If your ASM is using RAW devices then its pretty easy to find out by just looking into your /etc/sysconfig/rawdevices (hope your DBA had made some comments as to which RAW devices is for which).
If you are using ASMLib then its kinda hard to find out the exact mapping of your ORACLEASM stamped device to the Linux Physical Device.
To make it simple, here are the steps to find the Physical Device that is mapped to your ASMLib disk:
Run the following as a root:
1. /etc/init.d/oracleasm listdisks — lists all the disks that are in your ASM
2. /etc/init.d/oracleasm querydisk -d disk_name_that_you_wan_to_find_physical_device (this name is from the above result)
3. Note the values in braces [ n1, n2] from the above command result
4. do ls -al /dev/ |grep n1 |grep n2 — n1 and n2 are the values from the above step you noted
That gives you the device number its mapped to.
#/etc/init.d/oracleasm querydisk -d TIER2_DATA01
Disk “DG1_DISK1” is a valid ASM disk on device [253, 26]
#ls -l /dev |grep 253 |grep 26
brw-rw—- 1 root root 253, 26 Nov 3 12:03 dm-26
From the above dm-26 is the physical device that DG1_DISK1 ASM Disk is mapped to