fat zap should prefetch when iterating

When iterating over a ZAP object, we're almost always certain to iterate
over the entire object. If there are multiple leaf blocks, we can
realize a performance win by issuing reads for all the leaf blocks in
parallel when the iteration begins.

For example, if we have 10,000 snapshots, "zfs destroy -nv
pool/fs@1%9999" can take 30 minutes when the cache is cold. This change
provides a >3x performance improvement, by issuing the reads for all ~64
blocks of each ZAP object in parallel.

Reviewed-by: Andreas Dilger <andreas.dilger@whamcloud.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matthew Ahrens <mahrens@delphix.com>
External-issue: DLPX-58347
Closes #8862

man/man5/zfs-module-parameters.5

@@ -104,6 +104,18 @@ to a log2 fraction of the target arc size.
 Default value: \fB6\fR.
 .RE
 
+.sp
+.ne 2
+.na
+\fBdmu_prefetch_max\fR (int)
+.ad
+.RS 12n
+Limit the amount we can prefetch with one call to this amount (in bytes).
+This helps to limit the amount of memory that can be used by prefetching.
+.sp
+Default value: \fB134,217,728\fR (128MB).
+.RE
+
 .sp
 .ne 2
 .na
@@ -502,6 +514,19 @@ regular reads (but there's no reason it has to be the same).
 Default value: \fB32,768\fR.
 .RE
 
+.sp
+.ne 2
+.na
+\fBzap_iterate_prefetch\fR (int)
+.ad
+.RS 12n
+If this is set, when we start iterating over a ZAP object, zfs will prefetch
+the entire object (all leaf blocks).  However, this is limited by
+\fBdmu_prefetch_max\fR.
+.sp
+Use \fB1\fR for on (default) and \fB0\fR for off.
+.RE
+
 .sp
 .ne 2
 .na