Index: uspace/srv/bd/hr/hr.c
===================================================================
--- uspace/srv/bd/hr/hr.c	(revision f81960c5dd5ffc19dec3d22be6176802972b2639)
+++ uspace/srv/bd/hr/hr.c	(revision 30140c1ba6529eb7eea93638ce863d07b5612dd5)
@@ -202,4 +202,5 @@
 		new_volume->hr_ops.init = hr_raid4_init;
 		new_volume->hr_ops.status_event = hr_raid4_status_event;
+		new_volume->hr_ops.add_hotspare = hr_raid4_add_hotspare;
 		break;
 	case HR_LVL_5:
Index: uspace/srv/bd/hr/raid4.c
===================================================================
--- uspace/srv/bd/hr/raid4.c	(revision f81960c5dd5ffc19dec3d22be6176802972b2639)
+++ uspace/srv/bd/hr/raid4.c	(revision 30140c1ba6529eb7eea93638ce863d07b5612dd5)
@@ -58,4 +58,5 @@
 static ssize_t hr_raid4_get_bad_ext(hr_volume_t *);
 static errno_t hr_raid4_update_vol_status(hr_volume_t *);
+static void hr_raid4_handle_extent_error(hr_volume_t *, size_t, errno_t);
 static void xor(void *, const void *, size_t);
 static errno_t hr_raid4_read_degraded(hr_volume_t *, uint64_t, uint64_t,
@@ -67,4 +68,5 @@
 static errno_t hr_raid4_bd_op(hr_bd_op_type_t, bd_srv_t *, aoff64_t, size_t,
     void *, const void *, size_t);
+static errno_t hr_raid4_rebuild(void *);
 
 /* bdops */
@@ -145,4 +147,39 @@
 }
 
+errno_t hr_raid4_add_hotspare(hr_volume_t *vol, service_id_t hotspare)
+{
+	HR_DEBUG("hr_raid4_add_hotspare()\n");
+
+	fibril_mutex_lock(&vol->lock);
+
+	if (vol->hotspare_no >= HR_MAX_HOTSPARES) {
+		HR_ERROR("hr_raid4_add_hotspare(): cannot add more hotspares "
+		    "to \"%s\"\n", vol->devname);
+		fibril_mutex_unlock(&vol->lock);
+		return ELIMIT;
+	}
+
+	vol->hotspares[vol->hotspare_no].svc_id = hotspare;
+	vol->hotspares[vol->hotspare_no].status = HR_EXT_HOTSPARE;
+	vol->hotspare_no++;
+
+	/*
+	 * If the volume is degraded, start rebuild right away.
+	 */
+	if (vol->status == HR_VOL_DEGRADED) {
+		HR_DEBUG("hr_raid4_add_hotspare(): volume in DEGRADED state, "
+		    "spawning new rebuild fibril\n");
+		fid_t fib = fibril_create(hr_raid4_rebuild, vol);
+		if (fib == 0)
+			return EINVAL;
+		fibril_start(fib);
+		fibril_detach(fib);
+	}
+
+	fibril_mutex_unlock(&vol->lock);
+
+	return EOK;
+}
+
 static errno_t hr_raid4_bd_open(bd_srvs_t *bds, bd_srv_t *bd)
 {
@@ -228,9 +265,19 @@
 		return EOK;
 	case 1:
-		if (old_state != HR_VOL_DEGRADED) {
+		if (old_state != HR_VOL_DEGRADED &&
+		    old_state != HR_VOL_REBUILD) {
 			HR_WARN("RAID 4 array \"%s\" (%lu) has 1 extent "
 			    "inactive, marking as DEGRADED",
 			    vol->devname, vol->svc_id);
 			vol->status = HR_VOL_DEGRADED;
+			if (vol->hotspare_no > 0) {
+				fid_t fib = fibril_create(hr_raid4_rebuild,
+				    vol);
+				if (fib == 0) {
+					return EINVAL;
+				}
+				fibril_start(fib);
+				fibril_detach(fib);
+			}
 		}
 		return EOK;
@@ -238,5 +285,5 @@
 		if (old_state != HR_VOL_FAULTY) {
 			HR_WARN("RAID 4 array \"%s\" (%lu) has more "
-			    "than one 1 extent inactive, marking as FAULTY",
+			    "than one 1 extent unusable, marking as FAULTY",
 			    vol->devname, vol->svc_id);
 			vol->status = HR_VOL_FAULTY;
@@ -244,4 +291,13 @@
 		return EINVAL;
 	}
+}
+
+static void hr_raid4_handle_extent_error(hr_volume_t *vol, size_t extent,
+    errno_t rc)
+{
+	if (rc == ENOENT)
+		hr_update_ext_status(vol, extent, HR_EXT_MISSING);
+	else if (rc != EOK)
+		hr_update_ext_status(vol, extent, HR_EXT_FAILED);
 }
 
@@ -512,8 +568,5 @@
 			goto error;
 
