ceph-disk: is_mpath predicate for multipath devices

The is_mpath predicate returns True if a device is managed by
multipath. It is based on the devicemapper uuid content which is
expected to always contain the mpath- string to identify the multipath
subsystem.

The block_path helper is added to convert the path to a device to the
/sys directory that describes it. It uses the major and minor number
instead of the device name because it is more reliable. The rationale
including an actual example is added as a comment for future
maintainers.

Signed-off-by: Loic Dachary <ldachary@redhat.com>

diff --git a/src/ceph-disk b/src/ceph-disk
index d7b3233cff5c7e76f6db18767c98772693c38545..69f8b8c5f0d563c85cffdfc4a0395f2d2a71ea01 100755 (executable)
--- a/src/ceph-disk
+++ b/src/ceph-disk
@@ -90,6 +90,7 @@ DMCRYPT_TOBE_UUID =         '89c57f98-2fe5-4dc0-89c1-5ec00ceff2be'
 DMCRYPT_JOURNAL_TOBE_UUID = '89c57f98-2fe5-4dc0-89c1-35865ceff2be'
 
 DEFAULT_FS_TYPE = 'xfs'
+SYSFS = '/sys'
 
 MOUNT_OPTIONS = dict(
     btrfs='noatime,user_subvol_rm_allowed',
@@ -363,6 +364,56 @@ def platform_information():
         str(codename).strip()
     )
 
+#
+# An alternative block_path implementation would be
+#
+#   name = basename(dev)
+#   return /sys/devices/virtual/block/$name
+#
+# It is however more fragile because it relies on the fact
+# that the basename of the device the user will use always
+# matches the one the driver will use. On Ubuntu 14.04, for
+# instance, when multipath creates a partition table on
+#
+#   /dev/mapper/353333330000007d0 -> ../dm-0
+#
+# it will create partition devices named
+#
+#   /dev/mapper/353333330000007d0-part1
+#
+# which is the same device as /dev/dm-1 but not a symbolic
+# link to it:
+#
+#   ubuntu@other:~$ ls -l /dev/mapper /dev/dm-1
+#   brw-rw---- 1 root disk 252, 1 Aug 15 17:52 /dev/dm-1
+#   lrwxrwxrwx 1 root root        7 Aug 15 17:52 353333330000007d0 -> ../dm-0
+#   brw-rw---- 1 root disk 252,   1 Aug 15 17:52 353333330000007d0-part1
+#
+# Using the basename in this case fails.
+#
+def block_path(dev):
+    path = os.path.realpath(dev)
+    rdev = os.stat(path).st_rdev
+    (M, m) = (os.major(rdev), os.minor(rdev))
+    return "{sysfs}/dev/block/{M}:{m}".format(sysfs=SYSFS, M=M, m=m)
+
+def get_dm_uuid(dev):
+    uuid_path = os.path.join(block_path(dev), 'dm', 'uuid')
+    LOG.debug("get_dm_uuid " + dev + " uuid path is " + uuid_path)
+    if not os.path.exists(uuid_path):
+        return False
+    uuid = open(uuid_path, 'r').read()
+    LOG.debug("get_dm_uuid " + dev + " uuid is " + uuid)
+    return uuid
+
+def is_mpath(dev):
+    """
+    True if the path is managed by multipath
+    """
+    uuid = get_dm_uuid(dev)
+    return (uuid and
+            (re.match('part\d+-mpath-', uuid) or
+             re.match('mpath-', uuid)))
 
 def get_dev_name(path):
     """