dm raid: support metadata devices

Add the ability to parse and use metadata devices to dm-raid.  Although
not strictly required, without the metadata devices, many features of
RAID are unavailable.  They are used to store a superblock and bitmap.

The role, or position in the array, of each device must be recorded in
its superblock.  This is to help with fault handling, array reshaping,
and sanity checks.  RAID 4/5/6 devices must be loaded in a specific order:
in this way, the 'array_position' field helps validate the correctness
of the mapping when it is loaded.  It can be used during reshaping to
identify which devices are added/removed.  Fault handling is impossible
without this field.  For example, when a device fails it is recorded in
the superblock.  If this is a RAID1 device and the offending device is
removed from the array, there must be a way during subsequent array
assembly to determine that the failed device was the one removed.  This
is done by correlating the 'array_position' field and the bit-field
variable 'failed_devices'.

Signed-off-by: Jonathan Brassow <jbrassow@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

1 files changed, 3 insertions, 2 deletions

diff --git a/drivers/md/Kconfig b/drivers/md/Kconfig
index 8420129fc5ee..f75a66e7d312 100644
--- a/drivers/md/Kconfig
+++ b/drivers/md/Kconfig
@@ -241,12 +241,13 @@ config DM_MIRROR
          needed for live data migration tools such as 'pvmove'.
 
 config DM_RAID
-       tristate "RAID 4/5/6 target (EXPERIMENTAL)"
+       tristate "RAID 1/4/5/6 target (EXPERIMENTAL)"
        depends on BLK_DEV_DM && EXPERIMENTAL
+       select MD_RAID1
        select MD_RAID456
        select BLK_DEV_MD
        ---help---
-         A dm target that supports RAID4, RAID5 and RAID6 mappings
+         A dm target that supports RAID1, RAID4, RAID5 and RAID6 mappings
 
          A RAID-5 set of N drives with a capacity of C MB per drive provides
          the capacity of C * (N - 1) MB, and protects against a failure