Performance impact of unaligned disk partitions on ESX Server

By default, ESX Server exhibits the same disk partition alignment problems as seen on all Intel-based platforms. As is common with all Intel-based systems, the first track on a disk is reserved for the boot code or MBR. This means that the first partition of the disk starts at cylinder 0, head 1, sector 1. This is equivalent to LBA 63 and means the first partition of the disk, by default, is not aligned with the 64KB boundary.

While this mis-alignment is not a significant issue for the ESX operating system itself, it is and can become a significant performance limiting problem on VMFS volumes where guest operating systems live.

By default, ESX Server does NOT create properly aligned VMFS partitions unless the partition is created through Virtual Center or the VMware Virtual Infrastructure Client after initial system setup. VMware reports that proper alignment of VMFS and guest OS disk partitions along the 64KB boundary can increase ESX Server guest OS performance by approximately 10-15% by reducing latency and increasing disk I/O throughput.

Unaligned disk partitions create a performance hit because unaligned partitions mean the OS and VMFS must perform excessive disk operations. Note the image below. With unaligned partitions, when the guest OS reads a single cluster there are times when that disk operation will result in the read of two underlying blocks and three underlying chunks or stripes on the LUN or disk array. Clearly this is not optimal.

unaligned disk partitions in ESX Server

However, as can be seen above in a properly aligned partition, that same cluster read operation resulted in the read of only one underlying block and one chunk / stripe – a much more efficient disk operation. Similarly, a 3-cluster read operation in the aligned partition example above would result in a much more effective disk operation.

As a best practice approach we recommend initially installing ESX server without any VMFS partitions. Reserve space for the VMFS partition on the initial volume, or dedicate a separate disk volume for VMFS-only use, and then create the VMFS partitions through Virtual Center or the VMware Virtual Infrastructure Client application AFTER setup is complete. This will ensure that your VMFS partition is properly aligned.

VMware has an excellent document which summarizes Recommendations for Aligning VMFS partitions.