diff --git a/fs/btrfs/Makefile b/fs/btrfs/Makefile
index e5fc3cf..9dcfc2f 100644
--- a/fs/btrfs/Makefile
+++ b/fs/btrfs/Makefile
@@ -6,7 +6,7 @@
 	   hash.o file-item.o inode-item.o inode-map.o disk-io.o \
 	   transaction.o bit-radix.o inode.o file.o tree-defrag.o \
 	   extent_map.o sysfs.o struct-funcs.o xattr.o ordered-data.o \
-	   extent_io.o volumes.o
+	   extent_io.o volumes.o async-thread.o
 
 btrfs-$(CONFIG_FS_POSIX_ACL)	+= acl.o
 else
diff --git a/fs/btrfs/async-thread.c b/fs/btrfs/async-thread.c
new file mode 100644
index 0000000..2911b67
--- /dev/null
+++ b/fs/btrfs/async-thread.c
@@ -0,0 +1,288 @@
+/*
+ * Copyright (C) 2007 Oracle.  All rights reserved.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public
+ * License v2 as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public
+ * License along with this program; if not, write to the
+ * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
+ * Boston, MA 021110-1307, USA.
+ */
+
+#include <linux/kthread.h>
+#include <linux/list.h>
+#include <linux/spinlock.h>
+#include <linux/freezer.h>
+#include "async-thread.h"
+
+/*
+ * container for the kthread task pointer and the list of pending work
+ * One of these is allocated per thread.
+ */
+struct btrfs_worker_thread {
+	/* list of struct btrfs_work that are waiting for service */
+	struct list_head pending;
+
+	/* list of worker threads from struct btrfs_workers */
+	struct list_head worker_list;
+
+	/* kthread */
+	struct task_struct *task;
+
+	/* number of things on the pending list */
+	atomic_t num_pending;
+
+	/* protects the pending list. */
+	spinlock_t lock;
+
+	/* set to non-zero when this thread is already awake and kicking */
+	int working;
+};
+
+/*
+ * main loop for servicing work items
+ */
+static int worker_loop(void *arg)
+{
+	struct btrfs_worker_thread *worker = arg;
+	struct list_head *cur;
+	struct btrfs_work *work;
+	do {
+		spin_lock_irq(&worker->lock);
+		while(!list_empty(&worker->pending)) {
+			cur = worker->pending.next;
+			work = list_entry(cur, struct btrfs_work, list);
+			list_del(&work->list);
+			clear_bit(0, &work->flags);
+
+			work->worker = worker;
+			spin_unlock_irq(&worker->lock);
+
+			work->func(work);
+
+			atomic_dec(&worker->num_pending);
+			spin_lock_irq(&worker->lock);
+		}
+		worker->working = 0;
+		if (freezing(current)) {
+			refrigerator();
+		} else {
+			set_current_state(TASK_INTERRUPTIBLE);
+			spin_unlock_irq(&worker->lock);
+			schedule();
+			__set_current_state(TASK_RUNNING);
+		}
+	} while (!kthread_should_stop());
+	return 0;
+}
+
+/*
+ * this will wait for all the worker threads to shutdown
+ */
+int btrfs_stop_workers(struct btrfs_workers *workers)
+{
+	struct list_head *cur;
+	struct btrfs_worker_thread *worker;
+
+	while(!list_empty(&workers->worker_list)) {
+		cur = workers->worker_list.next;
+		worker = list_entry(cur, struct btrfs_worker_thread,
+				    worker_list);
+		kthread_stop(worker->task);
+		list_del(&worker->worker_list);
+		kfree(worker);
+	}
+	return 0;
+}
+
+/*
+ * simple init on struct btrfs_workers
+ */
+void btrfs_init_workers(struct btrfs_workers *workers, int max)
+{
+	workers->num_workers = 0;
+	INIT_LIST_HEAD(&workers->worker_list);
+	workers->last = NULL;
+	spin_lock_init(&workers->lock);
+	workers->max_workers = max;
+}
+
+/*
+ * starts new worker threads.  This does not enforce the max worker
+ * count in case you need to temporarily go past it.
+ */
+int btrfs_start_workers(struct btrfs_workers *workers, int num_workers)
+{
