Understanding Disk Alignment

The graphic above shows that SAN’s, VMFS, and NTFS virtual disks all have different block sizes.  Aligning all these layers to start their disk units at the same place can make disk operations more efficient and keep the physical disks from working harder than necessary.  For example, if these are not aligned, a call to read a single NTFS block may require the SAN to read three blocks as shown below:

The greatest efficiency is achieved when these layers are aligned so that the desired NTFS block requires only one SAN block to be read:



VMFS Alignment

When VMFS volumes are created by the vSphere client, they are aligned on a 64K boundary. Check your SAN vendor’s documentation but in most cases the default 64K boundary will work.

NTFS Alignment

Windows 2003 and older align on 32K which will not match up with the 64K for VMFS. Windows 2008 by default will align on a 1024K boundary – this works with VMFS because 1024K is divisible by 64K.

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