Metaslab max_size should be persisted while unloaded

When we unload metaslabs today in ZFS, the cached max_size value is
discarded. We instead use the histogram to determine whether or not we
think we can satisfy an allocation from the metaslab. This can result in
situations where, if we're doing I/Os of a size not aligned to a
histogram bucket, a metaslab is loaded even though it cannot satisfy the
allocation we think it can. For example, a metaslab with 16 entries in
the 16k-32k bucket may have entirely 16kB entries. If we try to allocate
a 24kB buffer, we will load that metaslab because we think it should be
able to handle the allocation. Doing so is expensive in CPU time, disk
reads, and average IO latency. This is exacerbated if the write being
attempted is a sync write.

This change makes ZFS cache the max_size after the metaslab is
unloaded. If we ever get a free (or a coalesced group of frees) larger
than the max_size, we will update it. Otherwise, we leave it as is. When
attempting to allocate, we use the max_size as a lower bound, and
respect it unless we are in try_hard. However, we do age the max_size
out at some point, since we expect the actual max_size to increase as we
do more frees. A more sophisticated algorithm here might be helpful, but
this works reasonably well.

Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Matt Ahrens <[email protected]>
Signed-off-by: Paul Dagnelie <[email protected]>
Closes #9055

1 files changed, 7 insertions, 0 deletions

diff --git a/include/sys/metaslab_impl.h b/include/sys/metaslab_impl.h
index 29bc8cd5e..08ee8d279 100644
--- a/include/sys/metaslab_impl.h
+++ b/include/sys/metaslab_impl.h
@@ -475,6 +475,12 @@ struct metaslab {
 	 * stay cached.
 	 */
 	uint64_t	ms_selected_txg;
+	/*
+	 * ms_load/unload_time can be used for performance monitoring
+	 * (e.g. by dtrace or mdb).
+	 */
+	hrtime_t	ms_load_time;	/* time last loaded */
+	hrtime_t	ms_unload_time;	/* time last unloaded */
 
 	uint64_t	ms_alloc_txg;	/* last successful alloc (debug only) */
 	uint64_t	ms_max_size;	/* maximum allocatable size	*/
@@ -495,6 +501,7 @@ struct metaslab {
 	 * segment sizes.
 	 */
 	avl_tree_t	ms_allocatable_by_size;
+	avl_tree_t	ms_unflushed_frees_by_size;
 	uint64_t	ms_lbas[MAX_LBAS];
 
 	metaslab_group_t *ms_group;	/* metaslab group		*/