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, 33 insertions, 24 deletions

diff --git a/drivers/md/raid5.h b/drivers/md/raid5.h
index e10c5531f9c5..43106f01862d 100644
--- a/drivers/md/raid5.h
+++ b/drivers/md/raid5.h
@@ -226,8 +226,11 @@ struct stripe_head {
                 #endif
         } ops;
         struct r5dev {
-                struct bio      req;
-                struct bio_vec  vec;
+                /* rreq and rvec are used for the replacement device when
+                 * writing data to both devices.
+                 */
+                struct bio      req, rreq;
+                struct bio_vec  vec, rvec;
                 struct page     *page;
                 struct bio      *toread, *read, *towrite, *written;
                 sector_t        sector;                 /* sector of this page */
@@ -252,29 +255,35 @@ struct stripe_head_state {
         int handle_bad_blocks;
 };
 
-/* Flags */
-#define R5_UPTODATE     0       /* page contains current data */
-#define R5_LOCKED       1       /* IO has been submitted on "req" */
-#define R5_OVERWRITE    2       /* towrite covers whole page */
+/* Flags for struct r5dev.flags */
+enum r5dev_flags {
+        R5_UPTODATE,    /* page contains current data */
+        R5_LOCKED,      /* IO has been submitted on "req" */
+        R5_OVERWRITE,   /* towrite covers whole page */
 /* and some that are internal to handle_stripe */
-#define R5_Insync       3       /* rdev && rdev->in_sync at start */
-#define R5_Wantread     4       /* want to schedule a read */
-#define R5_Wantwrite    5
-#define R5_Overlap      7       /* There is a pending overlapping request on this block */
-#define R5_ReadError    8       /* seen a read error here recently */
-#define R5_ReWrite      9       /* have tried to over-write the readerror */
+        R5_Insync,      /* rdev && rdev->in_sync at start */
+        R5_Wantread,    /* want to schedule a read */
+        R5_Wantwrite,
+        R5_Overlap,     /* There is a pending overlapping request
+                         * on this block */
+        R5_ReadError,   /* seen a read error here recently */
+        R5_ReWrite,     /* have tried to over-write the readerror */
 
-#define R5_Expanded     10      /* This block now has post-expand data */
-#define R5_Wantcompute  11      /* compute_block in progress treat as
-                                 * uptodate
-                                 */
-#define R5_Wantfill     12      /* dev->toread contains a bio that needs
-                                 * filling
-                                 */
-#define R5_Wantdrain    13      /* dev->towrite needs to be drained */
-#define R5_WantFUA      14      /* Write should be FUA */
-#define R5_WriteError   15      /* got a write error - need to record it */
-#define R5_MadeGood     16      /* A bad block has been fixed by writing to it*/
+        R5_Expanded,    /* This block now has post-expand data */
+        R5_Wantcompute, /* compute_block in progress treat as
+                         * uptodate
+                         */
+        R5_Wantfill,    /* dev->toread contains a bio that needs
+                         * filling
+                         */
+        R5_Wantdrain,   /* dev->towrite needs to be drained */
+        R5_WantFUA,     /* Write should be FUA */
+        R5_WriteError,  /* got a write error - need to record it */
+        R5_MadeGood,    /* A bad block has been fixed by writing to it */
+        R5_ReadRepl,    /* Will/did read from replacement rather than orig */
+        R5_MadeGoodRepl,/* A bad block on the replacement device has been
+                         * fixed by writing to it */
+};
 /*
  * Write method
  */
@@ -344,7 +353,7 @@ enum {
 
 
 struct disk_info {
-        struct md_rdev  *rdev;
+        struct md_rdev  *rdev, *replacement;
 };
 
 struct r5conf {