[PATCH] md: support write-mostly device in raid1

This allows a device in a raid1 to be marked as "write mostly".  Read requests
will only be sent if there is no other option.

Signed-off-by: Neil Brown <neilb@cse.unsw.edu.au>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

4 files changed, 82 insertions, 26 deletions

diff --git a/drivers/md/md.c b/drivers/md/md.c
index ae654466dc23..f1ac356e656d 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -670,6 +670,7 @@ static int super_90_validate(mddev_t *mddev, mdk_rdev_t *rdev)
 
         if (mddev->level != LEVEL_MULTIPATH) {
                 rdev->faulty = 0;
+                rdev->flags = 0;
                 desc = sb->disks + rdev->desc_nr;
 
                 if (desc->state & (1<<MD_DISK_FAULTY))
@@ -679,6 +680,8 @@ static int super_90_validate(mddev_t *mddev, mdk_rdev_t *rdev)
                         rdev->in_sync = 1;
                         rdev->raid_disk = desc->raid_disk;
                 }
+                if (desc->state & (1<<MD_DISK_WRITEMOSTLY))
+                        set_bit(WriteMostly, &rdev->flags);
         } else /* MULTIPATH are always insync */
                 rdev->in_sync = 1;
         return 0;
@@ -777,6 +780,8 @@ static void super_90_sync(mddev_t *mddev, mdk_rdev_t *rdev)
                         spare++;
                         working++;
                 }
+                if (test_bit(WriteMostly, &rdev2->flags))
+                        d->state |= (1<<MD_DISK_WRITEMOSTLY);
         }
         
         /* now set the "removed" and "faulty" bits on any missing devices */
@@ -990,6 +995,9 @@ static int super_1_validate(mddev_t *mddev, mdk_rdev_t *rdev)
                         rdev->raid_disk = role;
                         break;
                 }
+                rdev->flags = 0;
+                if (sb->devflags & WriteMostly1)
+                        set_bit(WriteMostly, &rdev->flags);
         } else /* MULTIPATH are always insync */
                 rdev->in_sync = 1;
 
@@ -2152,6 +2160,8 @@ static int get_disk_info(mddev_t * mddev, void __user * arg)
                         info.state |= (1<<MD_DISK_ACTIVE);
                         info.state |= (1<<MD_DISK_SYNC);
                 }
+                if (test_bit(WriteMostly, &rdev->flags))
+                        info.state |= (1<<MD_DISK_WRITEMOSTLY);
         } else {
                 info.major = info.minor = 0;
                 info.raid_disk = -1;
@@ -2237,6 +2247,9 @@ static int add_new_disk(mddev_t * mddev, mdu_disk_info_t *info)
                 rdev->saved_raid_disk = rdev->raid_disk;
 
                 rdev->in_sync = 0; /* just to be sure */
+                if (info->state & (1<<MD_DISK_WRITEMOSTLY))
+                        set_bit(WriteMostly, &rdev->flags);
+
                 rdev->raid_disk = -1;
                 err = bind_rdev_to_array(rdev, mddev);
                 if (err)
@@ -2277,6 +2290,9 @@ static int add_new_disk(mddev_t * mddev, mdu_disk_info_t *info)
                 else
                         rdev->in_sync = 0;
 
+                if (info->state & (1<<MD_DISK_WRITEMOSTLY))
+                        set_bit(WriteMostly, &rdev->flags);
+
                 err = bind_rdev_to_array(rdev, mddev);
                 if (err) {
                         export_rdev(rdev);
@@ -3329,6 +3345,8 @@ static int md_seq_show(struct seq_file *seq, void *v)
                         char b[BDEVNAME_SIZE];
                         seq_printf(seq, " %s[%d]",
                                 bdevname(rdev->bdev,b), rdev->desc_nr);
+                        if (test_bit(WriteMostly, &rdev->flags))
+                                seq_printf(seq, "(W)");
                         if (rdev->faulty) {
                                 seq_printf(seq, "(F)");
                                 continue;
diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c
index ba643e4bfac9..28839a8193f2 100644
--- a/drivers/md/raid1.c
+++ b/drivers/md/raid1.c
@@ -360,13 +360,14 @@ static int read_balance(conf_t *conf, r1bio_t *r1_bio)
 {
         const unsigned long this_sector = r1_bio->sector;
         int new_disk = conf->last_used, disk = new_disk;
+        int wonly_disk = -1;
         const int sectors = r1_bio->sectors;
         sector_t new_distance, current_distance;
-        mdk_rdev_t *new_rdev, *rdev;
+        mdk_rdev_t *rdev;
 
