Multipath - what is the differences between /dev/mapper/mpathxx and /dev/mapper/mpathxx1?

Question

For every multipath disk label in /dev/mapper I have another with 1 in the end. Are they the same? is there some relationship?

For example:

/dev/mapper/mpathaj and /dev/mapper/mpathaj1 or /dev/mapper/mpathai and /dev/mapper/mpathai1

when I issue the command od --read-bytes=128 --format=c /dev/mapper/mpathai, the disk seems clean:

[root@server02 ~]# od --read-bytes=128 --format=c /dev/mapper/mpathai
0000000  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0
*
0000200

But the other with 1 in the end show some rows:

[root@server02 ~]# od --read-bytes=128 --format=c /dev/mapper/mpathai1
0000000 001 202 001 001  \0  \0  \0  \0 003  \0  \0 200 220   .   5 213
0000020  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0
0000120   3  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0
   some characters I needed to remove to don't show costumer content.
0000160  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0
0000200

That happen to every disk. One is clean and the other no.

And, the reason I am asking it: Can I lost one (mpathaj) without lost the other (mpathaj1)? I've seen they point to different /dev/dm-xx.

ie: /dev/mapper/mpathaj is /dev/dm-18 and /dev/mapper/mpathaj1 is /dev/dm-19

Answer 1

I would expect that /dev/mapper/mpathai to be the whole disk/LUN, and /dev/mapper/mpathai1 the first partition on that disk/LUN. But it also could be something like a LUKS encryption layer with a confusingly chosen name.

On device-mapper based devices (multipaths, encrypted disks, software RAID...), partition detection is done in userspace (often via the command kpartx), and a new device-mapper entry (/dev/dm-<number>) is created for each of them.

The only way to be sure would be to use dmsetup ls and/or dmsetup table as root to view the mappings and see their relations to each other.

On a modern Linux system, you might begin with dmsetup ls --tree -o blkdevname: it's probably the easiest way to visualize the relationships between the different device-mapper entries, if there are any.

Unfortunately, the dmsetup ls --tree listing won't include the type of mapping, so you may still need to refer to dmsetup table to identify the type: if the mapping of mpathai1 is of type linear and refers to the mpathai device by major:minor numbers, then mpathai1 is a linear sub-mapping of mpathai, which usually means it's a partition within the disk device.

If mpathai1 is of type crypt, then mpathai could be an encrypted disk (LUKS or some other method that is understood by cryptsetup), which has been configured to have the decrypted view of the device appear as mpathai1 whenever encryption is unlocked. In other words, the encryption would be unlocked with a command like:

cryptsetup open /dev/mapper/mpathai mpathai1 --type <luks,loopaes,tcrypt,bitlk...>

If encryption is used, I would expect /etc/crypttab to also mention the device(s).

If you can't identify the mapping type on your own, please post the output of e.g. dmsetup table mpathai1 and dmsetup table mpathaj1.

If mpathaj1 is a partition of mpathaj, you could lose mpathaj1 by e.g. corrupting/overwriting the partition table. If the partition table no longer has a valid entry for the mpathaj1 partition, the system would no longer show it, even if the underlying disk mpathaj is 100% fine.

The same is true if mpathaj1 is the decrypted view of encrypted mpathaj, then if the encryption key (e.g. the encrypted master key within the LUKS header) is lost for any reason, you will no longer be able to unlock the encryption, and then mpathaj1 and all data within it are effectively lost to you.