Initial zio delay timing

While there is no right maximum timeout for a disk IO we can start
laying the ground work to measure how long they do take in practice.
This change simply measures the IO time and if it exceeds 30s an
event is posted for 'zpool events'.

This value was carefully selected because for sd devices it implies
that at least one timeout (SD_TIMEOUT) has occured.  Unfortunately,
even with FAILFAST set we may retry and request and not get an
error.  This behavior is strongly dependant on the device driver
and how it is hooked in to the scsi error handling stack.  However
by setting the limit at 30s we can log the event even if no error
was returned.

Slightly longer term we can start recording these delays perhaps
as a simple power-of-two histrogram.  This histogram can then be
reported as part of the 'zpool status' command when given an command
line option.

None of this code changes the internal behavior of ZFS.  Currently
it is simply for reporting excessively long delays.

include/sys/fm/fs/zfs.h

@@ -35,6 +35,7 @@ extern "C" {
 #define	FM_EREPORT_ZFS_CHECKSUM			"checksum"
 #define	FM_EREPORT_ZFS_IO			"io"
 #define	FM_EREPORT_ZFS_DATA			"data"
+#define	FM_EREPORT_ZFS_DELAY			"delay"
 #define	FM_EREPORT_ZFS_CONFIG_SYNC		"config.sync"
 #define	FM_EREPORT_ZFS_POOL			"zpool"
 #define	FM_EREPORT_ZFS_POOL_DESTROY		"zpool.destroy"
@@ -81,6 +82,7 @@ extern "C" {
 #define	FM_EREPORT_PAYLOAD_ZFS_ZIO_OFFSET	"zio_offset"
 #define	FM_EREPORT_PAYLOAD_ZFS_ZIO_SIZE		"zio_size"
 #define	FM_EREPORT_PAYLOAD_ZFS_ZIO_FLAGS	"zio_flags"
+#define	FM_EREPORT_PAYLOAD_ZFS_ZIO_DELAY	"zio_delay"
 #define	FM_EREPORT_PAYLOAD_ZFS_PREV_STATE	"prev_state"
 #define	FM_EREPORT_PAYLOAD_ZFS_CKSUM_EXPECTED	"cksum_expected"
 #define	FM_EREPORT_PAYLOAD_ZFS_CKSUM_ACTUAL	"cksum_actual"

include/sys/zio.h

@@ -116,6 +116,11 @@ enum zio_compress {
 	ZIO_COMPRESS_ON_VALUE == ZIO_COMPRESS_LZJB) ||	\
 	(compress) == ZIO_COMPRESS_OFF)
 
+/*
+ * Default Linux timeout for a sd device.
+ */
+#define ZIO_DELAY_MAX			(30 * MILLISEC)
+
 #define	ZIO_FAILURE_MODE_WAIT		0
 #define	ZIO_FAILURE_MODE_CONTINUE	1
 #define	ZIO_FAILURE_MODE_PANIC		2
@@ -401,6 +406,7 @@ struct zio {
 	enum zio_flag	io_orig_flags;
 	enum zio_stage	io_orig_stage;
 	enum zio_stage	io_orig_pipeline;
+	uint64_t	io_delay;
 	int		io_error;
 	int		io_child_error[ZIO_CHILD_TYPES];
 	uint64_t	io_children[ZIO_CHILD_TYPES][ZIO_WAIT_TYPES];

module/zfs/vdev_disk.c

@@ -95,10 +95,10 @@ static void
 vdev_disk_error(zio_t *zio)
 {
 #ifdef ZFS_DEBUG
-	printk("ZFS: zio error=%d type=%d offset=%llu "
-	    "size=%llu flags=%x\n", zio->io_error, zio->io_type,
+	printk("ZFS: zio error=%d type=%d offset=%llu size=%llu "
+	    "flags=%x delay=%llu\n", zio->io_error, zio->io_type,
 	    (u_longlong_t)zio->io_offset, (u_longlong_t)zio->io_size,
-	    zio->io_flags);
+	    zio->io_flags, (u_longlong_t)zio->io_delay);
 #endif
 }
 