+	struct btrfs_worker_thread *worker;
+	int ret = 0;
+	int i;
+
+	for (i = 0; i < num_workers; i++) {
+		worker = kzalloc(sizeof(*worker), GFP_NOFS);
+		if (!worker) {
+			ret = -ENOMEM;
+			goto fail;
+		}
+
+		INIT_LIST_HEAD(&worker->pending);
+		INIT_LIST_HEAD(&worker->worker_list);
+		spin_lock_init(&worker->lock);
+		atomic_set(&worker->num_pending, 0);
+		worker->task = kthread_run(worker_loop, worker, "btrfs");
+		if (IS_ERR(worker->task)) {
+			ret = PTR_ERR(worker->task);
+			goto fail;
+		}
+
+		spin_lock_irq(&workers->lock);
+		list_add_tail(&worker->worker_list, &workers->worker_list);
+		workers->last = worker;
+		workers->num_workers++;
+		spin_unlock_irq(&workers->lock);
+	}
+	return 0;
+fail:
+	btrfs_stop_workers(workers);
+	return ret;
+}
+
+/*
+ * run through the list and find a worker thread that doesn't have a lot
+ * to do right now.  This can return null if we aren't yet at the thread
+ * count limit and all of the threads are busy.
+ */
+static struct btrfs_worker_thread *next_worker(struct btrfs_workers *workers)
+{
+	struct btrfs_worker_thread *worker;
+	struct list_head *next;
+	struct list_head *start;
+	int enforce_min = workers->num_workers < workers->max_workers;
+
+	/* start with the last thread if it isn't busy */
+	worker = workers->last;
+	if (atomic_read(&worker->num_pending) < 64)
+		goto done;
+
+	next = worker->worker_list.next;
+	start = &worker->worker_list;
+
+	/*
+	 * check all the workers for someone that is bored.  FIXME, do
+	 * something smart here
+	 */
+	while(next != start) {
+		if (next == &workers->worker_list) {
+			next = workers->worker_list.next;
+			continue;
+		}
+		worker = list_entry(next, struct btrfs_worker_thread,
+				    worker_list);
+		if (atomic_read(&worker->num_pending) < 64 || !enforce_min)
+			goto done;
+		next = next->next;
+	}
+	/*
+	 * nobody was bored, if we're already at the max thread count,
+	 * use the last thread
+	 */
+	if (!enforce_min || atomic_read(&workers->last->num_pending) < 64) {
+		return workers->last;
+	}
+	return NULL;
+done:
+	workers->last = worker;
+	return worker;
+}
+
+static struct btrfs_worker_thread *find_worker(struct btrfs_workers *workers)
+{
+	struct btrfs_worker_thread *worker;
+	unsigned long flags;
+
+again:
+	spin_lock_irqsave(&workers->lock, flags);
+	worker = next_worker(workers);
+	spin_unlock_irqrestore(&workers->lock, flags);
+
+	if (!worker) {
+		spin_lock_irqsave(&workers->lock, flags);
+		if (workers->num_workers >= workers->max_workers) {
+			/*
+			 * we have failed to find any workers, just
+			 * return the force one
+			 */
+			worker = list_entry(workers->worker_list.next,
+				  struct btrfs_worker_thread, worker_list);
+			spin_unlock_irqrestore(&workers->lock, flags);
+		} else {
+			spin_unlock_irqrestore(&workers->lock, flags);
+			/* we're below the limit, start another worker */
+			btrfs_start_workers(workers, 1);
+			goto again;
+		}
+	}
+	return worker;
+}
+
+/*
+ * btrfs_requeue_work just puts the work item back on the tail of the list
+ * it was taken from.  It is intended for use with long running work functions
+ * that make some progress and want to give the cpu up for others.
+ */
+int btrfs_requeue_work(struct btrfs_work *work)
+{
+	struct btrfs_worker_thread *worker = work->worker;
+	unsigned long flags;
+
+	if (test_and_set_bit(0, &work->flags))
+		goto out;
+
+	spin_lock_irqsave(&worker->lock, flags);
+	atomic_inc(&worker->num_pending);
+	list_add_tail(&work->list, &worker->pending);
+	spin_unlock_irqrestore(&worker->lock, flags);
