diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index 83904b5..e17f211 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -1483,6 +1483,8 @@ struct btrfs_fs_info {
 	struct rw_semaphore scrub_super_lock;
 	int scrub_workers_refcnt;
 	struct btrfs_workers scrub_workers;
+	struct btrfs_workers scrub_wr_completion_workers;
+	struct btrfs_workers scrub_nocow_workers;
 
 #ifdef CONFIG_BTRFS_FS_CHECK_INTEGRITY
 	u32 check_integrity_print_mask;
diff --git a/fs/btrfs/reada.c b/fs/btrfs/reada.c
index 0ddc565..9f363e1 100644
--- a/fs/btrfs/reada.c
+++ b/fs/btrfs/reada.c
@@ -418,12 +418,17 @@ static struct reada_extent *reada_find_extent(struct btrfs_root *root,
 			 */
 			continue;
 		}
+		if (!dev->bdev) {
+			/* cannot read ahead on missing device */
+			continue;
+		}
 		prev_dev = dev;
 		ret = radix_tree_insert(&dev->reada_extents, index, re);
 		if (ret) {
 			while (--i >= 0) {
 				dev = bbio->stripes[i].dev;
 				BUG_ON(dev == NULL);
+				/* ignore whether the entry was inserted */
 				radix_tree_delete(&dev->reada_extents, index);
 			}
 			BUG_ON(fs_info == NULL);
@@ -914,7 +919,10 @@ struct reada_control *btrfs_reada_add(struct btrfs_root *root,
 	generation = btrfs_header_generation(node);
 	free_extent_buffer(node);
 
-	reada_add_block(rc, start, &max_key, level, generation);
+	if (reada_add_block(rc, start, &max_key, level, generation)) {
+		kfree(rc);
+		return ERR_PTR(-ENOMEM);
+	}
 
 	reada_start_machine(root->fs_info);
 
diff --git a/fs/btrfs/scrub.c b/fs/btrfs/scrub.c
index 460e30b..59c69e0 100644
--- a/fs/btrfs/scrub.c
+++ b/fs/btrfs/scrub.c
@@ -25,6 +25,7 @@
 #include "transaction.h"
 #include "backref.h"
 #include "extent_io.h"
+#include "dev-replace.h"
 #include "check-integrity.h"
 #include "rcu-string.h"
 
@@ -44,8 +45,15 @@
 struct scrub_block;
 struct scrub_ctx;
 
-#define SCRUB_PAGES_PER_BIO	16	/* 64k per bio */
-#define SCRUB_BIOS_PER_CTX	16	/* 1 MB per device in flight */
+/*
+ * the following three values only influence the performance.
+ * The last one configures the number of parallel and outstanding I/O
+ * operations. The first two values configure an upper limit for the number
+ * of (dynamically allocated) pages that are added to a bio.
+ */
+#define SCRUB_PAGES_PER_RD_BIO	32	/* 128k per bio */
+#define SCRUB_PAGES_PER_WR_BIO	32	/* 128k per bio */
+#define SCRUB_BIOS_PER_SCTX	64	/* 8MB per device in flight */
 
 /*
  * the following value times PAGE_SIZE needs to be large enough to match the
@@ -62,6 +70,7 @@ struct scrub_page {
 	u64			generation;
 	u64			logical;
 	u64			physical;
+	u64			physical_for_dev_replace;
 	atomic_t		ref_count;
 	struct {
 		unsigned int	mirror_num:8;
@@ -79,7 +88,11 @@ struct scrub_bio {
 	int			err;
 	u64			logical;
 	u64			physical;
-	struct scrub_page	*pagev[SCRUB_PAGES_PER_BIO];
+#if SCRUB_PAGES_PER_WR_BIO >= SCRUB_PAGES_PER_RD_BIO
+	struct scrub_page	*pagev[SCRUB_PAGES_PER_WR_BIO];
+#else
+	struct scrub_page	*pagev[SCRUB_PAGES_PER_RD_BIO];
+#endif
 	int			page_count;
 	int			next_free;
 	struct btrfs_work	work;
@@ -99,8 +112,16 @@ struct scrub_block {
 	};
 };
 
+struct scrub_wr_ctx {
+	struct scrub_bio *wr_curr_bio;
+	struct btrfs_device *tgtdev;
+	int pages_per_wr_bio; /* <= SCRUB_PAGES_PER_WR_BIO */
+	atomic_t flush_all_writes;
+	struct mutex wr_lock;
+};
+
 struct scrub_ctx {
-	struct scrub_bio	*bios[SCRUB_BIOS_PER_CTX];
+	struct scrub_bio	*bios[SCRUB_BIOS_PER_SCTX];
 	struct btrfs_root	*dev_root;
 	int			first_free;
 	int			curr;
@@ -112,12 +133,13 @@ struct scrub_ctx {
 	struct list_head	csum_list;
 	atomic_t		cancel_req;
 	int			readonly;
-	int			pages_per_bio; /* <= SCRUB_PAGES_PER_BIO */
+	int			pages_per_rd_bio;
 	u32			sectorsize;
 	u32			nodesize;
 	u32			leafsize;
 
 	int			is_dev_replace;
+	struct scrub_wr_ctx	wr_ctx;
 
 	/*
 	 * statistics
@@ -135,6 +157,15 @@ struct scrub_fixup_nodatasum {
 	int			mirror_num;
 };
 
+struct scrub_copy_nocow_ctx {
+	struct scrub_ctx	*sctx;
+	u64			logical;
+	u64			len;
+	int			mirror_num;
+	u64			physical_for_dev_replace;
+	struct btrfs_work	work;
+};
+
 struct scrub_warning {
 	struct btrfs_path	*path;
 	u64			extent_item_size;
@@ -156,8 +187,9 @@ static void scrub_pending_trans_workers_dec(struct scrub_ctx *sctx);
 static int scrub_handle_errored_block(struct scrub_block *sblock_to_check);
 static int scrub_setup_recheck_block(struct scrub_ctx *sctx,
 				     struct btrfs_fs_info *fs_info,
+				     struct scrub_block *original_sblock,
 				     u64 length, u64 logical,
-				     struct scrub_block *sblock);
+				     struct scrub_block *sblocks_for_recheck);
 static void scrub_recheck_block(struct btrfs_fs_info *fs_info,
 				struct scrub_block *sblock, int is_metadata,
 				int have_csum, u8 *csum, u64 generation,
@@ -174,6 +206,9 @@ static int scrub_repair_block_from_good_copy(struct scrub_block *sblock_bad,
 static int scrub_repair_page_from_good_copy(struct scrub_block *sblock_bad,
 					    struct scrub_block *sblock_good,
 					    int page_num, int force_write);
+static void scrub_write_block_to_dev_replace(struct scrub_block *sblock);
+static int scrub_write_page_to_dev_replace(struct scrub_block *sblock,
+					   int page_num);
 static int scrub_checksum_data(struct scrub_block *sblock);
 static int scrub_checksum_tree_block(struct scrub_block *sblock);
 static int scrub_checksum_super(struct scrub_block *sblock);
@@ -181,14 +216,38 @@ static void scrub_block_get(struct scrub_block *sblock);
 static void scrub_block_put(struct scrub_block *sblock);
 static void scrub_page_get(struct scrub_page *spage);
 static void scrub_page_put(struct scrub_page *spage);
-static int scrub_add_page_to_bio(struct scrub_ctx *sctx,
-				 struct scrub_page *spage);
+static int scrub_add_page_to_rd_bio(struct scrub_ctx *sctx,
+				    struct scrub_page *spage);
 static int scrub_pages(struct scrub_ctx *sctx, u64 logical, u64 len,
 		       u64 physical, struct btrfs_device *dev, u64 flags,
-		       u64 gen, int mirror_num, u8 *csum, int force);
+		       u64 gen, int mirror_num, u8 *csum, int force,
+		       u64 physical_for_dev_replace);
 static void scrub_bio_end_io(struct bio *bio, int err);
 static void scrub_bio_end_io_worker(struct btrfs_work *work);
 static void scrub_block_complete(struct scrub_block *sblock);
+static void scrub_remap_extent(struct btrfs_fs_info *fs_info,
+			       u64 extent_logical, u64 extent_len,
+			       u64 *extent_physical,
+			       struct btrfs_device **extent_dev,
+			       int *extent_mirror_num);
+static int scrub_setup_wr_ctx(struct scrub_ctx *sctx,
+			      struct scrub_wr_ctx *wr_ctx,
+			      struct btrfs_fs_info *fs_info,
+			      struct btrfs_device *dev,
+			      int is_dev_replace);
+static void scrub_free_wr_ctx(struct scrub_wr_ctx *wr_ctx);
+static int scrub_add_page_to_wr_bio(struct scrub_ctx *sctx,
+				    struct scrub_page *spage);
+static void scrub_wr_submit(struct scrub_ctx *sctx);
+static void scrub_wr_bio_end_io(struct bio *bio, int err);
+static void scrub_wr_bio_end_io_worker(struct btrfs_work *work);
+static int write_page_nocow(struct scrub_ctx *sctx,
+			    u64 physical_for_dev_replace, struct page *page);
+static int copy_nocow_pages_for_inode(u64 inum, u64 offset, u64 root,
+				      void *ctx);
+static int copy_nocow_pages(struct scrub_ctx *sctx, u64 logical, u64 len,
+			    int mirror_num, u64 physical_for_dev_replace);
+static void copy_nocow_pages_worker(struct btrfs_work *work);
 