-		if (rc == ENOENT)
-			hr_update_ext_status(vol, extent, HR_EXT_MISSING);
-		else if (rc != EOK)
-			hr_update_ext_status(vol, extent, HR_EXT_FAILED);
+		hr_raid4_handle_extent_error(vol, extent, rc);
 
 		if (rc != EOK) {
@@ -552,4 +605,137 @@
 }
 
+static errno_t hr_raid4_rebuild(void *arg)
+{
+	HR_DEBUG("hr_raid4_rebuild()\n");
+
+	hr_volume_t *vol = arg;
+	errno_t rc = EOK;
+	void *buf = NULL, *xorbuf = NULL;
+
+	fibril_mutex_lock(&vol->lock);
+
+	if (vol->hotspare_no == 0) {
+		HR_WARN("hr_raid4_rebuild(): no free hotspares on \"%s\", "
+		    "aborting rebuild\n", vol->devname);
+		/* retval isn't checked for now */
+		goto end;
+	}
+
+	size_t bad = vol->dev_no;
+	for (size_t i = 0; i < vol->dev_no; i++) {
+		if (vol->extents[i].status == HR_EXT_FAILED) {
+			bad = i;
+			break;
+		}
+	}
+
+	if (bad == vol->dev_no) {
+		HR_WARN("hr_raid4_rebuild(): no bad extent on \"%s\", "
+		    "aborting rebuild\n", vol->devname);
+		/* retval isn't checked for now */
+		goto end;
+	}
+
+	block_fini(vol->extents[bad].svc_id);
+
+	size_t hotspare_idx = vol->hotspare_no - 1;
+
+	vol->extents[bad].svc_id = vol->hotspares[hotspare_idx].svc_id;
+	hr_update_ext_status(vol, bad, HR_EXT_REBUILD);
+
+	vol->hotspares[hotspare_idx].svc_id = 0;
+	vol->hotspares[hotspare_idx].status = HR_EXT_MISSING;
+	vol->hotspare_no--;
+
+	HR_WARN("hr_raid4_rebuild(): changing volume \"%s\" (%lu) state "
+	    "from %s to %s\n", vol->devname, vol->svc_id,
+	    hr_get_vol_status_msg(vol->status),
+	    hr_get_vol_status_msg(HR_VOL_REBUILD));
+	vol->status = HR_VOL_REBUILD;
+
+	hr_extent_t *hotspare = &vol->extents[bad];
+
+	HR_DEBUG("hr_raid4_rebuild(): initing (%lu)\n", hotspare->svc_id);
+
+	rc = block_init(hotspare->svc_id);
+	if (rc != EOK) {
+		HR_ERROR("hr_raid4_rebuild(): initing (%lu) failed, "
+		    "aborting rebuild\n", hotspare->svc_id);
+		goto end;
+	}
+
+	uint64_t max_blks = DATA_XFER_LIMIT / vol->bsize;
+	uint64_t left = vol->data_blkno / (vol->dev_no - 1);
+	buf = malloc(max_blks * vol->bsize);
+	xorbuf = malloc(max_blks * vol->bsize);
+
+	uint64_t ba = 0, cnt;
+	hr_add_ba_offset(vol, &ba);
+	while (left != 0) {
+		cnt = min(left, max_blks);
+
+		/*
+		 * Almost the same as read_degraded,
+		 * but we don't want to allocate new
+		 * xorbuf each blk rebuild batch.
+		 */
+		bool first = true;
+		for (size_t i = 0; i < vol->dev_no; i++) {
+			if (i == bad)
+				continue;
+			rc = block_read_direct(vol->extents[i].svc_id, ba, cnt,
+			    buf);
+			if (rc != EOK) {
+				hr_raid4_handle_extent_error(vol, i, rc);
+				HR_ERROR("rebuild on \"%s\" (%lu), failed due "
+				    "to a failed ONLINE extent, number %lu\n",
+				    vol->devname, vol->svc_id, i);
+				goto end;
+			}
+
+			if (first)
+				memcpy(xorbuf, buf, cnt * vol->bsize);
+			else
+				xor(xorbuf, buf, cnt * vol->bsize);
+
+			first = false;
+		}
+
+		rc = block_write_direct(hotspare->svc_id, ba, cnt, xorbuf);
+		if (rc != EOK) {
+			hr_raid4_handle_extent_error(vol, bad, rc);
+			HR_ERROR("rebuild on \"%s\" (%lu), failed due to "
+			    "the rebuilt extent number %lu failing\n",
+			    vol->devname, vol->svc_id, bad);
+			goto end;
+		}
+
+		ba += cnt;
+		left -= cnt;
+	}
+
+	HR_DEBUG("hr_raid4_rebuild(): rebuild finished on \"%s\" (%lu), "
+	    "extent number %lu\n", vol->devname, vol->svc_id, hotspare_idx);
+
+	hr_update_ext_status(vol, bad, HR_EXT_ONLINE);
+	/*
+	 * For now write metadata at the end, because
+	 * we don't sync metada accross extents yet.
+	 */
+	hr_write_meta_to_ext(vol, bad);
+end:
+	(void) hr_raid4_update_vol_status(vol);
+
+	fibril_mutex_unlock(&vol->lock);
+
+	if (buf != NULL)
+		free(buf);
+
+	if (xorbuf != NULL)
+		free(xorbuf);
+
+	return rc;
+}
+
 /** @}
  */
Index: uspace/srv/bd/hr/var.h
===================================================================
--- uspace/srv/bd/hr/var.h	(revision f81960c5dd5ffc19dec3d22be6176802972b2639)
+++ uspace/srv/bd/hr/var.h	(revision 30140c1ba6529eb7eea93638ce863d07b5612dd5)
@@ -106,4 +106,5 @@
 
 extern errno_t hr_raid1_add_hotspare(hr_volume_t *, service_id_t);
+extern errno_t hr_raid4_add_hotspare(hr_volume_t *, service_id_t);
 
 #endif