Detect a slow raidz child during reads

A single slow responding disk can affect the overall read
performance of a raidz group.  When a raidz child disk is
determined to be a persistent slow outlier, then have it
sit out during reads for a period of time. The raidz group
can use parity to reconstruct the data that was skipped.

Each time a slow disk is placed into a sit out period, its
`vdev_stat.vs_slow_ios count` is incremented and a zevent
class `ereport.fs.zfs.delay` is posted.

The length of the sit out period can be changed using the
`raid_read_sit_out_secs` module parameter.  Setting it to
zero disables slow outlier detection.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Paul Dagnelie <paul.dagnelie@klarasystems.com>
Contributions-by: Don Brady <don.brady@klarasystems.com>
Contributions-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #17227

lib/libzfs/libzfs_util.c

@@ -776,6 +776,11 @@ zpool_standard_error_fmt(libzfs_handle_t *hdl, int error, const char *fmt, ...)
 	case ZFS_ERR_ASHIFT_MISMATCH:
 		zfs_verror(hdl, EZFS_ASHIFT_MISMATCH, fmt, ap);
 		break;
+	case ZFS_ERR_TOO_MANY_SITOUTS:
+		zfs_error_aux(hdl, dgettext(TEXT_DOMAIN, "too many disks "
+		    "already sitting out"));
+		zfs_verror(hdl, EZFS_BUSY, fmt, ap);
+		break;
 	default:
 		zfs_error_aux(hdl, "%s", zfs_strerror(error));
 		zfs_verror(hdl, EZFS_UNKNOWN, fmt, ap);