[Bug 222929] ZFS ARC stats have wrong count

[bugzilla-noreply at freebsd.org]

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

--- Comment #7 from Allan Jude <allanjude at FreeBSD.org> ---
I have confirmed the impact you are seeing, but also confirmed that is not a
major bug.

If you run: zpool iostat 1

You'll see that running your ruby script will not actually result in any reads
from the disk.

There is a small issue with the stats accounting in ZFS, where if the metadata
being read, happens to be stored in an "Embedded Block Pointer" (so instead of
being stored as a data block, the data is embedded in the parent block, to save
an entire sector, and to save an I/O to read that sector), then it is
incorrectly counted as a miss.

This is because to read the embedded block pointer, it has to create a read zio
and go through most of the process of doing a read, but then ends up copying
the data out of the block pointer instead of from disk.


Anyway, I am investigating a quick fixes to account for the cache hit
correctly, instead of as a cache miss.

I am also looking to see if it would be relatively simple to optimize the case
and return the data more directly in arc_read() instead of creating a zio and
going the currently more complicated path. This path mostly exists because it
makes it possible for other functions to not need to know about the embedded
block pointer feature.

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