+out:
+	return 0;
+}
+
+/*
+ * places a struct btrfs_work into the pending queue of one of the kthreads
+ */
+int btrfs_queue_worker(struct btrfs_workers *workers, struct btrfs_work *work)
+{
+	struct btrfs_worker_thread *worker;
+	unsigned long flags;
+	int wake = 0;
+
+	/* don't requeue something already on a list */
+	if (test_and_set_bit(0, &work->flags))
+		goto out;
+
+	worker = find_worker(workers);
+
+	spin_lock_irqsave(&worker->lock, flags);
+	atomic_inc(&worker->num_pending);
+	list_add_tail(&work->list, &worker->pending);
+
+	/*
+	 * avoid calling into wake_up_process if this thread has already
+	 * been kicked
+	 */
+	if (!worker->working)
+		wake = 1;
+	worker->working = 1;
+
+	spin_unlock_irqrestore(&worker->lock, flags);
+
+	if (wake)
+		wake_up_process(worker->task);
+out:
+	return 0;
+}
diff --git a/fs/btrfs/async-thread.h b/fs/btrfs/async-thread.h
new file mode 100644
index 0000000..52fc9da
--- /dev/null
+++ b/fs/btrfs/async-thread.h
@@ -0,0 +1,78 @@
+/*
+ * Copyright (C) 2007 Oracle.  All rights reserved.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public
+ * License v2 as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public
+ * License along with this program; if not, write to the
+ * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
+ * Boston, MA 021110-1307, USA.
+ */
+
+#ifndef __BTRFS_ASYNC_THREAD_
+#define __BTRFS_ASYNC_THREAD_
+
+struct btrfs_worker_thread;
+
+/*
+ * This is similar to a workqueue, but it is meant to spread the operations
+ * across all available cpus instead of just the CPU that was used to
+ * queue the work.  There is also some batching introduced to try and
+ * cut down on context switches.
+ *
+ * By default threads are added on demand up to 2 * the number of cpus.
+ * Changing struct btrfs_workers->max_workers is one way to prevent
+ * demand creation of kthreads.
+ *
+ * the basic model of these worker threads is to embed a btrfs_work
+ * structure in your own data struct, and use container_of in a
+ * work function to get back to your data struct.
+ */
+struct btrfs_work {
+	/*
+	 * only func should be set to the function you want called
+	 * your work struct is passed as the only arg
+	 */
+	void (*func)(struct btrfs_work *work);
+
+	/*
+	 * flags should be set to zero.  It is used to make sure the
+	 * struct is only inserted once into the list.
+	 */
+	unsigned long flags;
+
+	/* don't touch these */
+	struct btrfs_worker_thread *worker;
+	struct list_head list;
+};
+
+struct btrfs_workers {
+	/* current number of running workers */
+	int num_workers;
+
+	/* max number of workers allowed.  changed by btrfs_start_workers */
+	int max_workers;
+
+	/* list with all the work threads */
+	struct list_head worker_list;
+
+	/* the last worker thread to have something queued */
+	struct btrfs_worker_thread *last;
+
+	/* lock for finding the next worker thread to queue on */
+	spinlock_t lock;
+};
+
+int btrfs_queue_worker(struct btrfs_workers *workers, struct btrfs_work *work);
+int btrfs_start_workers(struct btrfs_workers *workers, int num_workers);
+int btrfs_stop_workers(struct btrfs_workers *workers);
+void btrfs_init_workers(struct btrfs_workers *workers, int max);
+int btrfs_requeue_work(struct btrfs_work *work);
+#endif
diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index 49cbc62..6c91a51 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -30,6 +30,7 @@
 #include "bit-radix.h"
 #include "extent_io.h"
 #include "extent_map.h"
+#include "async-thread.h"
 
