md/raid5: allow each slot to have an extra replacement device

Just enhance data structures to record a second device per slot to be
used as a 'replacement' device, replacing the original.
We also have a second bio in each slot in each stripe_head.  This will
only be used when writing to the array - we need to write to both the
original and the replacement at the same time, so will need two bios.

For now, only try using the replacement drive for aligned-reads.
In this case, we prefer the replacement if it has been recovered far
enough, otherwise use the original.

This includes a small enhancement.  Previously we would only do
aligned reads if the target device was fully recovered.  Now we also
do them if it has recovered far enough.

Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: NeilBrown <neilb@suse.de>

1 files changed, 13 insertions, 2 deletions

diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index 6b9fc58e8f2..94bc35ba4c8 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -3594,6 +3594,7 @@ static int chunk_aligned_read(struct mddev *mddev, struct bio * raid_bio)
         int dd_idx;
         struct bio* align_bi;
         struct md_rdev *rdev;
+        sector_t end_sector;
 
         if (!in_chunk_boundary(mddev, raid_bio)) {
                 pr_debug("chunk_aligned_read : non aligned\n");
@@ -3618,9 +3619,19 @@ static int chunk_aligned_read(struct mddev *mddev, struct bio * raid_bio)
                                                     0,
                                                     &dd_idx, NULL);
 
+        end_sector = align_bi->bi_sector + (align_bi->bi_size >> 9);
         rcu_read_lock();
-        rdev = rcu_dereference(conf->disks[dd_idx].rdev);
-        if (rdev && test_bit(In_sync, &rdev->flags)) {
+        rdev = rcu_dereference(conf->disks[dd_idx].replacement);
+        if (!rdev || test_bit(Faulty, &rdev->flags) ||
+            rdev->recovery_offset < end_sector) {
+                rdev = rcu_dereference(conf->disks[dd_idx].rdev);
+                if (rdev &&
+                    (test_bit(Faulty, &rdev->flags) ||
+                    !(test_bit(In_sync, &rdev->flags) ||
+                      rdev->recovery_offset >= end_sector)))
+                        rdev = NULL;
+        }
+        if (rdev) {
                 sector_t first_bad;
                 int bad_sectors;