Convert zfs_mg_noalloc_threshold to a module parameter and document

The parameter was added as illumos issue 4081 which was committed to
zfsonlinux in ac72fac3eaa569902cad88053167f7d74e7fe7e4.  This patch
documents the parameter and allows for it to be set as a module parameter.

Signed-off-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #2483
Illumos 4168, 4169, 4170: ztest, zdb and zhack fixes

4168 ztest assertion failure in dbuf_undirty
4169 verbatim import causes zdb to segfault
4170 zhack leaves pool in ACTIVE state
Reviewed by: Adam Leventhal <ahl@delphix.com>
Reviewed by: Eric Schrock <eric.schrock@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Approved by: Dan McDonald <danmcd@nexenta.com>

References:
    https://www.illumos.org/issues/4168
    https://www.illumos.org/issues/4169
    https://www.illumos.org/issues/4170
    https://github.com/illumos/illumos-gate/commit/7fdd916

Porting notes:

Of particular interest when troubleshooting corrupted pools, the
commonly-used "zdb -e" operation may perform verbatim imports and
furthermore, it will soon have direct support for verbatim imports via
a new "-V" option.  The 4169 fix eliminates a common segfault case in
which spa_history_log_version() tries to access an un-opened dsl_pool_t.

Ported by: Tim Chase <tim@chase2k.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #2451
Closes #2283
Closes #2467
zdb: Introduce -V for verbatim import

When given a pool name via -e, zdb would attempt an import. If it
failed, then it would attempt a verbatim import. This behavior is
not always desirable so a -V switch is added to zdb to control the
behavior. When specified, a verbatim import is done. Otherwise,
the behavior is as it was previously, except no verbatim import
is done on failure.

Signed-off-by: Richard Yao <ryao@gentoo.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #2372
Fix zil_commit() NULL dereference

Update the current code to ensure inodes are never dirtied if they are
part of a read-only file system or snapshot.  If they do somehow get
dirtied an attempt will make made to write them to disk.  In the case
of snapshots, which don't have a ZIL, this will result in a NULL
dereference in zil_commit().

Signed-off-by: Richard Yao <ryao@gentoo.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #2405
Move metaslab_group_alloc_update() call

This changes moves the called to metaslab_group_alloc_update() to the
metaslab_sync_reassess() function. The original placement of the call
within metaslab_sync_done() appears to have been a simple mistake,
introduced by ac72fac3eaa569902cad88053167f7d74e7fe7e4.

This aligns us more closely to the upstream illumos code base.

Signed-off-by: Prakash Surya <surya1@llnl.gov>
Illumos #4101, #4102, #4103, #4105, #4106

4101 metaslab_debug should allow for fine-grained control
4102 space_maps should store more information about themselves
4103 space map object blocksize should be increased
4105 removing a mirrored log device results in a leaked object
4106 asynchronously load metaslab
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Adam Leventhal <ahl@delphix.com>
Reviewed by: Sebastien Roy <seb@delphix.com>
Approved by: Garrett D'Amore <garrett@damore.org>

Prior to this patch, space_maps were preferred solely based on the
amount of free space left in each. Unfortunately, this heuristic didn't
contain any information about the make-up of that free space, which
meant we could keep preferring and loading a highly fragmented space map
that wouldn't actually have enough contiguous space to satisfy the
allocation; then unloading that space_map and repeating the process.

This change modifies the space_map's to store additional information
about the contiguous space in the space_map, so that we can use this
information to make a better decision about which space_map to load.
This requires reallocating all space_map objects to increase their
bonus buffer size sizes enough to fit the new metadata.

The above feature can be enabled via a new feature flag introduced by
this change: com.delphix:spacemap_histogram

In addition to the above, this patch allows the space_map block size to
be increase. Currently the block size is set to be 4K in size, which has
certain implications including the following:

    * 4K sector devices will not see any compression benefit
    * large space_maps require more metadata on-disk
    * large space_maps require more time to load (typically random reads)

Now the space_map block size can adjust as needed up to the maximum size
set via the space_map_max_blksz variable.

A bug was fixed which resulted in potentially leaking an object when
removing a mirrored log device. The previous logic for vdev_remove() did
not deal with removing top-level vdevs that are interior vdevs (i.e.
mirror) correctly. The problem would occur when removing a mirrored log
device, and result in the DTL space map object being leaked; because
top-level vdevs don't have DTL space map objects associated with them.

References:
  https://www.illumos.org/issues/4101
  https://www.illumos.org/issues/4102
  https://www.illumos.org/issues/4103
  https://www.illumos.org/issues/4105
  https://www.illumos.org/issues/4106
  https://github.com/illumos/illumos-gate/commit/0713e23

Porting notes:

A handful of kmem_alloc() calls were converted to kmem_zalloc(). Also,
the KM_PUSHPAGE and TQ_PUSHPAGE flags were used as necessary.

Ported-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Prakash Surya <surya1@llnl.gov>
Illumos #4730 destroy metaslab group taskq

4730 metaslab group taskq should be destroyed in metaslab_group_destroy()

Reviewed by: Alex Reece <alex.reece@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Sebastien Roy <sebastien.roy@delphix.com>
Reviewed by: Rich Lowe <richlowe@richlowe.net>
Reviewed by: Dan McDonald <danmcd@omniti.com>
Approved by: Dan McDonald <danmcd@omniti.com>

References:
  https://www.illumos.org/issues/4730
  https://github.com/illumos/illumos-gate/commit/be08211

Porting notes:

Under ZFSonlinux, one of the effects of not destroying the taskq is that
zdb would never exit (due to the SPL taskq implementation).

Ported-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Prakash Surya <surya1@llnl.gov>
Illumos #4756 Fix metaslab_group_preload deadlock

4756 metaslab_group_preload() could deadlock
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Christopher Siden <christopher.siden@delphix.com>
Reviewed by: Dan McDonald <danmcd@omniti.com>
Reviewed by: Saso Kiselkov <saso.kiselkov@nexenta.com>
Approved by: Garrett D'Amore <garrett@damore.org>

The metaslab_group_preload() function grabs the mg_lock and then later
tries to grab the metaslab lock. This lock ordering may lead to a
deadlock since other consumers of the mg_lock will grab the metaslab
lock first.

References:
  https://www.illumos.org/issues/4756
  https://github.com/illumos/illumos-gate/commit/30beaff

Ported-by: Prakash Surya <surya1@llnl.gov>
Signed-off-by: Prakash Surya <surya1@llnl.gov>