 struct btrfs_trans_handle;
 struct btrfs_transaction;
@@ -518,13 +519,20 @@
 	struct list_head hashers;
 	struct list_head dead_roots;
 	struct list_head end_io_work_list;
-	struct list_head async_submit_work_list;
 	struct work_struct end_io_work;
-	struct work_struct async_submit_work;
 	spinlock_t end_io_work_lock;
-	spinlock_t async_submit_work_lock;
 	atomic_t nr_async_submits;
 
+	/*
+	 * there is a pool of worker threads for checksumming during writes
+	 * and a pool for checksumming after reads.  This is because readers
+	 * can run with FS locks held, and the writers may be waiting for
+	 * those locks.  We don't want ordering in the pending list to cause
+	 * deadlocks, and so the two are serviced separately.
+	 */
+	struct btrfs_workers workers;
+	struct btrfs_workers endio_workers;
+
 #if LINUX_VERSION_CODE <= KERNEL_VERSION(2,6,18)
 	struct work_struct trans_work;
 #else
diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index b9a5364..98ff4fb 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -31,6 +31,7 @@
 #include "btrfs_inode.h"
 #include "volumes.h"
 #include "print-tree.h"
+#include "async-thread.h"
 
 #if 0
 static int check_tree_block(struct btrfs_root *root, struct extent_buffer *buf)
@@ -46,8 +47,7 @@
 #endif
 
 static struct extent_io_ops btree_extent_io_ops;
-static struct workqueue_struct *end_io_workqueue;
-static struct workqueue_struct *async_submit_workqueue;
+static void end_workqueue_fn(struct btrfs_work *work);
 
 struct end_io_wq {
 	struct bio *bio;
@@ -57,6 +57,7 @@
 	int error;
 	int metadata;
 	struct list_head list;
+	struct btrfs_work work;
 };
 
 struct async_submit_bio {
@@ -66,6 +67,7 @@
 	extent_submit_bio_hook_t *submit_bio_hook;
 	int rw;
 	int mirror_num;
+	struct btrfs_work work;
 };
 
 struct extent_map *btree_get_extent(struct inode *inode, struct page *page,
@@ -389,7 +391,6 @@
 {
 	struct end_io_wq *end_io_wq = bio->bi_private;
 	struct btrfs_fs_info *fs_info;
-	unsigned long flags;
 
 #if LINUX_VERSION_CODE <= KERNEL_VERSION(2,6,23)
 	if (bio->bi_size)
@@ -397,11 +398,10 @@
 #endif
 
 	fs_info = end_io_wq->info;
-	spin_lock_irqsave(&fs_info->end_io_work_lock, flags);
 	end_io_wq->error = err;
-	list_add_tail(&end_io_wq->list, &fs_info->end_io_work_list);
-	spin_unlock_irqrestore(&fs_info->end_io_work_lock, flags);
-	queue_work(end_io_workqueue, &fs_info->end_io_work);
+	end_io_wq->work.func = end_workqueue_fn;
+	end_io_wq->work.flags = 0;
+	btrfs_queue_worker(&fs_info->endio_workers, &end_io_wq->work);
 
 #if LINUX_VERSION_CODE <= KERNEL_VERSION(2,6,23)
 	return 0;
@@ -428,6 +428,19 @@
 	return 0;
 }
 
+static void run_one_async_submit(struct btrfs_work *work)
+{
+	struct btrfs_fs_info *fs_info;
+	struct async_submit_bio *async;
+
+	async = container_of(work, struct  async_submit_bio, work);
+	fs_info = BTRFS_I(async->inode)->root->fs_info;
+	atomic_dec(&fs_info->nr_async_submits);
+	async->submit_bio_hook(async->inode, async->rw, async->bio,
+			       async->mirror_num);
+	kfree(async);
+}
+
 int btrfs_wq_submit_bio(struct btrfs_fs_info *fs_info, struct inode *inode,
 			int rw, struct bio *bio, int mirror_num,
 			extent_submit_bio_hook_t *submit_bio_hook)
@@ -443,13 +456,10 @@
 	async->bio = bio;
 	async->mirror_num = mirror_num;
 	async->submit_bio_hook = submit_bio_hook;
-
-	spin_lock(&fs_info->async_submit_work_lock);
-	list_add_tail(&async->list, &fs_info->async_submit_work_list);
+	async->work.func = run_one_async_submit;
+	async->work.flags = 0;
 	atomic_inc(&fs_info->nr_async_submits);
-	spin_unlock(&fs_info->async_submit_work_lock);
-
-	queue_work(async_submit_workqueue, &fs_info->async_submit_work);
+	btrfs_queue_worker(&fs_info->workers, &async->work);
 	return 0;
 }
 
@@ -462,19 +472,32 @@
 
 	offset = bio->bi_sector << 9;
 
+	/*
+	 * when we're called for a write, we're already in the async
+	 * submission context.  Just jump ingo btrfs_map_bio
+	 */
 	if (rw & (1 << BIO_RW)) {
-		return btrfs_map_bio(BTRFS_I(inode)->root, rw, bio, mirror_num);
+		return btrfs_map_bio(BTRFS_I(inode)->root, rw, bio,
+				     mirror_num, 0);
 	}
 
+	/*
+	 * called for a read, do the setup so that checksum validation
+	 * can happen in the async kernel threads
+	 */
 	ret = btrfs_bio_wq_end_io(root->fs_info, bio, 1);
 	BUG_ON(ret);
 
-	return btrfs_map_bio(BTRFS_I(inode)->root, rw, bio, mirror_num);
+	return btrfs_map_bio(BTRFS_I(inode)->root, rw, bio, mirror_num, 1);
 }
 
 static int btree_submit_bio_hook(struct inode *inode, int rw, struct bio *bio,
 				 int mirror_num)
 {
+	/*
+	 * kthread helpers are used to submit writes so that checksumming
+	 * can happen in parallel across all CPUs
+	 */
 	if (!(rw & (1 << BIO_RW))) {
 		return __btree_submit_bio_hook(inode, rw, bio, mirror_num);
 	}
@@ -1036,95 +1059,40 @@
 	return ret;
 }
 
