Summary: How to check if your disks on Red Hat are aligned with the underlying SAN LUNs.
Date: Around 2014
Refactor: 29 March 2025: Checked links and formatting.

Red Hat Enterprise Linux 6 should align it's disk correctly to the underlying LUNs but to make sure you should always check:

First, check which partitions are seen by the kernel:

[root@redhatbox ~]# cat /proc/partitions
major minor  #blocks  name

   8        0   20971520 sda
   8        1     204800 sda1
   8        2   20765696 sda2
 253        0    2097152 dm-0
 253        1    5242880 dm-1
 253        2    5242880 dm-2
 253        3    5242880 dm-3
 253        4    2097152 dm-4

Then, check whether the partitions are aligned or not:

[root@redhatbox ~]# fdisk -lu /dev/sda

Disk /dev/sda: 21.5 GB, 21474836480 bytes
64 heads, 32 sectors/track, 20480 cylinders, total 41943040 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x000c0750

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *        2048      411647      204800   83  Linux
Partition 1 does not end on cylinder boundary.
/dev/sda2          411648    41943039    20765696   8e  Linux LVM
Partition 2 does not end on cylinder boundary.

The start sector should be cleanly divisible by 8, if so they are aligned. This counts for all partitions, not just the first one. Both starting sectors are, do the VM is aligned.