 
 static void scrub_pending_bio_inc(struct scrub_ctx *sctx)
@@ -262,19 +321,20 @@ static noinline_for_stack void scrub_free_ctx(struct scrub_ctx *sctx)
 	if (!sctx)
 		return;
 
+	scrub_free_wr_ctx(&sctx->wr_ctx);
+
 	/* this can happen when scrub is cancelled */
 	if (sctx->curr != -1) {
 		struct scrub_bio *sbio = sctx->bios[sctx->curr];
 
 		for (i = 0; i < sbio->page_count; i++) {
-			BUG_ON(!sbio->pagev[i]);
-			BUG_ON(!sbio->pagev[i]->page);
+			WARN_ON(!sbio->pagev[i]->page);
 			scrub_block_put(sbio->pagev[i]->sblock);
 		}
 		bio_put(sbio->bio);
 	}
 
-	for (i = 0; i < SCRUB_BIOS_PER_CTX; ++i) {
+	for (i = 0; i < SCRUB_BIOS_PER_SCTX; ++i) {
 		struct scrub_bio *sbio = sctx->bios[i];
 
 		if (!sbio)
@@ -292,18 +352,29 @@ struct scrub_ctx *scrub_setup_ctx(struct btrfs_device *dev, int is_dev_replace)
 	struct scrub_ctx *sctx;
 	int		i;
 	struct btrfs_fs_info *fs_info = dev->dev_root->fs_info;
-	int pages_per_bio;
+	int pages_per_rd_bio;
+	int ret;
 
-	pages_per_bio = min_t(int, SCRUB_PAGES_PER_BIO,
-			      bio_get_nr_vecs(dev->bdev));
+	/*
+	 * the setting of pages_per_rd_bio is correct for scrub but might
+	 * be wrong for the dev_replace code where we might read from
+	 * different devices in the initial huge bios. However, that
+	 * code is able to correctly handle the case when adding a page
+	 * to a bio fails.
+	 */
+	if (dev->bdev)
+		pages_per_rd_bio = min_t(int, SCRUB_PAGES_PER_RD_BIO,
+					 bio_get_nr_vecs(dev->bdev));
+	else
+		pages_per_rd_bio = SCRUB_PAGES_PER_RD_BIO;
 	sctx = kzalloc(sizeof(*sctx), GFP_NOFS);
 	if (!sctx)
 		goto nomem;
 	sctx->is_dev_replace = is_dev_replace;
-	sctx->pages_per_bio = pages_per_bio;
+	sctx->pages_per_rd_bio = pages_per_rd_bio;
 	sctx->curr = -1;
 	sctx->dev_root = dev->dev_root;
-	for (i = 0; i < SCRUB_BIOS_PER_CTX; ++i) {
+	for (i = 0; i < SCRUB_BIOS_PER_SCTX; ++i) {
 		struct scrub_bio *sbio;
 
 		sbio = kzalloc(sizeof(*sbio), GFP_NOFS);
@@ -316,7 +387,7 @@ struct scrub_ctx *scrub_setup_ctx(struct btrfs_device *dev, int is_dev_replace)
 		sbio->page_count = 0;
 		sbio->work.func = scrub_bio_end_io_worker;
 
-		if (i != SCRUB_BIOS_PER_CTX - 1)
+		if (i != SCRUB_BIOS_PER_SCTX - 1)
 			sctx->bios[i]->next_free = i + 1;
 		else
 			sctx->bios[i]->next_free = -1;
@@ -334,6 +405,13 @@ struct scrub_ctx *scrub_setup_ctx(struct btrfs_device *dev, int is_dev_replace)
 	spin_lock_init(&sctx->list_lock);
 	spin_lock_init(&sctx->stat_lock);
 	init_waitqueue_head(&sctx->list_wait);
+
+	ret = scrub_setup_wr_ctx(sctx, &sctx->wr_ctx, fs_info,
+				 fs_info->dev_replace.tgtdev, is_dev_replace);
+	if (ret) {
+		scrub_free_ctx(sctx);
+		return ERR_PTR(ret);
+	}
 	return sctx;
 
 nomem:
@@ -341,7 +419,8 @@ nomem:
 	return ERR_PTR(-ENOMEM);
 }
 
-static int scrub_print_warning_inode(u64 inum, u64 offset, u64 root, void *ctx)
+static int scrub_print_warning_inode(u64 inum, u64 offset, u64 root,
+				     void *warn_ctx)
 {
 	u64 isize;
 	u32 nlink;
@@ -349,7 +428,7 @@ static int scrub_print_warning_inode(u64 inum, u64 offset, u64 root, void *ctx)
 	int i;
 	struct extent_buffer *eb;
 	struct btrfs_inode_item *inode_item;
-	struct scrub_warning *swarn = ctx;
+	struct scrub_warning *swarn = warn_ctx;
 	struct btrfs_fs_info *fs_info = swarn->dev->dev_root->fs_info;
 	struct inode_fs_paths *ipath = NULL;
 	struct btrfs_root *local_root;
@@ -492,11 +571,11 @@ out:
 	kfree(swarn.msg_buf);
 }
 
-static int scrub_fixup_readpage(u64 inum, u64 offset, u64 root, void *ctx)
+static int scrub_fixup_readpage(u64 inum, u64 offset, u64 root, void *fixup_ctx)
 {
 	struct page *page = NULL;
 	unsigned long index;
-	struct scrub_fixup_nodatasum *fixup = ctx;
+	struct scrub_fixup_nodatasum *fixup = fixup_ctx;
 	int ret;
 	int corrected = 0;
 	struct btrfs_key key;
@@ -660,7 +739,9 @@ out:
 		spin_lock(&sctx->stat_lock);
 		++sctx->stat.uncorrectable_errors;
 		spin_unlock(&sctx->stat_lock);
-
+		btrfs_dev_replace_stats_inc(
+			&sctx->dev_root->fs_info->dev_replace.
+			num_uncorrectable_read_errors);
 		printk_ratelimited_in_rcu(KERN_ERR
 			"btrfs: unable to fixup (nodatasum) error at logical %llu on dev %s\n",
 			(unsigned long long)fixup->logical,
@@ -715,6 +796,11 @@ static int scrub_handle_errored_block(struct scrub_block *sblock_to_check)
 	csum = sblock_to_check->pagev[0]->csum;
 	dev = sblock_to_check->pagev[0]->dev;
 
