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How to Find the File Allocation Unit Size in Windows – Used for Disk IO Tuning

You want to know the block size that NTFS uses when writing data. This is useful for disk IO performance tuning.


Use fsutil! In the example below the bytes per sector is 4k as shown in Bytes Per Cluster. If this were a SQL server, we may want to evaluate whether 4k or 64k would be better. (For SQL, 64k blocks are usually best!)

C:>fsutil fsinfo ntfsinfo c:
NTFS Volume Serial Number : 0xd0783e69783e4e8a
Version : 3.1
Number Sectors : 0x000000003a1fa7ff
Total Clusters : 0x000000000743f4ff
Free Clusters : 0x0000000003a5de35
Total Reserved : 0x00000000000007b0
Bytes Per Sector : 512
Bytes Per Physical Sector : 4096
Bytes Per Cluster : 4096
Bytes Per FileRecord Segment : 1024
Clusters Per FileRecord Segment : 0
Mft Valid Data Length : 0x0000000027c80000
Mft Start Lcn : 0x00000000000c0000
Mft2 Start Lcn : 0x0000000000000002
Mft Zone Start : 0x000000000353c580
Mft Zone End : 0x0000000003543040
RM Identifier: E2203E62-8069-11E3-A5DF-F01FAF5D769C

Learn more on the TechNet article.

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