CA-403297: when storing read-through do not mirror

When the lcache driver has read from the remote storage and returned
the data to the caller it then stores the data into the local vhdcache
by forwarding a write to the vhdcache file. As the vhdcache file is
opened in TD_VBD_SECONDARY_MIRROR mode this also results in the data
being unnecessarily written back to the remote where it has just been
read from.

Signed-off-by: Mark Syms <[email protected]>

Author: MarkSymsCtx

drivers/block-lcache.c

@@ -284,6 +284,7 @@ lcache_store_read(td_lcache_t *cache, td_lcache_req_t *req)
 	vreq->iovcnt = 1;
 	vreq->cb     = __lcache_write_cb;
 	vreq->token  = cache;
+	vreq->skip_mirror = true;
 
 	vbd = req->treq.vreq->vbd;
 

drivers/tapdisk-vbd.c

@@ -1552,26 +1552,33 @@ tapdisk_vbd_issue_request(td_vbd_t *vbd, td_vbd_request_t *vreq)
 		vreq->secs_pending += iov->secs;
 		vbd->secs_pending  += iov->secs;
 		if (vbd->secondary_mode == TD_VBD_SECONDARY_MIRROR &&
-		    vreq->op == TD_OP_WRITE) {
+		    vreq->op == TD_OP_WRITE &&
+			likely(vreq->skip_mirror == false))
+		{
 			vreq->secs_pending += iov->secs;
 			vbd->secs_pending  += iov->secs;
 		}
 
 		switch (vreq->op) {
 		case TD_OP_WRITE:
 			treq.op = TD_OP_WRITE;
-                        vbd->vdi_stats.stats->write_reqs_submitted++;
+			vbd->vdi_stats.stats->write_reqs_submitted++;
 			/*
-			 * it's important to queue the mirror request before 
-			 * queuing the main one. If the main image runs into 
-			 * ENOSPC, the mirroring could be disabled before 
-			 * td_queue_write returns, so if the mirror request was 
-			 * queued after (which would then not happen), we'd 
-			 * lose that write and cause the process to hang with 
-			 * unacknowledged writes
+			 * it's important to queue the mirror request before
+			 * queuing the main one. If the main image runs into
+			 * ENOSPC, the mirroring could be disabled before
+			 * td_queue_write returns, so if the mirror request was
+			 * queued after (which would then not happen), we'd
+			 * lose that write and cause the process to hang with
+			 * unacknowledged writes.
+			 *
+			 * Skip when lcache is storing a read-through.
 			 */
-			if (vbd->secondary_mode == TD_VBD_SECONDARY_MIRROR)
-				queue_mirror_req(vbd, treq);
+			if (vbd->secondary_mode == TD_VBD_SECONDARY_MIRROR &&
+				likely(vreq->skip_mirror == false)) {
+					queue_mirror_req(vbd, treq);
+			}
+
 			td_queue_write(treq.image, treq);
 			break;
 

drivers/tapdisk.h

@@ -177,6 +177,8 @@ struct td_vbd_request {
 	int                         num_retries;
 	struct timeval		    ts;
 	struct timeval              last_try;
+	/* When "reading-through" the local cache, don't write back to the source */
+	bool                        skip_mirror;
 
 	td_vbd_t                   *vbd;
 	struct list_head            next;