zfetch: Don't issue new streams when old have not completed

The current dmu_zfetch code implicitly assumes that I/Os complete
within min_sec_reap seconds. With async dmu and a readonly workload
(and thus no exponential backoff in operations from the "write
throttle") such as L2ARC rebuild it is possible to saturate the drives
with I/O requests. These are then effectively compounded with prefetch
requests.

This change reference counts streams and prevents them from being
recycled after their min_sec_reap timeout if they still have
outstanding I/Os.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #10900

include/sys/dbuf.h

@@ -309,6 +309,8 @@ typedef struct dbuf_hash_table {
 	kmutex_t hash_mutexes[DBUF_MUTEXES];
 } dbuf_hash_table_t;
 
+typedef void (*dbuf_prefetch_fn)(void *, boolean_t);
+
 uint64_t dbuf_whichblock(const struct dnode *di, const int64_t level,
     const uint64_t offset);
 
@@ -324,7 +326,10 @@ int dbuf_hold_impl(struct dnode *dn, uint8_t level, uint64_t blkid,
     boolean_t fail_sparse, boolean_t fail_uncached,
     void *tag, dmu_buf_impl_t **dbp);
 
-void dbuf_prefetch(struct dnode *dn, int64_t level, uint64_t blkid,
+int dbuf_prefetch_impl(struct dnode *dn, int64_t level, uint64_t blkid,
+    zio_priority_t prio, arc_flags_t aflags, dbuf_prefetch_fn cb,
+    void *arg);
+int dbuf_prefetch(struct dnode *dn, int64_t level, uint64_t blkid,
     zio_priority_t prio, arc_flags_t aflags);
 
 void dbuf_add_ref(dmu_buf_impl_t *db, void *tag);

include/sys/dmu_zfetch.h

@@ -40,6 +40,13 @@ extern unsigned long	zfetch_array_rd_sz;
 
 struct dnode;				/* so we can reference dnode */
 
+typedef struct zfetch {
+	kmutex_t	zf_lock;	/* protects zfetch structure */
+	list_t		zf_stream;	/* list of zstream_t's */
+	struct dnode	*zf_dnode;	/* dnode that owns this zfetch */
+	int		zf_numstreams;	/* number of zstream_t's */
+} zfetch_t;
+
 typedef struct zstream {
 	uint64_t	zs_blkid;	/* expect next access at this blkid */
 	uint64_t	zs_pf_blkid;	/* next block to prefetch */
@@ -52,15 +59,12 @@ typedef struct zstream {
 
 	kmutex_t	zs_lock;	/* protects stream */
 	hrtime_t	zs_atime;	/* time last prefetch issued */
+	hrtime_t	zs_start_time;	/* start of last prefetch */
 	list_node_t	zs_node;	/* link for zf_stream */
+	zfetch_t	*zs_fetch;	/* parent fetch */
+	zfs_refcount_t	zs_blocks; /* number of pending blocks in the stream */
 } zstream_t;
 
-typedef struct zfetch {
-	kmutex_t	zf_lock;	/* protects zfetch structure */
-	list_t		zf_stream;	/* list of zstream_t's */
-	struct dnode	*zf_dnode;	/* dnode that owns this zfetch */
-} zfetch_t;
-
 void		zfetch_init(void);
 void		zfetch_fini(void);