+	if (sctx->is_dev_replace && !is_metadata && !have_csum) {
+		sblocks_for_recheck = NULL;
+		goto nodatasum_case;
+	}
+
 	/*
 	 * read all mirrors one after the other. This includes to
 	 * re-read the extent or metadata block that failed (that was
@@ -758,7 +844,7 @@ static int scrub_handle_errored_block(struct scrub_block *sblock_to_check)
 	}
 
 	/* setup the context, map the logical blocks and alloc the pages */
-	ret = scrub_setup_recheck_block(sctx, fs_info, length,
+	ret = scrub_setup_recheck_block(sctx, fs_info, sblock_to_check, length,
 					logical, sblocks_for_recheck);
 	if (ret) {
 		spin_lock(&sctx->stat_lock);
@@ -789,6 +875,8 @@ static int scrub_handle_errored_block(struct scrub_block *sblock_to_check)
 		sctx->stat.unverified_errors++;
 		spin_unlock(&sctx->stat_lock);
 
+		if (sctx->is_dev_replace)
+			scrub_write_block_to_dev_replace(sblock_bad);
 		goto out;
 	}
 
@@ -822,12 +910,15 @@ static int scrub_handle_errored_block(struct scrub_block *sblock_to_check)
 				BTRFS_DEV_STAT_CORRUPTION_ERRS);
 	}
 
-	if (sctx->readonly)
+	if (sctx->readonly && !sctx->is_dev_replace)
 		goto did_not_correct_error;
 
 	if (!is_metadata && !have_csum) {
 		struct scrub_fixup_nodatasum *fixup_nodatasum;
 
+nodatasum_case:
+		WARN_ON(sctx->is_dev_replace);
+
 		/*
 		 * !is_metadata and !have_csum, this means that the data
 		 * might not be COW'ed, that it might be modified
@@ -883,18 +974,79 @@ static int scrub_handle_errored_block(struct scrub_block *sblock_to_check)
 		if (!sblock_other->header_error &&
 		    !sblock_other->checksum_error &&
 		    sblock_other->no_io_error_seen) {
-			int force_write = is_metadata || have_csum;
-
-			ret = scrub_repair_block_from_good_copy(sblock_bad,
-								sblock_other,
-								force_write);
+			if (sctx->is_dev_replace) {
+				scrub_write_block_to_dev_replace(sblock_other);
+			} else {
+				int force_write = is_metadata || have_csum;
+
+				ret = scrub_repair_block_from_good_copy(
+						sblock_bad, sblock_other,
+						force_write);
+			}
 			if (0 == ret)
 				goto corrected_error;
 		}
 	}
 
 	/*
-	 * in case of I/O errors in the area that is supposed to be
+	 * for dev_replace, pick good pages and write to the target device.
+	 */
+	if (sctx->is_dev_replace) {
+		success = 1;
+		for (page_num = 0; page_num < sblock_bad->page_count;
+		     page_num++) {
+			int sub_success;
+
+			sub_success = 0;
+			for (mirror_index = 0;
+			     mirror_index < BTRFS_MAX_MIRRORS &&
+			     sblocks_for_recheck[mirror_index].page_count > 0;
+			     mirror_index++) {
+				struct scrub_block *sblock_other =
+					sblocks_for_recheck + mirror_index;
+				struct scrub_page *page_other =
+					sblock_other->pagev[page_num];
+
+				if (!page_other->io_error) {
+					ret = scrub_write_page_to_dev_replace(
+							sblock_other, page_num);
+					if (ret == 0) {
+						/* succeeded for this page */
+						sub_success = 1;
+						break;
+					} else {
+						btrfs_dev_replace_stats_inc(
+							&sctx->dev_root->
+							fs_info->dev_replace.
+							num_write_errors);
+					}
+				}
+			}
+
+			if (!sub_success) {
+				/*
+				 * did not find a mirror to fetch the page
+				 * from. scrub_write_page_to_dev_replace()
+				 * handles this case (page->io_error), by
+				 * filling the block with zeros before
+				 * submitting the write request
+				 */
+				success = 0;
+				ret = scrub_write_page_to_dev_replace(
+						sblock_bad, page_num);
+				if (ret)
+					btrfs_dev_replace_stats_inc(
+						&sctx->dev_root->fs_info->
+						dev_replace.num_write_errors);
+			}
+		}
+
+		goto out;
+	}
+
+	/*
+	 * for regular scrub, repair those pages that are errored.
+	 * In case of I/O errors in the area that is supposed to be
 	 * repaired, continue by picking good copies of those pages.
 	 * Select the good pages from mirrors to rewrite bad pages from
 	 * the area to fix. Afterwards verify the checksum of the block
@@ -1017,6 +1169,7 @@ out:
 
 static int scrub_setup_recheck_block(struct scrub_ctx *sctx,
 				     struct btrfs_fs_info *fs_info,
+				     struct scrub_block *original_sblock,
 				     u64 length, u64 logical,
 				     struct scrub_block *sblocks_for_recheck)
 {
@@ -1047,7 +1200,7 @@ static int scrub_setup_recheck_block(struct scrub_ctx *sctx,
 			return -EIO;
 		}
 
-		BUG_ON(page_index >= SCRUB_PAGES_PER_BIO);
+		BUG_ON(page_index >= SCRUB_PAGES_PER_RD_BIO);
 		for (mirror_index = 0; mirror_index < (int)bbio->num_stripes;
 		     mirror_index++) {
 			struct scrub_block *sblock;
@@ -1071,6 +1224,10 @@ leave_nomem:
 			sblock->pagev[page_index] = page;
 			page->logical = logical;
 			page->physical = bbio->stripes[mirror_index].physical;
+			BUG_ON(page_index >= original_sblock->page_count);
+			page->physical_for_dev_replace =
+				original_sblock->pagev[page_index]->
+				physical_for_dev_replace;
 			/* for missing devices, dev->bdev is NULL */
 			page->dev = bbio->stripes[mirror_index].dev;
 			page->mirror_num = mirror_index + 1;
@@ -1249,6 +1406,12 @@ static int scrub_repair_page_from_good_copy(struct scrub_block *sblock_bad,
 		int ret;
 		DECLARE_COMPLETION_ONSTACK(complete);
 
+		if (!page_bad->dev->bdev) {
+			printk_ratelimited(KERN_WARNING
+				"btrfs: scrub_repair_page_from_good_copy(bdev == NULL) is unexpected!\n");
+			return -EIO;
+		}
+
 		bio = bio_alloc(GFP_NOFS, 1);
 		if (!bio)
 			return -EIO;
@@ -1269,6 +1432,9 @@ static int scrub_repair_page_from_good_copy(struct scrub_block *sblock_bad,
 		if (!bio_flagged(bio, BIO_UPTODATE)) {
 			btrfs_dev_stat_inc_and_print(page_bad->dev,
 				BTRFS_DEV_STAT_WRITE_ERRS);
+			btrfs_dev_replace_stats_inc(
+				&sblock_bad->sctx->dev_root->fs_info->
+				dev_replace.num_write_errors);
 			bio_put(bio);
 			return -EIO;
 		}
@@ -1278,7 +1444,166 @@ static int scrub_repair_page_from_good_copy(struct scrub_block *sblock_bad,
 	return 0;
 }
 
