Picking Allocation Size for NTFS

Picking Allocation Size for NTFS

Q: What allocation unit size should I use for my NTFS volume that will be running Hyper-V virtual machines (VMs) and SQL Server databases?

A: When formatting a partition, you can select an allocation unit size (cluster size) which has a default value (as documented at Microsoft Support). However, if you are running Hyper-V VMs or a SQL Server database, for best performance, set the allocation unit size to 64K.

This can only be set when the volume is formatted, so if you have a smaller value, you would need to move the data, then perform a fresh format. To check the cluster size, use fsutil and look at the Bytes Per Cluster value:

C:\>fsutil fsinfo ntfsinfo c:
NTFS Volume Serial Number : 0xd40efd830efd5f44
NTFS Version : 3.1
LFS Version : 2.0
Number Sectors : 0x000000000945f7ff
Total Clusters : 0x000000000128beff
Free Clusters : 0x0000000000b942e8
Total Reserved : 0x0000000000000a40
Bytes Per Sector : 512
Bytes Per Physical Sector : 512
Bytes Per Cluster : 4096
Bytes Per FileRecord Segment : 1024
Clusters Per FileRecord Segment : 0
Mft Valid Data Length : 0x000000000b5c0000
Mft Start Lcn : 0x00000000000c0000
Mft2 Start Lcn : 0x0000000000000002
Mft Zone Start : 0x00000000000cb5c0
Mft Zone End : 0x00000000000cc820
Resource Manager Identifier : 1CDFE057-1264-11E2-B782-C689895D03AC


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