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4.6 Volume alignment in disks with a 4-KB sector size

When a new volume is created, its beginning is aligned with the disk's physical sector boundaries.

It ensures that each file system allocation unit (cluster) on the volume starts and ends on the

boundaries of the disk's physical sectors. If the volume clusters are aligned with sectors, this volume

and all following volumes are also aligned. If clusters are not aligned with sectors, the volumes are

misaligned. Misalignment decreases the overall system performance and hardware lifetime.

When misalignment occurs

Volume misalignment occurs when you create a volume on a modern HDD or SSD drive that has a 4-

KB sector size using a Windows operating systems earlier than Vista.

What is the cause of misalignment

All Windows operating systems earlier than Vista use a factor of 512 bytes to create volume clusters.

The volume start is aligned to 512-byte sectors. Also, these operating systems use the

Cylinder/Head/Sector (CHS) addressing scheme. Volumes created with this scheme are aligned by

cylinders/tracks of the disk.

Usually, a track consists of 63 physical sectors. Since the first track is reserved for the master boot

record (MBR) and other service purposes, the first volume starts from the beginning of the second

track of the disk. Therefore, volumes aligned by 63 sectors are not aligned with 4-KB sectors: 63

sectors by 512 bytes do not match with the integer number of 4-KB sectors.

Thus, the first created volume and all of the following volumes on the hard disk drive will be

misaligned.

Why misalignment is an important issue for hard disk drives

When a single bit of data is changed, the operating system entirely overwrites the cluster that

contains the changed data. But if misalignment occurs, the cluster will overlap more physical sectors

than it would have occupied if aligned. As a result, more physical sectors need to be erased and

rewritten each time data changes.

The redundant read/write operations noticeably slow down the disk speed and overall system

performance.

The same is true for SSD drives that have a 4-KB or larger sector (memory page) size. For SSD drives

misalignment decreases not only system performance, but also drive lifetime. SSD memory cells are

designed for a certain amount of read/write operations. Therefore, redundant read/write operations

lead to early degradation of the SSD drive.

How to avoid volume misalignment

The latest operating systems, starting from Windows Vista, already support the new sector size.

Thus, volumes created with these operating systems will be properly aligned.