Finally drop long disabled vdev cache.

It was a vdev level read cache, designed to aggregate many small
reads by speculatively issuing bigger reads instead and caching
the result.  But since it has almost no idea about what is going
on with exception of ZIO_FLAG_DONT_CACHE flag set by higher layers,
it was found to make more harm than good, for which reason it was
disabled for the past 12 years.  These days we have much better
instruments to enlarge the I/Os, such as speculative and prescient
prefetches, I/O scheduler, I/O aggregation etc.

Besides just the dead code removal this removes one extra mutex
lock/unlock per write inside vdev_cache_write(), not otherwise
disabled and trying to do some work.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by:	Alexander Motin <mav@FreeBSD.org>
Sponsored by:	iXsystems, Inc.
Closes #14953

cmd/arc_summary

@@ -64,7 +64,6 @@ SECTION_HELP = 'print info from one section ('+' '.join(SECTIONS)+')'
 SECTION_PATHS = {'arc': 'arcstats',
                  'dmu': 'dmu_tx',
                  'l2arc': 'arcstats',  # L2ARC stuff lives in arcstats
-                 'vdev': 'vdev_cache_stats',
                  'zfetch': 'zfetchstats',
                  'zil': 'zil'}
 
@@ -90,8 +89,6 @@ if sys.platform.startswith('freebsd'):
     # Requires py36-sysctl on FreeBSD
     import sysctl
 
-    VDEV_CACHE_SIZE = 'vdev.cache_size'
-
     def is_value(ctl):
         return ctl.type != sysctl.CTLTYPE_NODE
 
@@ -135,8 +132,6 @@ elif sys.platform.startswith('linux'):
     SPL_PATH = '/sys/module/spl/parameters'
     TUNABLES_PATH = '/sys/module/zfs/parameters'
 
-    VDEV_CACHE_SIZE = 'zfs_vdev_cache_size'
-
     def load_kstats(section):
         path = os.path.join(KSTAT_PATH, section)
         with open(path) as f:
@@ -952,35 +947,6 @@ def section_tunables(*_):
     print()
 
 
-def section_vdev(kstats_dict):
-    """Collect information on VDEV caches"""
-
-    # Currently [Nov 2017] the VDEV cache is disabled, because it is actually
-    # harmful. When this is the case, we just skip the whole entry. See
-    # https://github.com/openzfs/zfs/blob/master/module/zfs/vdev_cache.c
-    # for details
-    tunables = get_vdev_params()
-
-    if tunables[VDEV_CACHE_SIZE] == '0':
-        print('VDEV cache disabled, skipping section\n')
-        return
-
-    vdev_stats = isolate_section('vdev_cache_stats', kstats_dict)
-
-    vdev_cache_total = int(vdev_stats['hits']) +\
-        int(vdev_stats['misses']) +\
-        int(vdev_stats['delegations'])
-
-    prt_1('VDEV cache summary:', f_hits(vdev_cache_total))
-    prt_i2('Hit ratio:', f_perc(vdev_stats['hits'], vdev_cache_total),
-           f_hits(vdev_stats['hits']))
-    prt_i2('Miss ratio:', f_perc(vdev_stats['misses'], vdev_cache_total),
-           f_hits(vdev_stats['misses']))
-    prt_i2('Delegations:', f_perc(vdev_stats['delegations'], vdev_cache_total),
-           f_hits(vdev_stats['delegations']))
-    print()
-
-
 def section_zil(kstats_dict):
     """Collect information on the ZFS Intent Log. Some of the information
     taken from https://github.com/openzfs/zfs/blob/master/include/sys/zil.h
@@ -1008,7 +974,6 @@ section_calls = {'arc': section_arc,
                  'l2arc': section_l2arc,
                  'spl': section_spl,
                  'tunables': section_tunables,
-                 'vdev': section_vdev,
                  'zil': section_zil}