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 

2 files changed, 2 insertions, 0 deletions

diff --git a/include/sys/arc.h b/include/sys/arc.h
index f322d6328..3fdf36e2a 100644
--- a/include/sys/arc.h
+++ b/include/sys/arc.h
@@ -203,6 +203,7 @@ typedef enum arc_space_type {
 	ARC_SPACE_DBUF,
 	ARC_SPACE_DNODE,
 	ARC_SPACE_BONUS,
+	ARC_SPACE_ABD_CHUNK_WASTE,
 	ARC_SPACE_NUMTYPES
 } arc_space_type_t;
 
diff --git a/include/sys/arc_impl.h b/include/sys/arc_impl.h
index bb9163ba7..d07791d07 100644
--- a/include/sys/arc_impl.h
+++ b/include/sys/arc_impl.h
@@ -844,6 +844,7 @@ typedef struct arc_stats {
 	kstat_named_t arcstat_sys_free;
 	kstat_named_t arcstat_raw_size;
 	kstat_named_t arcstat_cached_only_in_progress;
+	kstat_named_t arcstat_abd_chunk_waste_size;
 } arc_stats_t;
 
 typedef struct arc_evict_waiter {