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Commit 67f45548 authored by NeilBrown's avatar NeilBrown
Browse files

md/raid56: Don't perform reads to support writes until stripe is ready. 



If it is found that we need to pre-read some blocks before a write
can succeed, we normally set STRIPE_DELAYED and don't actually perform
the read until STRIPE_PREREAD_ACTIVE subsequently gets set.

However for a degraded RAID6 we currently perform the reads as soon
as we see that a write is pending.  This significantly hurts
throughput.

So:
 - when handle_stripe_dirtying find a block that it wants on a device
   that is failed, set STRIPE_DELAY, instead of doing nothing, and
 - when fetch_block detects that a read might be required to satisfy a
   write, only perform the read if STRIPE_PREREAD_ACTIVE is set,
   and if we would actually need to read something to complete the write.

This also helps RAID5, though less often as RAID5 supports a
read-modify-write cycle.  For RAID5 the read is performed too early
only if the write is not a full 4K aligned write (i.e. no an
R5_OVERWRITE).

Also clean up a couple of horrible bits of formatting.

Reported-by: default avatarPatrik Horník <patrik@dsl.sk>
Signed-off-by: default avatarNeilBrown <neilb@suse.de>
parent bd8839e0

drivers/md/raid5.c

+18 −12
Original line number Diff line number Diff line
@@ -292,9 +292,12 @@ static void do_release_stripe(struct r5conf *conf, struct stripe_head *sh, 
	BUG_ON(atomic_read(&conf->active_stripes)==0);

	if (test_bit(STRIPE_HANDLE, &sh->state)) {

		if (test_bit(STRIPE_DELAYED, &sh->state) &&

		    !test_bit(STRIPE_PREREAD_ACTIVE, &sh->state))

		    !test_bit(STRIPE_PREREAD_ACTIVE, &sh->state)) {

			list_add_tail(&sh->lru, &conf->delayed_list);

		else if (test_bit(STRIPE_BIT_DELAY, &sh->state) &&

			if (atomic_read(&conf->preread_active_stripes)

			    < IO_THRESHOLD)

				md_wakeup_thread(conf->mddev->thread);

		} else if (test_bit(STRIPE_BIT_DELAY, &sh->state) &&

			   sh->bm_seq - conf->seq_write > 0)

			list_add_tail(&sh->lru, &conf->bitmap_list);

		else {
@@ -2886,8 +2889,11 @@ static int fetch_block(struct stripe_head *sh, struct stripe_head_state *s, 
	     (s->failed >= 1 && fdev[0]->toread) ||

	     (s->failed >= 2 && fdev[1]->toread) ||

	     (sh->raid_conf->level <= 5 && s->failed && fdev[0]->towrite &&

	      (!test_bit(R5_Insync, &dev->flags) || test_bit(STRIPE_PREREAD_ACTIVE, &sh->state)) &&

	      !test_bit(R5_OVERWRITE, &fdev[0]->flags)) ||

	     (sh->raid_conf->level == 6 && s->failed && s->to_write))) {

	     (sh->raid_conf->level == 6 && s->failed && s->to_write &&

	      s->to_write < sh->raid_conf->raid_disks - 2 &&

	      (!test_bit(R5_Insync, &dev->flags) || test_bit(STRIPE_PREREAD_ACTIVE, &sh->state))))) {

		/* we would like to get this block, possibly by computing it,

		 * otherwise read it if the backing disk is insync

		 */
@@ -3086,7 +3092,8 @@ static void handle_stripe_dirtying(struct r5conf *conf, 
		    !test_bit(R5_LOCKED, &dev->flags) &&

		    !(test_bit(R5_UPTODATE, &dev->flags) ||

		    test_bit(R5_Wantcompute, &dev->flags))) {

			if (test_bit(R5_Insync, &dev->flags)) rcw++;

			if (test_bit(R5_Insync, &dev->flags))

				rcw++;

			else

				rcw += 2*disks;

		}
@@ -3107,10 +3114,10 @@ static void handle_stripe_dirtying(struct r5conf *conf, 
			    !(test_bit(R5_UPTODATE, &dev->flags) ||

			    test_bit(R5_Wantcompute, &dev->flags)) &&

			    test_bit(R5_Insync, &dev->flags)) {

				if (

				  test_bit(STRIPE_PREREAD_ACTIVE, &sh->state)) {

					pr_debug("Read_old block "

						 "%d for r-m-w\n", i);

				if (test_bit(STRIPE_PREREAD_ACTIVE,

					     &sh->state)) {

					pr_debug("Read_old block %d for r-m-w\n",

						 i);

					set_bit(R5_LOCKED, &dev->flags);

					set_bit(R5_Wantread, &dev->flags);

					s->locked++;
@@ -3133,10 +3140,9 @@ static void handle_stripe_dirtying(struct r5conf *conf, 
			    !(test_bit(R5_UPTODATE, &dev->flags) ||

			      test_bit(R5_Wantcompute, &dev->flags))) {

				rcw++;

				if (!test_bit(R5_Insync, &dev->flags))

					continue; /* it's a failed drive */

				if (

				  test_bit(STRIPE_PREREAD_ACTIVE, &sh->state)) {

				if (test_bit(R5_Insync, &dev->flags) &&

				    test_bit(STRIPE_PREREAD_ACTIVE,

					     &sh->state)) {

					pr_debug("Read_old block "

						"%d for Reconstruct\n", i);

					set_bit(R5_LOCKED, &dev->flags);