-static void scrub_checksum(struct scrub_block *sblock)
+static void scrub_write_block_to_dev_replace(struct scrub_block *sblock)
+{
+	int page_num;
+
+	for (page_num = 0; page_num < sblock->page_count; page_num++) {
+		int ret;
+
+		ret = scrub_write_page_to_dev_replace(sblock, page_num);
+		if (ret)
+			btrfs_dev_replace_stats_inc(
+				&sblock->sctx->dev_root->fs_info->dev_replace.
+				num_write_errors);
+	}
+}
+
+static int scrub_write_page_to_dev_replace(struct scrub_block *sblock,
+					   int page_num)
+{
+	struct scrub_page *spage = sblock->pagev[page_num];
+
+	BUG_ON(spage->page == NULL);
+	if (spage->io_error) {
+		void *mapped_buffer = kmap_atomic(spage->page);
+
+		memset(mapped_buffer, 0, PAGE_CACHE_SIZE);
+		flush_dcache_page(spage->page);
+		kunmap_atomic(mapped_buffer);
+	}
+	return scrub_add_page_to_wr_bio(sblock->sctx, spage);
+}
+
+static int scrub_add_page_to_wr_bio(struct scrub_ctx *sctx,
+				    struct scrub_page *spage)
+{
+	struct scrub_wr_ctx *wr_ctx = &sctx->wr_ctx;
+	struct scrub_bio *sbio;
+	int ret;
+
+	mutex_lock(&wr_ctx->wr_lock);
+again:
+	if (!wr_ctx->wr_curr_bio) {
+		wr_ctx->wr_curr_bio = kzalloc(sizeof(*wr_ctx->wr_curr_bio),
+					      GFP_NOFS);
+		if (!wr_ctx->wr_curr_bio)
+			return -ENOMEM;
+		wr_ctx->wr_curr_bio->sctx = sctx;
+		wr_ctx->wr_curr_bio->page_count = 0;
+	}
+	sbio = wr_ctx->wr_curr_bio;
+	if (sbio->page_count == 0) {
+		struct bio *bio;
+
+		sbio->physical = spage->physical_for_dev_replace;
+		sbio->logical = spage->logical;
+		sbio->dev = wr_ctx->tgtdev;
+		bio = sbio->bio;
+		if (!bio) {
+			bio = bio_alloc(GFP_NOFS, wr_ctx->pages_per_wr_bio);
+			if (!bio) {
+				mutex_unlock(&wr_ctx->wr_lock);
+				return -ENOMEM;
+			}
+			sbio->bio = bio;
+		}
+
+		bio->bi_private = sbio;
+		bio->bi_end_io = scrub_wr_bio_end_io;
+		bio->bi_bdev = sbio->dev->bdev;
+		bio->bi_sector = sbio->physical >> 9;
+		sbio->err = 0;
+	} else if (sbio->physical + sbio->page_count * PAGE_SIZE !=
+		   spage->physical_for_dev_replace ||
+		   sbio->logical + sbio->page_count * PAGE_SIZE !=
+		   spage->logical) {
+		scrub_wr_submit(sctx);
+		goto again;
+	}
+
+	ret = bio_add_page(sbio->bio, spage->page, PAGE_SIZE, 0);
+	if (ret != PAGE_SIZE) {
+		if (sbio->page_count < 1) {
+			bio_put(sbio->bio);
+			sbio->bio = NULL;
+			mutex_unlock(&wr_ctx->wr_lock);
+			return -EIO;
+		}
+		scrub_wr_submit(sctx);
+		goto again;
+	}
+
+	sbio->pagev[sbio->page_count] = spage;
+	scrub_page_get(spage);
+	sbio->page_count++;
+	if (sbio->page_count == wr_ctx->pages_per_wr_bio)
+		scrub_wr_submit(sctx);
+	mutex_unlock(&wr_ctx->wr_lock);
+
+	return 0;
+}
+
+static void scrub_wr_submit(struct scrub_ctx *sctx)
+{
+	struct scrub_wr_ctx *wr_ctx = &sctx->wr_ctx;
+	struct scrub_bio *sbio;
+
+	if (!wr_ctx->wr_curr_bio)
+		return;
+
+	sbio = wr_ctx->wr_curr_bio;
+	wr_ctx->wr_curr_bio = NULL;
+	WARN_ON(!sbio->bio->bi_bdev);
+	scrub_pending_bio_inc(sctx);
+	/* process all writes in a single worker thread. Then the block layer
+	 * orders the requests before sending them to the driver which
+	 * doubled the write performance on spinning disks when measured
+	 * with Linux 3.5 */
+	btrfsic_submit_bio(WRITE, sbio->bio);
+}
+
+static void scrub_wr_bio_end_io(struct bio *bio, int err)
+{
+	struct scrub_bio *sbio = bio->bi_private;
+	struct btrfs_fs_info *fs_info = sbio->dev->dev_root->fs_info;
+
+	sbio->err = err;
+	sbio->bio = bio;
+
+	sbio->work.func = scrub_wr_bio_end_io_worker;
+	btrfs_queue_worker(&fs_info->scrub_wr_completion_workers, &sbio->work);
+}
+
+static void scrub_wr_bio_end_io_worker(struct btrfs_work *work)
+{
+	struct scrub_bio *sbio = container_of(work, struct scrub_bio, work);
+	struct scrub_ctx *sctx = sbio->sctx;
+	int i;
+
+	WARN_ON(sbio->page_count > SCRUB_PAGES_PER_WR_BIO);
+	if (sbio->err) {
+		struct btrfs_dev_replace *dev_replace =
+			&sbio->sctx->dev_root->fs_info->dev_replace;
+
+		for (i = 0; i < sbio->page_count; i++) {
+			struct scrub_page *spage = sbio->pagev[i];
+
+			spage->io_error = 1;
+			btrfs_dev_replace_stats_inc(&dev_replace->
+						    num_write_errors);
+		}
+	}
+
+	for (i = 0; i < sbio->page_count; i++)
+		scrub_page_put(sbio->pagev[i]);
+
+	bio_put(sbio->bio);
+	kfree(sbio);
+	scrub_pending_bio_dec(sctx);
+}
+
+static int scrub_checksum(struct scrub_block *sblock)
 {
 	u64 flags;
 	int ret;
@@ -1296,6 +1621,8 @@ static void scrub_checksum(struct scrub_block *sblock)
 		WARN_ON(1);
 	if (ret)
 		scrub_handle_errored_block(sblock);
+
+	return ret;
 }
 
 static int scrub_checksum_data(struct scrub_block *sblock)
@@ -1386,7 +1713,7 @@ static int scrub_checksum_tree_block(struct scrub_block *sblock)
 		   BTRFS_UUID_SIZE))
 		++fail;
 
-	BUG_ON(sctx->nodesize != sctx->leafsize);
+	WARN_ON(sctx->nodesize != sctx->leafsize);
 	len = sctx->nodesize - BTRFS_CSUM_SIZE;
 	mapped_size = PAGE_SIZE - BTRFS_CSUM_SIZE;
 	p = ((u8 *)mapped_buffer) + BTRFS_CSUM_SIZE;
@@ -1534,11 +1861,24 @@ static void scrub_submit(struct scrub_ctx *sctx)
 	sctx->curr = -1;
 	scrub_pending_bio_inc(sctx);
 
-	btrfsic_submit_bio(READ, sbio->bio);
+	if (!sbio->bio->bi_bdev) {
+		/*
+		 * this case should not happen. If btrfs_map_block() is
+		 * wrong, it could happen for dev-replace operations on
+		 * missing devices when no mirrors are available, but in
+		 * this case it should already fail the mount.
+		 * This case is handled correctly (but _very_ slowly).
+		 */
+		printk_ratelimited(KERN_WARNING
+			"btrfs: scrub_submit(bio bdev == NULL) is unexpected!\n");
+		bio_endio(sbio->bio, -EIO);
+	} else {
+		btrfsic_submit_bio(READ, sbio->bio);
+	}
 }
 
