Include scatter_chunk_waste in arc_size

The ARC caches data in scatter ABD's, which are collections of pages,
which are typically 4K.  Therefore, the space used to cache each block
is rounded up to a multiple of 4K.  The ABD subsystem tracks this wasted
memory in the `scatter_chunk_waste` kstat.  However, the ARC's `size` is
not aware of the memory used by this round-up, it only accounts for the
size that it requested from the ABD subsystem.

Therefore, the ARC is effectively using more memory than it is aware of,
due to the `scatter_chunk_waste`.  This impacts observability, e.g.
`arcstat` will show that the ARC is using less memory than it
effectively is.  It also impacts how the ARC responds to memory
pressure.  As the amount of `scatter_chunk_waste` changes, it appears to
the ARC as memory pressure, so it needs to resize `arc_c`.

If the sector size (`1<<ashift`) is the same as the page size (or
larger), there won't be any waste.  If the (compressed) block size is
relatively large compared to the page size, the amount of
`scatter_chunk_waste` will be small, so the problematic effects are
minimal.

However, if using 512B sectors (`ashift=9`), and the (compressed) block
size is small (e.g. `compression=on` with the default `volblocksize=8k`
or a decreased `recordsize`), the amount of `scatter_chunk_waste` can be
very large.  On a production system, with `arc_size` at a constant 50%
of memory, `scatter_chunk_waste` has been been observed to be 10-30% of
memory.

This commit adds `scatter_chunk_waste` to `arc_size`, and adds a new
`waste` field to `arcstat`.  As a result, the ARC's memory usage is more
observable, and `arc_c` does not need to be adjusted as frequently.

Reviewed-by: Pavel Zakharov <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: George Wilson <[email protected]>
Reviewed-by: Ryan Moeller <[email protected]>
Signed-off-by: Matthew Ahrens <[email protected]>
Closes #10701 

1 files changed, 2 insertions, 0 deletions

diff --git a/cmd/arcstat/arcstat.in b/cmd/arcstat/arcstat.in
index b230e4b85..c83a1c745 100755
--- a/cmd/arcstat/arcstat.in
+++ b/cmd/arcstat/arcstat.in
@@ -103,6 +103,7 @@ cols = {
     "need":       [4, 1024, "ARC reclaim need"],
     "free":       [4, 1024, "ARC free memory"],
     "avail":      [5, 1024, "ARC available memory"],
+    "waste":      [5, 1024, "Wasted memory due to round up to pagesize"],
 }
 
 v = {}
@@ -452,6 +453,7 @@ def calculate():
     v["need"] = cur["arc_need_free"]
     v["free"] = cur["memory_free_bytes"]
     v["avail"] = cur["memory_available_bytes"]
+    v["waste"] = cur["abd_chunk_waste_size"]
 
 
 def main():