[Bug 222929] ZFS ARC stats have wrong count

[bugzilla-noreply at freebsd.org]

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=222929

            Bug ID: 222929
           Summary: ZFS ARC stats have wrong count
           Product: Base System
           Version: 11.1-RELEASE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Many People
          Priority: ---
         Component: kern
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: naito.yuichiro at gmail.com

I'm using FreeBSD-11.1R amd64 with ZFS root-fs on my 8GB ram PC.
I think zfs arcstats miss count shows wrong value.

For example,
I checked out FreeBSD ports tree from svn repository.

```
$ svn co svn://svn.freebsd.org/ports/head ports-head
```

and then execute following ruby script to stat(2) all files.

```
Dir.glob("ports-head/**/*").each do |file|
  File.stat(file)
end
```

After 3-5 times running it,
I expect all metadata cached on ARC.
But zfs-mon -a shows as follows.

```
ZFS real-time cache activity monitor
Seconds elapsed:   9

Cache hits and misses:
                                  1s    10s    60s    tot
                     ARC hits:     0      0      0      0
                   ARC misses: 28294   7716   7716   7716
         ARC demand data hits:     0      0      0      0
       ARC demand data misses:    28      7      7      7
     ARC demand metadata hits:     0      0      0      0
   ARC demand metadata misses: 28267   7709   7709   7709
       ARC prefetch data hits:     0      0      0      0
     ARC prefetch data misses:     0      0      0      0
   ARC prefetch metadata hits:     0      0      0      0
 ARC prefetch metadata misses:     0      0      0      0
                  ZFETCH hits:    25      7      7      7
                ZFETCH misses: 51529  14062  14062  14062

Cache efficiency percentage:
                          10s    60s    tot
                  ARC:   0.00   0.00   0.00
      ARC demand data:   0.00   0.00   0.00
  ARC demand metadata:   0.00   0.00   0.00
    ARC prefetch data:   0.00   0.00   0.00
ARC prefetch metadata:   0.00   0.00   0.00
               ZFETCH:   0.05   0.05   0.05

```

I think `ARC demand metadata misses` shows hit count.

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