-static int scrub_add_page_to_bio(struct scrub_ctx *sctx,
-				 struct scrub_page *spage)
+static int scrub_add_page_to_rd_bio(struct scrub_ctx *sctx,
+				    struct scrub_page *spage)
 {
 	struct scrub_block *sblock = spage->sblock;
 	struct scrub_bio *sbio;
@@ -1570,7 +1910,7 @@ again:
 		sbio->dev = spage->dev;
 		bio = sbio->bio;
 		if (!bio) {
-			bio = bio_alloc(GFP_NOFS, sctx->pages_per_bio);
+			bio = bio_alloc(GFP_NOFS, sctx->pages_per_rd_bio);
 			if (!bio)
 				return -ENOMEM;
 			sbio->bio = bio;
@@ -1602,10 +1942,10 @@ again:
 		goto again;
 	}
 
-	scrub_block_get(sblock); /* one for the added page */
+	scrub_block_get(sblock); /* one for the page added to the bio */
 	atomic_inc(&sblock->outstanding_pages);
 	sbio->page_count++;
-	if (sbio->page_count == sctx->pages_per_bio)
+	if (sbio->page_count == sctx->pages_per_rd_bio)
 		scrub_submit(sctx);
 
 	return 0;
@@ -1613,7 +1953,8 @@ again:
 
 static int scrub_pages(struct scrub_ctx *sctx, u64 logical, u64 len,
 		       u64 physical, struct btrfs_device *dev, u64 flags,
-		       u64 gen, int mirror_num, u8 *csum, int force)
+		       u64 gen, int mirror_num, u8 *csum, int force,
+		       u64 physical_for_dev_replace)
 {
 	struct scrub_block *sblock;
 	int index;
@@ -1654,6 +1995,7 @@ leave_nomem:
 		spage->generation = gen;
 		spage->logical = logical;
 		spage->physical = physical;
+		spage->physical_for_dev_replace = physical_for_dev_replace;
 		spage->mirror_num = mirror_num;
 		if (csum) {
 			spage->have_csum = 1;
@@ -1668,6 +2010,7 @@ leave_nomem:
 		len -= l;
 		logical += l;
 		physical += l;
+		physical_for_dev_replace += l;
 	}
 
 	WARN_ON(sblock->page_count == 0);
@@ -1675,7 +2018,7 @@ leave_nomem:
 		struct scrub_page *spage = sblock->pagev[index];
 		int ret;
 
-		ret = scrub_add_page_to_bio(sctx, spage);
+		ret = scrub_add_page_to_rd_bio(sctx, spage);
 		if (ret) {
 			scrub_block_put(sblock);
 			return ret;
@@ -1707,7 +2050,7 @@ static void scrub_bio_end_io_worker(struct btrfs_work *work)
 	struct scrub_ctx *sctx = sbio->sctx;
 	int i;
 
-	BUG_ON(sbio->page_count > SCRUB_PAGES_PER_BIO);
+	BUG_ON(sbio->page_count > SCRUB_PAGES_PER_RD_BIO);
 	if (sbio->err) {
 		for (i = 0; i < sbio->page_count; i++) {
 			struct scrub_page *spage = sbio->pagev[i];
@@ -1733,15 +2076,30 @@ static void scrub_bio_end_io_worker(struct btrfs_work *work)
 	sbio->next_free = sctx->first_free;
 	sctx->first_free = sbio->index;
 	spin_unlock(&sctx->list_lock);
+
+	if (sctx->is_dev_replace &&
+	    atomic_read(&sctx->wr_ctx.flush_all_writes)) {
+		mutex_lock(&sctx->wr_ctx.wr_lock);
+		scrub_wr_submit(sctx);
+		mutex_unlock(&sctx->wr_ctx.wr_lock);
+	}
+
 	scrub_pending_bio_dec(sctx);
 }
 
 static void scrub_block_complete(struct scrub_block *sblock)
 {
-	if (!sblock->no_io_error_seen)
+	if (!sblock->no_io_error_seen) {
 		scrub_handle_errored_block(sblock);
-	else
-		scrub_checksum(sblock);
+	} else {
+		/*
+		 * if has checksum error, write via repair mechanism in
+		 * dev replace case, otherwise write here in dev replace
+		 * case.
+		 */
+		if (!scrub_checksum(sblock) && sblock->sctx->is_dev_replace)
+			scrub_write_block_to_dev_replace(sblock);
+	}
 }
 
 static int scrub_find_csum(struct scrub_ctx *sctx, u64 logical, u64 len,
@@ -1786,7 +2144,7 @@ static int scrub_find_csum(struct scrub_ctx *sctx, u64 logical, u64 len,
 /* scrub extent tries to collect up to 64 kB for each bio */
 static int scrub_extent(struct scrub_ctx *sctx, u64 logical, u64 len,
 			u64 physical, struct btrfs_device *dev, u64 flags,
-			u64 gen, int mirror_num)
+			u64 gen, int mirror_num, u64 physical_for_dev_replace)
 {
 	int ret;
 	u8 csum[BTRFS_CSUM_SIZE];
@@ -1799,7 +2157,7 @@ static int scrub_extent(struct scrub_ctx *sctx, u64 logical, u64 len,
 		sctx->stat.data_bytes_scrubbed += len;
 		spin_unlock(&sctx->stat_lock);
 	} else if (flags & BTRFS_EXTENT_FLAG_TREE_BLOCK) {
-		BUG_ON(sctx->nodesize != sctx->leafsize);
+		WARN_ON(sctx->nodesize != sctx->leafsize);
 		blocksize = sctx->nodesize;
 		spin_lock(&sctx->stat_lock);
 		sctx->stat.tree_extents_scrubbed++;
@@ -1807,7 +2165,7 @@ static int scrub_extent(struct scrub_ctx *sctx, u64 logical, u64 len,
 		spin_unlock(&sctx->stat_lock);
 	} else {
 		blocksize = sctx->sectorsize;
-		BUG_ON(1);
+		WARN_ON(1);
 	}
 
 	while (len) {
@@ -1819,14 +2177,23 @@ static int scrub_extent(struct scrub_ctx *sctx, u64 logical, u64 len,
 			have_csum = scrub_find_csum(sctx, logical, l, csum);
 			if (have_csum == 0)
 				++sctx->stat.no_csum;
+			if (sctx->is_dev_replace && !have_csum) {
+				ret = copy_nocow_pages(sctx, logical, l,
+						       mirror_num,
+						      physical_for_dev_replace);
+				goto behind_scrub_pages;
+			}
 		}
 		ret = scrub_pages(sctx, logical, l, physical, dev, flags, gen,
-				  mirror_num, have_csum ? csum : NULL, 0);
+				  mirror_num, have_csum ? csum : NULL, 0,
+				  physical_for_dev_replace);
+behind_scrub_pages:
 		if (ret)
 			return ret;
 		len -= l;
 		logical += l;
 		physical += l;
+		physical_for_dev_replace += l;
 	}
 	return 0;
 }
@@ -1834,7 +2201,8 @@ static int scrub_extent(struct scrub_ctx *sctx, u64 logical, u64 len,
 static noinline_for_stack int scrub_stripe(struct scrub_ctx *sctx,
 					   struct map_lookup *map,
 					   struct btrfs_device *scrub_dev,
-					   int num, u64 base, u64 length)
+					   int num, u64 base, u64 length,
+					   int is_dev_replace)
 {
 	struct btrfs_path *path;
 	struct btrfs_fs_info *fs_info = sctx->dev_root->fs_info;
@@ -1859,6 +2227,11 @@ static noinline_for_stack int scrub_stripe(struct scrub_ctx *sctx,
 	struct btrfs_key key_end;
 	u64 increment = map->stripe_len;
 	u64 offset;
+	u64 extent_logical;
+	u64 extent_physical;
+	u64 extent_len;
+	struct btrfs_device *extent_dev;
+	int extent_mirror_num;
 