         rcu_read_lock();
         /*
-         * Check if it if we can balance. We can balance on the whole
+         * Check if we can balance. We can balance on the whole
          * device if no resync is going on, or below the resync window.
          * We take the first readable disk when above the resync window.
          */
@@ -376,11 +377,16 @@ static int read_balance(conf_t *conf, r1bio_t *r1_bio)
                 /* Choose the first operation device, for consistancy */
                 new_disk = 0;
 
-                while ((new_rdev=conf->mirrors[new_disk].rdev) == NULL ||
-                       !new_rdev->in_sync) {
-                        new_disk++;
-                        if (new_disk == conf->raid_disks) {
-                                new_disk = -1;
+                for (rdev = conf->mirrors[new_disk].rdev;
+                     !rdev || !rdev->in_sync
+                             || test_bit(WriteMostly, &rdev->flags);
+                     rdev = conf->mirrors[++new_disk].rdev) {
+
+                        if (rdev && rdev->in_sync)
+                                wonly_disk = new_disk;
+
+                        if (new_disk == conf->raid_disks - 1) {
+                                new_disk = wonly_disk;
                                 break;
                         }
                 }
@@ -389,16 +395,26 @@ static int read_balance(conf_t *conf, r1bio_t *r1_bio)
 
 
         /* make sure the disk is operational */
-        while ((new_rdev=conf->mirrors[new_disk].rdev) == NULL ||
-               !new_rdev->in_sync) {
+        for (rdev = conf->mirrors[new_disk].rdev;
+             !rdev || !rdev->in_sync ||
+                     test_bit(WriteMostly, &rdev->flags);
+             rdev = conf->mirrors[new_disk].rdev) {
+
+                if (rdev && rdev->in_sync)
+                        wonly_disk = new_disk;
+
                 if (new_disk <= 0)
                         new_disk = conf->raid_disks;
                 new_disk--;
                 if (new_disk == disk) {
-                        new_disk = -1;
-                        goto rb_out;
+                        new_disk = wonly_disk;
+                        break;
                 }
         }
+
+        if (new_disk < 0)
+                goto rb_out;
+
         disk = new_disk;
         /* now disk == new_disk == starting point for search */
 
@@ -419,37 +435,41 @@ static int read_balance(conf_t *conf, r1bio_t *r1_bio)
                         disk = conf->raid_disks;
                 disk--;
 
-                if ((rdev=conf->mirrors[disk].rdev) == NULL ||
-                    !rdev->in_sync)
+                rdev = conf->mirrors[disk].rdev;
+
+                if (!rdev ||
+                    !rdev->in_sync ||
+                    test_bit(WriteMostly, &rdev->flags))
                         continue;
 
                 if (!atomic_read(&rdev->nr_pending)) {
                         new_disk = disk;
-                        new_rdev = rdev;
                         break;
                 }
                 new_distance = abs(this_sector - conf->mirrors[disk].head_position);
                 if (new_distance < current_distance) {
                         current_distance = new_distance;
                         new_disk = disk;
-                        new_rdev = rdev;
                 }
         } while (disk != conf->last_used);
 
-rb_out:
+ rb_out:
 
 
         if (new_disk >= 0) {
-                conf->next_seq_sect = this_sector + sectors;
-                conf->last_used = new_disk;
-                atomic_inc(&new_rdev->nr_pending);
-                if (!new_rdev->in_sync) {
+                rdev = conf->mirrors[new_disk].rdev;
+                if (!rdev)
+                        goto retry;
+                atomic_inc(&rdev->nr_pending);
+                if (!rdev->in_sync) {
                         /* cannot risk returning a device that failed
                          * before we inc'ed nr_pending
                          */
-                        atomic_dec(&new_rdev->nr_pending);
+                        atomic_dec(&rdev->nr_pending);
                         goto retry;
                 }
+                conf->next_seq_sect = this_sector + sectors;
+                conf->last_used = new_disk;
         }
         rcu_read_unlock();
 
