Grow: report better message when --grow --chunk cannot work.

When changing the chunksize of an array, the new chunksize must
divide the device size.
If it doesn't we report a very brief message.
Make this message a bit longer and suggest a way forward be reducing
the size of the array.

Reported-by: Mark Knecht <markknecht@gmail.com>
Signed-off-by: NeilBrown <neilb@suse.de>

diff --git a/Grow.c b/Grow.c
index 35584cd907bdb515497d514d27e4b5b602711ed6..d939850c5e92b9a3c2d9ef6a26c0bbc69e4c2715 100644 (file)
--- a/Grow.c
+++ b/Grow.c
@@ -963,7 +963,7 @@ unsigned long compute_backup_blocks(int nchunk, int ochunk,
        return blocks;
 }
 
-char *analyse_change(struct mdinfo *info, struct reshape *re)
+char *analyse_change(char *devname, struct mdinfo *info, struct reshape *re)
 {
        /* Based on the current array state in info->array and
         * the changes in info->new_* etc, determine:
@@ -1004,9 +1004,16 @@ char *analyse_change(struct mdinfo *info, struct reshape *re)
                        /* chunk size is meaningful, must divide component_size
                         * evenly
                         */
-                       if (info->component_size % (info->new_chunk/512))
-                               return "New chunk size does not"
-                                       " divide component size";
+                       if (info->component_size % (info->new_chunk/512)) {
+                               unsigned long long shrink = info->component_size;
+                               shrink &= ~(unsigned long long)(info->new_chunk/512-1);
+                               pr_err("New chunk size (%dK) does not evenly divide device size (%lluk)\n",
+                                      info->new_chunk/1024, info->component_size/2);
+                               pr_err("After shrinking any filesystem, \"mdadm --grow %s --size %llu\"\n",
+                                      devname, shrink/2);
+                               pr_err("will shrink the array so the given chunk size would work.\n");
+                               return "";
+                       }
                        break;
                default:
                        return "chunk size not meaningful for this level";
@@ -2775,7 +2782,7 @@ static int reshape_array(char *container, int fd, char *devname,
                info->new_level = UnSet;
                if (info->delta_disks > 0)
                        info->array.raid_disks -= info->delta_disks;
-               msg = analyse_change(info, &reshape);
+               msg = analyse_change(devname, info, &reshape);
                info->new_level = new_level;
                if (info->delta_disks > 0)
                        info->array.raid_disks += info->delta_disks;
@@ -2783,9 +2790,11 @@ static int reshape_array(char *container, int fd, char *devname,
                        /* Make sure the array isn't read-only */
                        ioctl(fd, RESTART_ARRAY_RW, 0);
        } else
-               msg = analyse_change(info, &reshape);
+               msg = analyse_change(devname, info, &reshape);
        if (msg) {
-               pr_err("%s\n", msg);
+               /* if msg == "", error has already been printed */
+               if (msg[0])
+                       pr_err("%s\n", msg);
                goto release;
        }
        if (restart &&