 	nstripes = length;
 	offset = 0;
@@ -1966,9 +2339,14 @@ static noinline_for_stack int scrub_stripe(struct scrub_ctx *sctx,
 		 */
 		if (atomic_read(&fs_info->scrub_pause_req)) {
 			/* push queued extents */
+			atomic_set(&sctx->wr_ctx.flush_all_writes, 1);
 			scrub_submit(sctx);
+			mutex_lock(&sctx->wr_ctx.wr_lock);
+			scrub_wr_submit(sctx);
+			mutex_unlock(&sctx->wr_ctx.wr_lock);
 			wait_event(sctx->list_wait,
 				   atomic_read(&sctx->bios_in_flight) == 0);
+			atomic_set(&sctx->wr_ctx.flush_all_writes, 0);
 			atomic_inc(&fs_info->scrubs_paused);
 			wake_up(&fs_info->scrub_pause_wait);
 			mutex_lock(&fs_info->scrub_lock);
@@ -2063,10 +2441,20 @@ static noinline_for_stack int scrub_stripe(struct scrub_ctx *sctx,
 					     key.objectid;
 			}
 
-			ret = scrub_extent(sctx, key.objectid, key.offset,
-					   key.objectid - logical + physical,
-					   scrub_dev, flags, generation,
-					   mirror_num);
+			extent_logical = key.objectid;
+			extent_physical = key.objectid - logical + physical;
+			extent_len = key.offset;
+			extent_dev = scrub_dev;
+			extent_mirror_num = mirror_num;
+			if (is_dev_replace)
+				scrub_remap_extent(fs_info, extent_logical,
+						   extent_len, &extent_physical,
+						   &extent_dev,
+						   &extent_mirror_num);
+			ret = scrub_extent(sctx, extent_logical, extent_len,
+					   extent_physical, extent_dev, flags,
+					   generation, extent_mirror_num,
+					   key.objectid - logical + physical);
 			if (ret)
 				goto out;
 
@@ -2080,10 +2468,13 @@ next:
 		sctx->stat.last_physical = physical;
 		spin_unlock(&sctx->stat_lock);
 	}
+out:
 	/* push queued extents */
 	scrub_submit(sctx);
+	mutex_lock(&sctx->wr_ctx.wr_lock);
+	scrub_wr_submit(sctx);
+	mutex_unlock(&sctx->wr_ctx.wr_lock);
 
-out:
 	blk_finish_plug(&plug);
 	btrfs_free_path(path);
 	return ret < 0 ? ret : 0;
@@ -2093,14 +2484,14 @@ static noinline_for_stack int scrub_chunk(struct scrub_ctx *sctx,
 					  struct btrfs_device *scrub_dev,
 					  u64 chunk_tree, u64 chunk_objectid,
 					  u64 chunk_offset, u64 length,
-					  u64 dev_offset)
+					  u64 dev_offset, int is_dev_replace)
 {
 	struct btrfs_mapping_tree *map_tree =
 		&sctx->dev_root->fs_info->mapping_tree;
 	struct map_lookup *map;
 	struct extent_map *em;
 	int i;
-	int ret = -EINVAL;
+	int ret = 0;
 
 	read_lock(&map_tree->map_tree.lock);
 	em = lookup_extent_mapping(&map_tree->map_tree, chunk_offset, 1);
@@ -2120,7 +2511,8 @@ static noinline_for_stack int scrub_chunk(struct scrub_ctx *sctx,
 		if (map->stripes[i].dev->bdev == scrub_dev->bdev &&
 		    map->stripes[i].physical == dev_offset) {
 			ret = scrub_stripe(sctx, map, scrub_dev, i,
-					   chunk_offset, length);
+					   chunk_offset, length,
+					   is_dev_replace);
 			if (ret)
 				goto out;
 		}
@@ -2133,7 +2525,8 @@ out:
 
 static noinline_for_stack
 int scrub_enumerate_chunks(struct scrub_ctx *sctx,
-			   struct btrfs_device *scrub_dev, u64 start, u64 end)
+			   struct btrfs_device *scrub_dev, u64 start, u64 end,
+			   int is_dev_replace)
 {
 	struct btrfs_dev_extent *dev_extent = NULL;
 	struct btrfs_path *path;
@@ -2149,6 +2542,7 @@ int scrub_enumerate_chunks(struct scrub_ctx *sctx,
 	struct btrfs_key key;
 	struct btrfs_key found_key;
 	struct btrfs_block_group_cache *cache;
+	struct btrfs_dev_replace *dev_replace = &fs_info->dev_replace;
 
 	path = btrfs_alloc_path();
 	if (!path)
@@ -2214,11 +2608,61 @@ int scrub_enumerate_chunks(struct scrub_ctx *sctx,
 			ret = -ENOENT;
 			break;
 		}
+		dev_replace->cursor_right = found_key.offset + length;
+		dev_replace->cursor_left = found_key.offset;
+		dev_replace->item_needs_writeback = 1;
 		ret = scrub_chunk(sctx, scrub_dev, chunk_tree, chunk_objectid,
-				  chunk_offset, length, found_key.offset);
+				  chunk_offset, length, found_key.offset,
+				  is_dev_replace);
+
+		/*
+		 * flush, submit all pending read and write bios, afterwards
+		 * wait for them.
+		 * Note that in the dev replace case, a read request causes
+		 * write requests that are submitted in the read completion
+		 * worker. Therefore in the current situation, it is required
+		 * that all write requests are flushed, so that all read and
+		 * write requests are really completed when bios_in_flight
+		 * changes to 0.
+		 */
+		atomic_set(&sctx->wr_ctx.flush_all_writes, 1);
+		scrub_submit(sctx);
+		mutex_lock(&sctx->wr_ctx.wr_lock);
+		scrub_wr_submit(sctx);
+		mutex_unlock(&sctx->wr_ctx.wr_lock);
+
+		wait_event(sctx->list_wait,
+			   atomic_read(&sctx->bios_in_flight) == 0);
+		atomic_set(&sctx->wr_ctx.flush_all_writes, 0);
+		atomic_inc(&fs_info->scrubs_paused);
+		wake_up(&fs_info->scrub_pause_wait);
+		wait_event(sctx->list_wait,
+			   atomic_read(&sctx->workers_pending) == 0);
+
+		mutex_lock(&fs_info->scrub_lock);
+		while (atomic_read(&fs_info->scrub_pause_req)) {
+			mutex_unlock(&fs_info->scrub_lock);
+			wait_event(fs_info->scrub_pause_wait,
+			   atomic_read(&fs_info->scrub_pause_req) == 0);
+			mutex_lock(&fs_info->scrub_lock);
+		}
+		atomic_dec(&fs_info->scrubs_paused);
+		mutex_unlock(&fs_info->scrub_lock);
+		wake_up(&fs_info->scrub_pause_wait);
+
+		dev_replace->cursor_left = dev_replace->cursor_right;
+		dev_replace->item_needs_writeback = 1;
 		btrfs_put_block_group(cache);
 		if (ret)
 			break;
+		if (atomic64_read(&dev_replace->num_write_errors) > 0) {
+			ret = -EIO;
+			break;
+		}
+		if (sctx->stat.malloc_errors > 0) {
+			ret = -ENOMEM;
+			break;
+		}
 
 		key.offset = found_key.offset + length;
 		btrfs_release_path(path);
@@ -2254,7 +2698,7 @@ static noinline_for_stack int scrub_supers(struct scrub_ctx *sctx,
 