-#if LINUX_VERSION_CODE <= KERNEL_VERSION(2,6,18)
-static void btrfs_end_io_csum(void *p)
-#else
-static void btrfs_end_io_csum(struct work_struct *work)
-#endif
+/*
+ * called by the kthread helper functions to finally call the bio end_io
+ * functions.  This is where read checksum verification actually happens
+ */
+static void end_workqueue_fn(struct btrfs_work *work)
 {
-#if LINUX_VERSION_CODE <= KERNEL_VERSION(2,6,18)
-	struct btrfs_fs_info *fs_info = p;
-#else
-	struct btrfs_fs_info *fs_info = container_of(work,
-						     struct btrfs_fs_info,
-						     end_io_work);
-#endif
-	unsigned long flags;
-	struct end_io_wq *end_io_wq;
 	struct bio *bio;
-	struct list_head *next;
+	struct end_io_wq *end_io_wq;
+	struct btrfs_fs_info *fs_info;
 	int error;
-	int was_empty;
 
-	while(1) {
-		spin_lock_irqsave(&fs_info->end_io_work_lock, flags);
-		if (list_empty(&fs_info->end_io_work_list)) {
-			spin_unlock_irqrestore(&fs_info->end_io_work_lock,
-					       flags);
-			return;
-		}
-		next = fs_info->end_io_work_list.next;
-		list_del(next);
-		spin_unlock_irqrestore(&fs_info->end_io_work_lock, flags);
+	end_io_wq = container_of(work, struct end_io_wq, work);
+	bio = end_io_wq->bio;
+	fs_info = end_io_wq->info;
 
-		end_io_wq = list_entry(next, struct end_io_wq, list);
-
-		bio = end_io_wq->bio;
-		if (end_io_wq->metadata && !bio_ready_for_csum(bio)) {
-			spin_lock_irqsave(&fs_info->end_io_work_lock, flags);
-			was_empty = list_empty(&fs_info->end_io_work_list);
-			list_add_tail(&end_io_wq->list,
-				      &fs_info->end_io_work_list);
-			spin_unlock_irqrestore(&fs_info->end_io_work_lock,
-					       flags);
-			if (was_empty)
-				return;
-			continue;
-		}
-		error = end_io_wq->error;
-		bio->bi_private = end_io_wq->private;
-		bio->bi_end_io = end_io_wq->end_io;
-		kfree(end_io_wq);
+	/* metadata bios are special because the whole tree block must
+	 * be checksummed at once.  This makes sure the entire block is in
+	 * ram and up to date before trying to verify things.  For
+	 * blocksize <= pagesize, it is basically a noop
+	 */
+	if (end_io_wq->metadata && !bio_ready_for_csum(bio)) {
+		btrfs_queue_worker(&fs_info->endio_workers,
+				   &end_io_wq->work);
+		return;
+	}
+	error = end_io_wq->error;
+	bio->bi_private = end_io_wq->private;
+	bio->bi_end_io = end_io_wq->end_io;
+	kfree(end_io_wq);
 #if LINUX_VERSION_CODE <= KERNEL_VERSION(2,6,23)
-		bio_endio(bio, bio->bi_size, error);
+	bio_endio(bio, bio->bi_size, error);
 #else
-		bio_endio(bio, error);
+	bio_endio(bio, error);
 #endif
-	}
-}
-
-#if LINUX_VERSION_CODE <= KERNEL_VERSION(2,6,18)
-static void btrfs_async_submit_work(void *p)
-#else
-static void btrfs_async_submit_work(struct work_struct *work)
-#endif
-{
-#if LINUX_VERSION_CODE <= KERNEL_VERSION(2,6,18)
-	struct btrfs_fs_info *fs_info = p;
-#else
-	struct btrfs_fs_info *fs_info = container_of(work,
-						     struct btrfs_fs_info,
-						     async_submit_work);
-#endif
-	struct async_submit_bio *async;
-	struct list_head *next;
-
-	while(1) {
-		spin_lock(&fs_info->async_submit_work_lock);
-		if (list_empty(&fs_info->async_submit_work_list)) {
-			spin_unlock(&fs_info->async_submit_work_lock);
-			return;
-		}
-		next = fs_info->async_submit_work_list.next;
-		list_del(next);
-		atomic_dec(&fs_info->nr_async_submits);
-		spin_unlock(&fs_info->async_submit_work_lock);
-
-		async = list_entry(next, struct async_submit_bio, list);
-		async->submit_bio_hook(async->inode, async->rw, async->bio,
-				       async->mirror_num);
-		kfree(async);
-	}
 }
 
