.
+ * On RAIDZ, this space is overwritten during RAIDZ expansion.
  */
 #define	VDEV_BOOT_SIZE		(7ULL << 19)			/* 3.5M */
 
@@ -608,7 +610,7 @@ extern vdev_ops_t vdev_indirect_ops;
  */
 extern void vdev_default_xlate(vdev_t *vd, const range_seg64_t *logical_rs,
     range_seg64_t *physical_rs, range_seg64_t *remain_rs);
-extern uint64_t vdev_default_asize(vdev_t *vd, uint64_t psize);
+extern uint64_t vdev_default_asize(vdev_t *vd, uint64_t psize, uint64_t txg);
 extern uint64_t vdev_default_min_asize(vdev_t *vd);
 extern uint64_t vdev_get_min_asize(vdev_t *vd);
 extern void vdev_set_min_asize(vdev_t *vd);