@@ -1109,6 +1129,7 @@ static sector_t sync_request(mddev_t *mddev, sector_t sector_nr, int *skipped, i
         sector_t max_sector, nr_sectors;
         int disk;
         int i;
+        int wonly;
         int write_targets = 0;
         int sync_blocks;
         int still_degraded = 0;
@@ -1164,14 +1185,21 @@ static sector_t sync_request(mddev_t *mddev, sector_t sector_nr, int *skipped, i
          */
         disk = conf->last_used;
         /* make sure disk is operational */
-
+        wonly = disk;
         while (conf->mirrors[disk].rdev == NULL ||
-               !conf->mirrors[disk].rdev->in_sync) {
+               !conf->mirrors[disk].rdev->in_sync ||
+               test_bit(WriteMostly, &conf->mirrors[disk].rdev->flags)
+                ) {
+                if (conf->mirrors[disk].rdev  &&
+                    conf->mirrors[disk].rdev->in_sync)
+                        wonly = disk;
                 if (disk <= 0)
                         disk = conf->raid_disks;
                 disk--;
-                if (disk == conf->last_used)
+                if (disk == conf->last_used) {
+                        disk = wonly;
                         break;
+                }
         }
         conf->last_used = disk;
         atomic_inc(&conf->mirrors[disk].rdev->nr_pending);
diff --git a/include/linux/raid/md_k.h b/include/linux/raid/md_k.h
index 817062bf7352..7ef78e15ce04 100644
--- a/include/linux/raid/md_k.h
+++ b/include/linux/raid/md_k.h
@@ -181,6 +181,9 @@ struct mdk_rdev_s
         int faulty;                     /* if faulty do not issue IO requests */
         int in_sync;                    /* device is a full member of the array */
 
+        unsigned long   flags;          /* Should include faulty and in_sync here. */
+#define WriteMostly     4               /* Avoid reading if at all possible */
+
         int desc_nr;                    /* descriptor index in the superblock */
         int raid_disk;                  /* role of device in array */
         int saved_raid_disk;            /* role that device used to have in the
diff --git a/include/linux/raid/md_p.h b/include/linux/raid/md_p.h
index dc65cd435494..4f047f84fb1f 100644
--- a/include/linux/raid/md_p.h
+++ b/include/linux/raid/md_p.h
@@ -79,6 +79,11 @@
 #define MD_DISK_SYNC            2 /* disk is in sync with the raid set */
 #define MD_DISK_REMOVED         3 /* disk is in sync with the raid set */
 
+#define MD_DISK_WRITEMOSTLY     9 /* disk is "write-mostly" is RAID1 config.
+                                   * read requests will only be sent here in
+                                   * dire need
+                                   */
+
 typedef struct mdp_device_descriptor_s {
         __u32 number;           /* 0 Device number in the entire set          */
         __u32 major;            /* 1 Device major number                      */
@@ -193,7 +198,7 @@ struct mdp_superblock_1 {
 
         __u64   ctime;          /* lo 40 bits are seconds, top 24 are microseconds or 0*/
         __u32   level;          /* -4 (multipath), -1 (linear), 0,1,4,5 */
-        __u32   layout;         /* only for raid5 currently */
+        __u32   layout;         /* only for raid5 and raid10 currently */
         __u64   size;           /* used size of component devices, in 512byte sectors */
 
         __u32   chunksize;      /* in 512byte sectors */
@@ -212,7 +217,9 @@ struct mdp_superblock_1 {
         __u32   dev_number;     /* permanent identifier of this  device - not role in raid */
         __u32   cnt_corrected_read; /* number of read errors that were corrected by re-writing */
         __u8    device_uuid[16]; /* user-space setable, ignored by kernel */
-        __u8    pad2[64-56];    /* set to 0 when writing */
+        __u8    devflags;       /* per-device flags.  Only one defined...*/
+#define WriteMostly1    1       /* mask for writemostly flag in above */
+        __u8    pad2[64-57];    /* set to 0 when writing */
 
         /* array state information - 64 bytes */
         __u64   utime;          /* 40 bits second, 24 btes microseconds */