 struct btrfs_root *open_ctree(struct super_block *sb,
@@ -1155,19 +1123,11 @@
 		err = -ENOMEM;
 		goto fail;
 	}
-	end_io_workqueue = create_workqueue("btrfs-end-io");
-	BUG_ON(!end_io_workqueue);
-	async_submit_workqueue = create_workqueue("btrfs-async-submit");
-
 	INIT_RADIX_TREE(&fs_info->fs_roots_radix, GFP_NOFS);
 	INIT_LIST_HEAD(&fs_info->trans_list);
 	INIT_LIST_HEAD(&fs_info->dead_roots);
 	INIT_LIST_HEAD(&fs_info->hashers);
-	INIT_LIST_HEAD(&fs_info->end_io_work_list);
-	INIT_LIST_HEAD(&fs_info->async_submit_work_list);
 	spin_lock_init(&fs_info->hash_lock);
-	spin_lock_init(&fs_info->end_io_work_lock);
-	spin_lock_init(&fs_info->async_submit_work_lock);
 	spin_lock_init(&fs_info->delalloc_lock);
 	spin_lock_init(&fs_info->new_trans_lock);
 
@@ -1222,13 +1182,8 @@
 	fs_info->do_barriers = 1;
 
 #if LINUX_VERSION_CODE <= KERNEL_VERSION(2,6,18)
-	INIT_WORK(&fs_info->end_io_work, btrfs_end_io_csum, fs_info);
-	INIT_WORK(&fs_info->async_submit_work, btrfs_async_submit_work,
-		  fs_info);
 	INIT_WORK(&fs_info->trans_work, btrfs_transaction_cleaner, fs_info);
 #else
-	INIT_WORK(&fs_info->end_io_work, btrfs_end_io_csum);
-	INIT_WORK(&fs_info->async_submit_work, btrfs_async_submit_work);
 	INIT_DELAYED_WORK(&fs_info->trans_work, btrfs_transaction_cleaner);
 #endif
 	BTRFS_I(fs_info->btree_inode)->root = tree_root;
@@ -1240,6 +1195,19 @@
 	mutex_init(&fs_info->trans_mutex);
 	mutex_init(&fs_info->fs_mutex);
 
+	/* we need to start all the end_io workers up front because the
+	 * queue work function gets called at interrupt time.  The endio
+	 * workers don't normally start IO, so some number of them <= the
+	 * number of cpus is fine.  They handle checksumming after a read.
+	 *
+	 * The other worker threads do start IO, so the max is larger than
+	 * the number of CPUs.  FIXME, tune this for huge machines
+	 */
+	btrfs_init_workers(&fs_info->workers, num_online_cpus() * 2);
+	btrfs_init_workers(&fs_info->endio_workers, num_online_cpus());
+	btrfs_start_workers(&fs_info->workers, 1);
+	btrfs_start_workers(&fs_info->endio_workers, num_online_cpus());
+
 #if 0
 	ret = add_hasher(fs_info, "crc32c");
 	if (ret) {
@@ -1375,6 +1343,8 @@
 	extent_io_tree_empty_lru(&BTRFS_I(fs_info->btree_inode)->io_tree);
 fail_iput:
 	iput(fs_info->btree_inode);
+	btrfs_stop_workers(&fs_info->workers);
+	btrfs_stop_workers(&fs_info->endio_workers);
 fail:
 	btrfs_close_devices(fs_info->fs_devices);
 	btrfs_mapping_tree_free(&fs_info->mapping_tree);
@@ -1623,16 +1593,10 @@
 	extent_io_tree_empty_lru(&fs_info->extent_ins);
 	extent_io_tree_empty_lru(&BTRFS_I(fs_info->btree_inode)->io_tree);
 
-	flush_workqueue(async_submit_workqueue);
-	flush_workqueue(end_io_workqueue);
-
 	truncate_inode_pages(fs_info->btree_inode->i_mapping, 0);
 
-	flush_workqueue(async_submit_workqueue);
-	destroy_workqueue(async_submit_workqueue);
-
-	flush_workqueue(end_io_workqueue);
-	destroy_workqueue(end_io_workqueue);
+	btrfs_stop_workers(&fs_info->workers);
+	btrfs_stop_workers(&fs_info->endio_workers);
 
 	iput(fs_info->btree_inode);
 #if 0
diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 0f14697b..7daef8d 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -359,7 +359,7 @@
 
 	kfree(sums);
 
