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Dell PowerMax storage arrays manage the data on physical disk using a logical construct 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 created by VMware ESXi 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. Because the VMFS on the data partition of the disk uses a 1 MB block size which is a multiple of the track size, file allocations are in even multiples of the track size. Thus, virtual disks created on those VMFS formats are always aligned.
While the VMFS and thus the virtual disks created on that VMFS are aligned, the operating systems of the virtual machines created on those virtual disks will not be aligned if they use an MBR. An unaligned partition results in a track crossing and an additional IO for the additional track, incurring a penalty on latency and throughput. The additional IO (especially if small) can impact system resources significantly. An aligned partition eliminates the need for the additional IO and results in an overall performance improvement.
Prior experience with misaligned Windows partitions and file systems has shown as much as 20 to 30 percent degradation in performance. Aligning the data partitions on 64 KB boundary results in positive improvements in overall IO response time experienced by all hosts connected to the shared storage array.
Therefore, Dell recommends aligning the virtual disk on a track boundary to ensure the optimal performance from the storage subsystem if MBR is in use by the OS. Most Windows (e.g., Windows 2008) and Linux (e.g., RHEL) distributions are aligned by default now and therefore no additional action is required. Microsoft on its support site provides alignment instructions for older operating systems. A similar procedure utilizing fdisk or sfdisk can be used to align disks in virtual machines with Linux as the guest operating system.
Note: Due to a number of known issues, Dell 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 to 128 KB.
Dell PowerMax devices accessed by virtual machines using raw device mapping (RDM) do not contain VMFS volumes. In this configuration, the alignment problem is the same as that seen on physical servers. The process employed for aligning partitions on physical servers needs to be used in the virtual machines.