Home > Storage > PowerMax and VMAX > Storage Admin > Dell PowerMax and VMware vSphere Configuration Guide > Partition alignment
Dell PowerMax storage arrays manage the data on hard drive using a logical construct that is known as a track. A track is 128 KB in size. A device can thus be viewed as a collection of 128 KB tracks. A data partition that VMware ESXi creates consists of all or a subset of the tracks that represent the device.
VMFS 5 and VMFS 6 datastores employ a GUID partition table (GPT) when formatting a disk, as opposed to an MBR. The GPT occupies the first 1 MB of the disk. The VMFS on the data partition of the disk uses a 1 MB block size which is a multiple of the track size. Therefore, file allocations are in even multiples of the track size. Thus, virtual disks that are created on those VMFS formats are always aligned.
The VMFS and thus the virtual disks that are created on that VMFS are aligned. However, if the virtual disks use MBR, the operating systems of the virtual machines that are created on those virtual disks are not aligned. An unaligned partition results in a track crossing and an additional I/O for the additional track, incurring a penalty on latency and throughput. The additional I/O (especially if small) can have a significant impact on system resources. An aligned partition eliminates the need for the additional I/O and results in an overall performance improvement.
Prior experience with misaligned Windows partitions and file systems has shown as 20 to 30 percent degradation in performance. Aligning the data partitions on a 64 KB boundary improves the overall I/O response time that all the hosts that are connected to the shared storage array experience.
If MBR is in use by the operating system, Dell Technologies recommends aligning the virtual disk on a track boundary. Doing so ensures optimal performance from the storage subsystem. Most Windows (for example, Windows 2008) and Linux (for example, Red Hat Enterprise Linux) distributions are aligned by default now and no additional action is required. Microsoft on its support site provides alignment instructions for older operating systems. A similar procedure using fdisk or sfdisk can be used to align disks in virtual machines with Linux as the guest operating system.
Note: Due to several known issues, Dell Technologies recommends using a 64 KB Allocation Unit for NTFS rather than the default of 4 KB. In Windows 2019, it is possible to increase this size to 128 KB.
Dell PowerMax devices that virtual machines access using raw device mapping (RDM) do not contain VMFS volumes. In this configuration, the alignment problem is the same as seen on physical servers. The process that is employed for aligning partitions on physical servers must be used in the virtual machines.