 		ret = scrub_pages(sctx, bytenr, BTRFS_SUPER_INFO_SIZE, bytenr,
 				  scrub_dev, BTRFS_EXTENT_FLAG_SUPER, gen, i,
-				  NULL, 1);
+				  NULL, 1, bytenr);
 		if (ret)
 			return ret;
 	}
@@ -2266,18 +2710,38 @@ static noinline_for_stack int scrub_supers(struct scrub_ctx *sctx,
 /*
  * get a reference count on fs_info->scrub_workers. start worker if necessary
  */
-static noinline_for_stack int scrub_workers_get(struct btrfs_fs_info *fs_info)
+static noinline_for_stack int scrub_workers_get(struct btrfs_fs_info *fs_info,
+						int is_dev_replace)
 {
 	int ret = 0;
 
 	mutex_lock(&fs_info->scrub_lock);
 	if (fs_info->scrub_workers_refcnt == 0) {
-		btrfs_init_workers(&fs_info->scrub_workers, "scrub",
-			   fs_info->thread_pool_size, &fs_info->generic_worker);
+		if (is_dev_replace)
+			btrfs_init_workers(&fs_info->scrub_workers, "scrub", 1,
+					&fs_info->generic_worker);
+		else
+			btrfs_init_workers(&fs_info->scrub_workers, "scrub",
+					fs_info->thread_pool_size,
+					&fs_info->generic_worker);
 		fs_info->scrub_workers.idle_thresh = 4;
 		ret = btrfs_start_workers(&fs_info->scrub_workers);
 		if (ret)
 			goto out;
+		btrfs_init_workers(&fs_info->scrub_wr_completion_workers,
+				   "scrubwrc",
+				   fs_info->thread_pool_size,
+				   &fs_info->generic_worker);
+		fs_info->scrub_wr_completion_workers.idle_thresh = 2;
+		ret = btrfs_start_workers(
+				&fs_info->scrub_wr_completion_workers);
+		if (ret)
+			goto out;
+		btrfs_init_workers(&fs_info->scrub_nocow_workers, "scrubnc", 1,
+				   &fs_info->generic_worker);
+		ret = btrfs_start_workers(&fs_info->scrub_nocow_workers);
+		if (ret)
+			goto out;
 	}
 	++fs_info->scrub_workers_refcnt;
 out:
@@ -2289,8 +2753,11 @@ out:
 static noinline_for_stack void scrub_workers_put(struct btrfs_fs_info *fs_info)
 {
 	mutex_lock(&fs_info->scrub_lock);
-	if (--fs_info->scrub_workers_refcnt == 0)
+	if (--fs_info->scrub_workers_refcnt == 0) {
 		btrfs_stop_workers(&fs_info->scrub_workers);
+		btrfs_stop_workers(&fs_info->scrub_wr_completion_workers);
+		btrfs_stop_workers(&fs_info->scrub_nocow_workers);
+	}
 	WARN_ON(fs_info->scrub_workers_refcnt < 0);
 	mutex_unlock(&fs_info->scrub_lock);
 }
@@ -2354,7 +2821,7 @@ int btrfs_scrub_dev(struct btrfs_fs_info *fs_info, u64 devid, u64 start,
 		return -EINVAL;
 	}
 
-	ret = scrub_workers_get(fs_info);
+	ret = scrub_workers_get(fs_info, is_dev_replace);
 	if (ret)
 		return ret;
 
@@ -2394,12 +2861,15 @@ int btrfs_scrub_dev(struct btrfs_fs_info *fs_info, u64 devid, u64 start,
 	mutex_unlock(&fs_info->scrub_lock);
 	mutex_unlock(&fs_info->fs_devices->device_list_mutex);
 
-	down_read(&fs_info->scrub_super_lock);
-	ret = scrub_supers(sctx, dev);
-	up_read(&fs_info->scrub_super_lock);
+	if (!is_dev_replace) {
+		down_read(&fs_info->scrub_super_lock);
+		ret = scrub_supers(sctx, dev);
+		up_read(&fs_info->scrub_super_lock);
+	}
 
 	if (!ret)
-		ret = scrub_enumerate_chunks(sctx, dev, start, end);
+		ret = scrub_enumerate_chunks(sctx, dev, start, end,
+					     is_dev_replace);
 
 	wait_event(sctx->list_wait, atomic_read(&sctx->bios_in_flight) == 0);
 	atomic_dec(&fs_info->scrubs_running);
@@ -2537,3 +3007,272 @@ int btrfs_scrub_progress(struct btrfs_root *root, u64 devid,
 