-	return btrfs_map_bio(root, rw, bio, mirror_num);
+	return btrfs_map_bio(root, rw, bio, mirror_num, 1);
 }
 
 int btrfs_submit_bio_hook(struct inode *inode, int rw, struct bio *bio,
@@ -383,7 +383,7 @@
 				   inode, rw, bio, mirror_num,
 				   __btrfs_submit_bio_hook);
 mapit:
-	return btrfs_map_bio(root, rw, bio, mirror_num);
+	return btrfs_map_bio(root, rw, bio, mirror_num, 0);
 }
 
 int btrfs_readpage_io_hook(struct page *page, u64 start, u64 end)
diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 722eb45..c57458c 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -27,6 +27,7 @@
 #include "transaction.h"
 #include "print-tree.h"
 #include "volumes.h"
+#include "async-thread.h"
 
 struct map_lookup {
 	u64 type;
@@ -110,6 +111,101 @@
 	return NULL;
 }
 
+/*
+ * we try to collect pending bios for a device so we don't get a large
+ * number of procs sending bios down to the same device.  This greatly
+ * improves the schedulers ability to collect and merge the bios.
+ *
+ * But, it also turns into a long list of bios to process and that is sure
+ * to eventually make the worker thread block.  The solution here is to
+ * make some progress and then put this work struct back at the end of
+ * the list if the block device is congested.  This way, multiple devices
+ * can make progress from a single worker thread.
+ */
+int run_scheduled_bios(struct btrfs_device *device)
+{
+	struct bio *pending;
+	struct backing_dev_info *bdi;
+	struct bio *tail;
+	struct bio *cur;
+	int again = 0;
+	unsigned long num_run = 0;
+
+	bdi = device->bdev->bd_inode->i_mapping->backing_dev_info;
+loop:
+	spin_lock(&device->io_lock);
+
+	/* take all the bios off the list at once and process them
+	 * later on (without the lock held).  But, remember the
+	 * tail and other pointers so the bios can be properly reinserted
+	 * into the list if we hit congestion
+	 */
+	pending = device->pending_bios;
+	tail = device->pending_bio_tail;
+	WARN_ON(pending && !tail);
+	device->pending_bios = NULL;
+	device->pending_bio_tail = NULL;
+
+	/*
+	 * if pending was null this time around, no bios need processing
+	 * at all and we can stop.  Otherwise it'll loop back up again
+	 * and do an additional check so no bios are missed.
+	 *
+	 * device->running_pending is used to synchronize with the
+	 * schedule_bio code.
+	 */
+	if (pending) {
+		again = 1;
+		device->running_pending = 1;
+	} else {
+		again = 0;
+		device->running_pending = 0;
+	}
+	spin_unlock(&device->io_lock);
+
+	while(pending) {
+		cur = pending;
+		pending = pending->bi_next;
+		cur->bi_next = NULL;
+		atomic_dec(&device->dev_root->fs_info->nr_async_submits);
+		submit_bio(cur->bi_rw, cur);
+		num_run++;
+
+		/*
+		 * we made progress, there is more work to do and the bdi
+		 * is now congested.  Back off and let other work structs
+		 * run instead
+		 */
+		if (pending && num_run && bdi_write_congested(bdi)) {
+			struct bio *old_head;
+
+			spin_lock(&device->io_lock);
+			old_head = device->pending_bios;
+			device->pending_bios = pending;
+			if (device->pending_bio_tail)
+				tail->bi_next = old_head;
+			else
+				device->pending_bio_tail = tail;
+
+			spin_unlock(&device->io_lock);
+			btrfs_requeue_work(&device->work);
+			goto done;
+		}
+	}
+	if (again)
+		goto loop;
+done:
+	return 0;
+}
+
+void pending_bios_fn(struct btrfs_work *work)
+{
+	struct btrfs_device *device;
+
+	device = container_of(work, struct btrfs_device, work);
+	run_scheduled_bios(device);
+}
+
 static int device_list_add(const char *path,
 			   struct btrfs_super_block *disk_super,
 			   u64 devid, struct btrfs_fs_devices **fs_devices_ret)
@@ -141,6 +237,7 @@
 			return -ENOMEM;
 		}
 		device->devid = devid;
+		device->work.func = pending_bios_fn;
 		memcpy(device->uuid, disk_super->dev_item.uuid,
 		       BTRFS_UUID_SIZE);
 		device->barriers = 1;
@@ -925,6 +1022,7 @@
 	}
 
 	device->barriers = 1;
+	device->work.func = pending_bios_fn;
 	generate_random_uuid(device->uuid);
 	spin_lock_init(&device->io_lock);
 	device->name = kstrdup(device_path, GFP_NOFS);
@@ -1965,8 +2063,61 @@
 #endif
 }
 
