See patch to md.txt for more details
Signed-off-by: Neil Brown <neilb@xxxxxxx>
### Diffstat output
./Documentation/md.txt | 24 ++++++++++++++++++++++++
./drivers/md/md.c | 4 ++++
./drivers/md/raid5.c | 15 +++++++++++----
./drivers/md/raid6main.c | 13 +++++++++----
./include/linux/raid/md_k.h | 1 +
5 files changed, 49 insertions(+), 8 deletions(-)
diff ./Documentation/md.txt~current~ ./Documentation/md.txt
--- ./Documentation/md.txt~current~ 2005-11-28 10:08:19.000000000 +1100
+++ ./Documentation/md.txt 2005-11-28 10:12:33.000000000 +1100
@@ -51,6 +51,30 @@ superblock can be autodetected and run a
The kernel parameter "raid=partitionable" (or "raid=part") means
that all auto-detected arrays are assembled as partitionable.
+Boot time assembly of degraded/dirty arrays
+-------------------------------------------
+
+If a raid5 or raid6 array is both dirty and degraded, it could have
+undetectable data corruption. This is because the fact that it is
+'dirty' means that the parity cannot be trusted, and the fact that it
+is degraded means that some datablocks are missing and cannot reliably
+be reconstructed (due to no parity).
+
+For this reason, md will normally refuse to start such an array. This
+requires the sysadmin to take action to explicitly start the array
+desipite possible corruption. This is normally done with
+ mdadm --assemble --force ....
+
+This option is not really available if the array has the root
+filesystem on it. In order to support this booting from such an
+array, md supports a module parameter "start_dirty_degraded" which,
+when set to 1, bypassed the checks and will allows dirty degraded
+arrays to be started.
+
+So, to boot with a root filesystem of a dirty degraded raid[56], use
+
+ md-mod.start_dirty_degraded=1
+
Superblock formats
------------------
diff ./drivers/md/md.c~current~ ./drivers/md/md.c
--- ./drivers/md/md.c~current~ 2005-11-28 10:10:54.000000000 +1100
+++ ./drivers/md/md.c 2005-11-28 10:12:33.000000000 +1100
@@ -1937,6 +1937,7 @@ static void md_safemode_timeout(unsigned
md_wakeup_thread(mddev->thread);
}
+static int start_dirty_degraded;
static int do_md_run(mddev_t * mddev)
{
@@ -2048,6 +2049,7 @@ static int do_md_run(mddev_t * mddev)
mddev->recovery = 0;
mddev->resync_max_sectors = mddev->size << 1; /* may be over-ridden by personality */
mddev->barriers_work = 1;
+ mddev->ok_start_degraded = start_dirty_degraded;
if (start_readonly)
mddev->ro = 2; /* read-only, but switch on first write */
@@ -4501,6 +4503,8 @@ static int set_ro(const char *val, struc
}
module_param_call(start_ro, set_ro, get_ro, NULL, 0600);
+module_param(start_dirty_degraded, int, 0644);
+
EXPORT_SYMBOL(register_md_personality);
EXPORT_SYMBOL(unregister_md_personality);
diff ./drivers/md/raid5.c~current~ ./drivers/md/raid5.c
--- ./drivers/md/raid5.c~current~ 2005-11-28 10:12:30.000000000 +1100
+++ ./drivers/md/raid5.c 2005-11-28 10:12:33.000000000 +1100
@@ -1903,10 +1903,17 @@ static int run(mddev_t *mddev)
if (mddev->degraded == 1 &&
mddev->recovery_cp != MaxSector) {
- printk(KERN_ERR
- "raid5: cannot start dirty degraded array for %s\n",
- mdname(mddev));
- goto abort;
+ if (mddev->ok_start_degraded)
+ printk(KERN_WARNING
+ "raid5: starting dirty degraded array: %s"
+ "- data corruption possible.\n",
+ mdname(mddev));
+ else {
+ printk(KERN_ERR
+ "raid5: cannot start dirty degraded array for %s\n",
+ mdname(mddev));
+ goto abort;
+ }
}
{
diff ./drivers/md/raid6main.c~current~ ./drivers/md/raid6main.c
--- ./drivers/md/raid6main.c~current~ 2005-11-28 10:11:52.000000000 +1100
+++ ./drivers/md/raid6main.c 2005-11-28 10:12:33.000000000 +1100
@@ -1929,13 +1929,18 @@ static int run(mddev_t *mddev)
goto abort;
}
-#if 0 /* FIX: For now */
if (mddev->degraded > 0 &&
mddev->recovery_cp != MaxSector) {
- printk(KERN_ERR "raid6: cannot start dirty degraded array for %s\n", mdname(mddev));
- goto abort;
+ if (mddev->ok_start_degraded)
+ printk(KERN_WARNING "raid6: starting dirty degraded array:%s"
+ "- data corruption possible.\n",
+ mdname(mddev));
+ else {
+ printk(KERN_ERR "raid6: cannot start dirty degraded array"
+ " for %s\n", mdname(mddev));
+ goto abort;
+ }
}
-#endif
{
mddev->thread = md_register_thread(raid6d, mddev, "%s_raid6");
diff ./include/linux/raid/md_k.h~current~ ./include/linux/raid/md_k.h
--- ./include/linux/raid/md_k.h~current~ 2005-11-28 10:08:19.000000000 +1100
+++ ./include/linux/raid/md_k.h 2005-11-28 10:12:33.000000000 +1100
@@ -183,6 +183,7 @@ struct mddev_s
sector_t resync_mismatches; /* count of sectors where
* parity/replica mismatch found
*/
+ int ok_start_degraded;
/* recovery/resync flags
* NEEDED: we might need to start a resync/recover
* RUNNING: a thread is running, or about to be started
-
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