@@ -242,6 +242,8 @@ vdev_disk_dio_put(dio_request_t *dr)
 		vdev_disk_dio_free(dr);
 
 		if (zio) {
+			zio->io_delay = jiffies_to_msecs(
+			    jiffies_64 - zio->io_delay);
 			zio->io_error = error;
 			ASSERT3S(zio->io_error, >=, 0);
 			if (zio->io_error)
@@ -403,6 +405,8 @@ retry:
 
 	/* Extra reference to protect dio_request during submit_bio */
 	vdev_disk_dio_get(dr);
+	if (zio)
+		zio->io_delay = jiffies_64;
 
 	/* Submit all bio's associated with this dio */
 	for (i = 0; i < dr->dr_bio_count; i++)
@@ -442,6 +446,7 @@ BIO_END_IO_PROTO(vdev_disk_io_flush_completion, bio, size, rc)
 {
 	zio_t *zio = bio->bi_private;
 
+	zio->io_delay = jiffies_to_msecs(jiffies_64 - zio->io_delay);
 	zio->io_error = -rc;
 	if (rc && (rc == -EOPNOTSUPP))
 		zio->io_vd->vdev_nowritecache = B_TRUE;
@@ -472,6 +477,7 @@ vdev_disk_io_flush(struct block_device *bdev, zio_t *zio)
 	bio->bi_end_io = vdev_disk_io_flush_completion;
 	bio->bi_private = zio;
 	bio->bi_bdev = bdev;
+	zio->io_delay = jiffies_64;
 	submit_bio(WRITE_BARRIER, bio);
 
 	return 0;

module/zfs/zfs_fm.c

@@ -290,6 +290,8 @@ zfs_ereport_start(nvlist_t **ereport_out, nvlist_t **detector_out,
 		    DATA_TYPE_INT32, zio->io_error, NULL);
 		fm_payload_set(ereport, FM_EREPORT_PAYLOAD_ZFS_ZIO_FLAGS,
 		    DATA_TYPE_INT32, zio->io_flags, NULL);
+		fm_payload_set(ereport, FM_EREPORT_PAYLOAD_ZFS_ZIO_DELAY,
+		    DATA_TYPE_UINT64, zio->io_delay, NULL);
 
 		/*
 		 * If the 'size' parameter is non-zero, it indicates this is a

module/zfs/zio.c

@@ -75,6 +75,7 @@ kmem_cache_t *zio_link_cache;
 kmem_cache_t *zio_buf_cache[SPA_MAXBLOCKSIZE >> SPA_MINBLOCKSHIFT];
 kmem_cache_t *zio_data_buf_cache[SPA_MAXBLOCKSIZE >> SPA_MINBLOCKSHIFT];
 int zio_bulk_flags = 0;
+int zio_delay_max = ZIO_DELAY_MAX;
 
 #ifdef _KERNEL
 extern vmem_t *zio_alloc_arena;
@@ -2754,6 +2755,17 @@ zio_done(zio_t *zio)
 
 	vdev_stat_update(zio, zio->io_size);
 
+	/*
+	 * If this I/O is attached to a particular vdev is slow, exeeding
+	 * 30 seconds to complete, post an error described the I/O delay.
+	 * We ignore these errors if the device is currently unavailable.
+	 */
+	if (zio->io_delay >= zio_delay_max) {
+		if (zio->io_vd != NULL && !vdev_is_dead(zio->io_vd))
+			zfs_ereport_post(FM_EREPORT_ZFS_DELAY, zio->io_spa,
+                                         zio->io_vd, zio, 0, 0);
+	}
+
 	if (zio->io_error) {
 		/*
 		 * If this I/O is attached to a particular vdev,
@@ -2993,4 +3005,7 @@ EXPORT_SYMBOL(zio_type_name);
 
 module_param(zio_bulk_flags, int, 0644);
 MODULE_PARM_DESC(zio_bulk_flags, "Additional flags to pass to bulk buffers");
+
+module_param(zio_delay_max, int, 0644);
+MODULE_PARM_DESC(zio_delay_max, "Max zio delay before posting an event (ms)");
 #endif