What are 4K Native and 512e drives?
Industry standard disk drives have been using a native (physical) 512 bytes sector size.
However, due to the increasing demand for larger capacities, the storage industry introduced new advanced formats that use 4KB (4096 bytes) physical sectors.
The disk sector size is an important factor in the design of Operating System and Hypervisor (collectively called OS here) software such as device drivers and file systems, because it represents the atomic unit of I/O operations on a disk drive. Not all OS versions have been modified to utilize 4KB sectors in the disk drives. Thus, the firmware of these newer devices may expose a logical sector size, which is either 4KB Native (4Kn) or 512B Emulation (512e).
512e is the advanced format in which the physical sector size is 4,096 bytes, but the logical sector size emulates 512 bytes sector size. The purpose of 512e is for the new devices to be used with OSs that do not support 4Kn sectors yet. However, inherently, 512-byte emulation involves a read-modify-write process in the device firmware for every write operation that is not 4KB aligned.
For example, a workload that does I/O operations that are either not aligned at a 4KB offset in bytes from the start of the disk or that are not 4KB multiples in length, one gets an alignment penalty caused by a read-modify-write process taking place in the drive for every write operation. The penalty is more pronounced for smaller operations. For larger I/Os, the per-operation latency is dominated by transfer times. Because many 512e drives are slightly faster than their older 512n counterparts, the alignment penalty is typically cancelled out after a certain operation size (say 256KB and larger, sometimes less).
In other words, even with 512e sectors, it is still preferable for the applications and the OS to perform 4KB aligned I/O for predicable performance. This is a general problem and not particular to any specific OS.
Also, the read-modify-write penalty applies to both magnetic disks (HDD) and solid-state disks (SSD), but due to the much lower number of IOPS of magnetic disks, the performance impact is more pronounced in that case.
This table compares native 512-byte sectors to the new advanced formats:
Format |
Logical Sector Size |
Physical Sector Size |
512n |
512 |
512 |
512e |
512 |
4,096 |
4Kn |
4,096 |
4,096 |
vSphere/vSAN 6.0 and earlier versions have not been designed to use 4Kn/512e direct attached disk drives.
512e drives are supported only in version 6.5, 6.7, 7.x and later.
vSAN is optimized for 4K aligned I/O operations since version 6.0, including the new vsanSparse snapshot format.
However, due to limitations in older builds of ESXi, full utilization of 4k alignment was not possible.