+struct async_sched {
+	struct bio *bio;
+	int rw;
+	struct btrfs_fs_info *info;
+	struct btrfs_work work;
+};
+
+/*
+ * see run_scheduled_bios for a description of why bios are collected for
+ * async submit.
+ *
+ * This will add one bio to the pending list for a device and make sure
+ * the work struct is scheduled.
+ */
+int schedule_bio(struct btrfs_root *root, struct btrfs_device *device,
+		 int rw, struct bio *bio)
+{
+	int should_queue = 1;
+
+	/* don't bother with additional async steps for reads, right now */
+	if (!(rw & (1 << BIO_RW))) {
+		submit_bio(rw, bio);
+		return 0;
+	}
+
+	/*
+	 * nr_async_sumbits allows us to reliably return congestion to the
+	 * higher layers.  Otherwise, the async bio makes it appear we have
+	 * made progress against dirty pages when we've really just put it
+	 * on a queue for later
+	 */
+	atomic_inc(&root->fs_info->nr_async_submits);
+	bio->bi_next = NULL;
+	bio->bi_rw |= rw;
+
+	spin_lock(&device->io_lock);
+
+	if (device->pending_bio_tail)
+		device->pending_bio_tail->bi_next = bio;
+
+	device->pending_bio_tail = bio;
+	if (!device->pending_bios)
+		device->pending_bios = bio;
+	if (device->running_pending)
+		should_queue = 0;
+
+	spin_unlock(&device->io_lock);
+
+	if (should_queue)
+		btrfs_queue_worker(&root->fs_info->workers, &device->work);
+	return 0;
+}
+
 int btrfs_map_bio(struct btrfs_root *root, int rw, struct bio *bio,
-		  int mirror_num)
+		  int mirror_num, int async_submit)
 {
 	struct btrfs_mapping_tree *map_tree;
 	struct btrfs_device *dev;
@@ -2012,10 +2163,10 @@
 		dev = multi->stripes[dev_nr].dev;
 		if (dev && dev->bdev) {
 			bio->bi_bdev = dev->bdev;
-			spin_lock(&dev->io_lock);
-			dev->total_ios++;
-			spin_unlock(&dev->io_lock);
-			submit_bio(rw, bio);
+			if (async_submit)
+				schedule_bio(root, dev, rw, bio);
+			else
+				submit_bio(rw, bio);
 		} else {
 			bio->bi_bdev = root->fs_info->fs_devices->latest_bdev;
 			bio->bi_sector = logical >> 9;
@@ -2054,6 +2205,7 @@
 	device->barriers = 1;
 	device->dev_root = root->fs_info->dev_root;
 	device->devid = devid;
+	device->work.func = pending_bios_fn;
 	fs_devices->num_devices++;
 	spin_lock_init(&device->io_lock);
 	memcpy(device->uuid, dev_uuid, BTRFS_UUID_SIZE);
diff --git a/fs/btrfs/volumes.h b/fs/btrfs/volumes.h
index 4df6b16..48a44f7 100644
--- a/fs/btrfs/volumes.h
+++ b/fs/btrfs/volumes.h
@@ -20,6 +20,7 @@
 #define __BTRFS_VOLUMES_
 
 #include <linux/bio.h>
+#include "async-thread.h"
 
 struct buffer_head;
 struct btrfs_device {
@@ -27,6 +28,9 @@
 	struct list_head dev_alloc_list;
 	struct btrfs_root *dev_root;
 	struct buffer_head *pending_io;
+	struct bio *pending_bios;
+	struct bio *pending_bio_tail;
+	int running_pending;
 	u64 generation;
 
 	int barriers;
@@ -36,8 +40,6 @@
 
 	struct block_device *bdev;
 
-	u64 total_ios;
-
 	char *name;
 
 	/* the internal btrfs device id */
@@ -63,6 +65,8 @@
 
 	/* physical drive uuid (or lvm uuid) */
 	u8 uuid[BTRFS_UUID_SIZE];
+
+	struct btrfs_work work;
 };
 
 struct btrfs_fs_devices {
@@ -117,7 +121,7 @@
 void btrfs_mapping_init(struct btrfs_mapping_tree *tree);
 void btrfs_mapping_tree_free(struct btrfs_mapping_tree *tree);
 int btrfs_map_bio(struct btrfs_root *root, int rw, struct bio *bio,
-		  int mirror_num);
+		  int mirror_num, int async_submit);
 int btrfs_read_super_device(struct btrfs_root *root, struct extent_buffer *buf);
 int btrfs_open_devices(struct btrfs_fs_devices *fs_devices,
 		       int flags, void *holder);