Filesystem Alignment

You’re likely to have seen the filesystem alignment check fail on most, if not all, of the EMC HEAT reports that you run on your windows 2003 servers.  The starting offset for partition 1 should optimally be a multiple of 128 sectors.  So, how do you fix this problem, and what does it mean?

If you align the partition to 128 blocks (or 64KB as each block is 512bytes) then you don’t cross a track boundary and thereby issue the minimum number of IOs.   Issuing the minimum number of IOs sounds good, right? 🙂

Because NTFS reserves 31.5 KB of signature space, if a LUN has an element size of 64 KB with the default alignment offset of 0 (both are default Navisphere settings), a 64 KB write to that LUN would result in a disk crossing even though it would seem to fit perfectly on
the disk.  A disk crossing can also be referred to as a split IO because the read or write must be split into two or more segments. In this case, 32.5 KB would be written to the first disk and 31.5 KB would be written to the following disk, because the beginning of the stripe is offset by 31.5 KB of signature space. This problem can be avoided by providing the correct alignment offset.  Each alignment offset value represents one block.  Therefore, EMC recommends setting the alignment offset value to 63, because 63 times 512 bytes is 31.5 KB.

Checking your offset:

1. Launch System Information in windows (msinfo32.exe)

2. Select Components -> Storage -> Disks.

3. Scroll to the bottom and you will see the partition starting offset information.  This number needs to be perfectly divisible by 4096, if it’s not then your partition is not properly aligned.

Correcting your starting offset:

Launch diskpart:

C:\>diskpart

DISKPART> list disk

Two disks should be listed

DISKPART> select disk 1

This selects the second disk drive

DISKPART> list partitions

This step should give a message “There are no partitions on this disk to show”.  This confirms a blank disk.

DISKPART> create partition primary align=64

That’s it.  You now have a perfectly aligned disk.

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