 	return dev ? (sctx ? 0 : -ENOTCONN) : -ENODEV;
 }
+
+static void scrub_remap_extent(struct btrfs_fs_info *fs_info,
+			       u64 extent_logical, u64 extent_len,
+			       u64 *extent_physical,
+			       struct btrfs_device **extent_dev,
+			       int *extent_mirror_num)
+{
+	u64 mapped_length;
+	struct btrfs_bio *bbio = NULL;
+	int ret;
+
+	mapped_length = extent_len;
+	ret = btrfs_map_block(fs_info, READ, extent_logical,
+			      &mapped_length, &bbio, 0);
+	if (ret || !bbio || mapped_length < extent_len ||
+	    !bbio->stripes[0].dev->bdev) {
+		kfree(bbio);
+		return;
+	}
+
+	*extent_physical = bbio->stripes[0].physical;
+	*extent_mirror_num = bbio->mirror_num;
+	*extent_dev = bbio->stripes[0].dev;
+	kfree(bbio);
+}
+
+static int scrub_setup_wr_ctx(struct scrub_ctx *sctx,
+			      struct scrub_wr_ctx *wr_ctx,
+			      struct btrfs_fs_info *fs_info,
+			      struct btrfs_device *dev,
+			      int is_dev_replace)
+{
+	WARN_ON(wr_ctx->wr_curr_bio != NULL);
+
+	mutex_init(&wr_ctx->wr_lock);
+	wr_ctx->wr_curr_bio = NULL;
+	if (!is_dev_replace)
+		return 0;
+
+	WARN_ON(!dev->bdev);
+	wr_ctx->pages_per_wr_bio = min_t(int, SCRUB_PAGES_PER_WR_BIO,
+					 bio_get_nr_vecs(dev->bdev));
+	wr_ctx->tgtdev = dev;
+	atomic_set(&wr_ctx->flush_all_writes, 0);
+	return 0;
+}
+
+static void scrub_free_wr_ctx(struct scrub_wr_ctx *wr_ctx)
+{
+	mutex_lock(&wr_ctx->wr_lock);
+	kfree(wr_ctx->wr_curr_bio);
+	wr_ctx->wr_curr_bio = NULL;
+	mutex_unlock(&wr_ctx->wr_lock);
+}
+
+static int copy_nocow_pages(struct scrub_ctx *sctx, u64 logical, u64 len,
+			    int mirror_num, u64 physical_for_dev_replace)
+{
+	struct scrub_copy_nocow_ctx *nocow_ctx;
+	struct btrfs_fs_info *fs_info = sctx->dev_root->fs_info;
+
+	nocow_ctx = kzalloc(sizeof(*nocow_ctx), GFP_NOFS);
+	if (!nocow_ctx) {
+		spin_lock(&sctx->stat_lock);
+		sctx->stat.malloc_errors++;
+		spin_unlock(&sctx->stat_lock);
+		return -ENOMEM;
+	}
+
+	scrub_pending_trans_workers_inc(sctx);
+
+	nocow_ctx->sctx = sctx;
+	nocow_ctx->logical = logical;
+	nocow_ctx->len = len;
+	nocow_ctx->mirror_num = mirror_num;
+	nocow_ctx->physical_for_dev_replace = physical_for_dev_replace;
+	nocow_ctx->work.func = copy_nocow_pages_worker;
+	btrfs_queue_worker(&fs_info->scrub_nocow_workers,
+			   &nocow_ctx->work);
+
+	return 0;
+}
+
+static void copy_nocow_pages_worker(struct btrfs_work *work)
+{
+	struct scrub_copy_nocow_ctx *nocow_ctx =
+		container_of(work, struct scrub_copy_nocow_ctx, work);
+	struct scrub_ctx *sctx = nocow_ctx->sctx;
+	u64 logical = nocow_ctx->logical;
+	u64 len = nocow_ctx->len;
+	int mirror_num = nocow_ctx->mirror_num;
+	u64 physical_for_dev_replace = nocow_ctx->physical_for_dev_replace;
+	int ret;
+	struct btrfs_trans_handle *trans = NULL;
+	struct btrfs_fs_info *fs_info;
+	struct btrfs_path *path;
+	struct btrfs_root *root;
+	int not_written = 0;
+
+	fs_info = sctx->dev_root->fs_info;
+	root = fs_info->extent_root;
+
+	path = btrfs_alloc_path();
+	if (!path) {
+		spin_lock(&sctx->stat_lock);
+		sctx->stat.malloc_errors++;
+		spin_unlock(&sctx->stat_lock);
+		not_written = 1;
+		goto out;
+	}
+
+	trans = btrfs_join_transaction(root);
+	if (IS_ERR(trans)) {
+		not_written = 1;
+		goto out;
+	}
+
+	ret = iterate_inodes_from_logical(logical, fs_info, path,
+					  copy_nocow_pages_for_inode,
+					  nocow_ctx);
+	if (ret != 0 && ret != -ENOENT) {
+		pr_warn("iterate_inodes_from_logical() failed: log %llu, phys %llu, len %llu, mir %llu, ret %d\n",
+			(unsigned long long)logical,
+			(unsigned long long)physical_for_dev_replace,
+			(unsigned long long)len,
+			(unsigned long long)mirror_num, ret);
+		not_written = 1;
+		goto out;
+	}
+
+out:
+	if (trans && !IS_ERR(trans))
+		btrfs_end_transaction(trans, root);
+	if (not_written)
+		btrfs_dev_replace_stats_inc(&fs_info->dev_replace.
+					    num_uncorrectable_read_errors);
+
+	btrfs_free_path(path);
+	kfree(nocow_ctx);
+
+	scrub_pending_trans_workers_dec(sctx);
+}
+
+static int copy_nocow_pages_for_inode(u64 inum, u64 offset, u64 root, void *ctx)
+{
+	unsigned long index;
+	struct scrub_copy_nocow_ctx *nocow_ctx = ctx;
+	int ret = 0;
+	struct btrfs_key key;
+	struct inode *inode = NULL;
+	struct btrfs_root *local_root;
+	u64 physical_for_dev_replace;
+	u64 len;
+	struct btrfs_fs_info *fs_info = nocow_ctx->sctx->dev_root->fs_info;
+
+	key.objectid = root;
+	key.type = BTRFS_ROOT_ITEM_KEY;
+	key.offset = (u64)-1;
+	local_root = btrfs_read_fs_root_no_name(fs_info, &key);
+	if (IS_ERR(local_root))
+		return PTR_ERR(local_root);
+
+	key.type = BTRFS_INODE_ITEM_KEY;
+	key.objectid = inum;
+	key.offset = 0;
+	inode = btrfs_iget(fs_info->sb, &key, local_root, NULL);
+	if (IS_ERR(inode))
+		return PTR_ERR(inode);
+
+	physical_for_dev_replace = nocow_ctx->physical_for_dev_replace;
+	len = nocow_ctx->len;
+	while (len >= PAGE_CACHE_SIZE) {
+		struct page *page = NULL;
+		int ret_sub;
+
+		index = offset >> PAGE_CACHE_SHIFT;
+
+		page = find_or_create_page(inode->i_mapping, index, GFP_NOFS);
+		if (!page) {
+			pr_err("find_or_create_page() failed\n");
+			ret = -ENOMEM;
+			goto next_page;
+		}
+
+		if (PageUptodate(page)) {
+			if (PageDirty(page))
+				goto next_page;
+		} else {
+			ClearPageError(page);
+			ret_sub = extent_read_full_page(&BTRFS_I(inode)->
+							 io_tree,
+							page, btrfs_get_extent,
+							nocow_ctx->mirror_num);
+			if (ret_sub) {
+				ret = ret_sub;
+				goto next_page;
+			}
+			wait_on_page_locked(page);
+			if (!PageUptodate(page)) {
+				ret = -EIO;
+				goto next_page;
+			}
+		}
+		ret_sub = write_page_nocow(nocow_ctx->sctx,
+					   physical_for_dev_replace, page);
+		if (ret_sub) {
+			ret = ret_sub;
+			goto next_page;
+		}
+
+next_page:
+		if (page) {
+			unlock_page(page);
+			put_page(page);
+		}
+		offset += PAGE_CACHE_SIZE;
+		physical_for_dev_replace += PAGE_CACHE_SIZE;
+		len -= PAGE_CACHE_SIZE;
+	}
+
+	if (inode)
+		iput(inode);
+	return ret;
+}
+
+static int write_page_nocow(struct scrub_ctx *sctx,
+			    u64 physical_for_dev_replace, struct page *page)
+{
+	struct bio *bio;
+	struct btrfs_device *dev;
+	int ret;
+	DECLARE_COMPLETION_ONSTACK(compl);
+
+	dev = sctx->wr_ctx.tgtdev;
+	if (!dev)
+		return -EIO;
+	if (!dev->bdev) {
+		printk_ratelimited(KERN_WARNING
+			"btrfs: scrub write_page_nocow(bdev == NULL) is unexpected!\n");
+		return -EIO;
+	}
+	bio = bio_alloc(GFP_NOFS, 1);
+	if (!bio) {
+		spin_lock(&sctx->stat_lock);
+		sctx->stat.malloc_errors++;
+		spin_unlock(&sctx->stat_lock);
+		return -ENOMEM;
+	}
+	bio->bi_private = &compl;
+	bio->bi_end_io = scrub_complete_bio_end_io;
+	bio->bi_size = 0;
+	bio->bi_sector = physical_for_dev_replace >> 9;
+	bio->bi_bdev = dev->bdev;
+	ret = bio_add_page(bio, page, PAGE_CACHE_SIZE, 0);
+	if (ret != PAGE_CACHE_SIZE) {
+leave_with_eio:
+		bio_put(bio);
+		btrfs_dev_stat_inc_and_print(dev, BTRFS_DEV_STAT_WRITE_ERRS);
+		return -EIO;
+	}
+	btrfsic_submit_bio(WRITE_SYNC, bio);
+	wait_for_completion(&compl);
+
+	if (!test_bit(BIO_UPTODATE, &bio->bi_flags))
+		goto leave_with_eio;
+
+	bio_put(bio);
+	return 0;
+}
diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 789c9b2..8ad5f15 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -1203,7 +1203,8 @@ static void btrfs_resize_thread_pool(struct btrfs_fs_info *fs_info,
 	btrfs_set_max_workers(&fs_info->endio_freespace_worker, new_pool_size);
 	btrfs_set_max_workers(&fs_info->delayed_workers, new_pool_size);
 	btrfs_set_max_workers(&fs_info->readahead_workers, new_pool_size);
-	btrfs_set_max_workers(&fs_info->scrub_workers, new_pool_size);
+	btrfs_set_max_workers(&fs_info->scrub_wr_completion_workers,
+			      new_pool_size);
 }
 
 static int btrfs_remount(struct super_block *sb, int *flags, char *data)
