Define block device of an ASM disk
Newer versions of ASMLib support oracleasm querydisk -p DATA01 which displays the underlying physicaldisk.

oracleasm-querydisk

Example:

# oracleasm querydisk -p SSDDATA1
Disk "SSDDATA1" is a valid ASM disk
/dev/sdl1: LABEL="SSDDATA1" TYPE="oracleasm"
/dev/sdar1: LABEL="SSDDATA1" TYPE="oracleasm"
/dev/sdbn1: LABEL="SSDDATA1" TYPE="oracleasm"
/dev/sddf1: LABEL="SSDDATA1" TYPE="oracleasm"
/dev/mapper/mpathqp1: LABEL="SSDDATA1" TYPE="oracleasm"
#


Older versions may not have the -p option and they display only the major and minor numbers which you can use to find the physical disk:

Mapping ASM disks to Physical Devices

Example:

[root@orcldb2 ~]# /etc/init.d/oracleasm querydisk VOL1
Disk "VOL1" is a valid ASM disk on device [8, 97]

[root@orcldb2 ~]# ls -l /dev | grep 8, | grep 97
brw-rw----   1 root disk     8,      81 Nov  4 13:02 sdg1


This example I copied from the above blog post seems to be not totally accurate (grep 97 returns minor number 81?), the concept however works.