16 Copyright © Acronis, Inc., 2000-2010
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.
Many hard disk drive manufacturers supply their modern drives with controllers that can shift
addressing offset to one sector (63 sector becomes 64 sector), so volumes will appear aligned.
How to work with 4-KB sector size disks using Acronis Disk Director
Suppose that you added a new 4-KB sector size hard disk drive with to a machine that is running
Windows XP only. There are no volumes on this drive yet. If you start creating volumes on this disk
using Windows XP, you may experience some slowdown of the system performance while accessing
the disk. To ensure proper volume alignment and normal access to volumes on this disk, perform the
following steps:
1. Create a bootable media with Acronis Disk Director—see How to create bootable media (p. 50).
2. Run Acronis Disk Director from a bootable media—see Running Acronis Disk Director (p. 16).
3. Select the Bootable media OS disk layout—see Disk layout (p. 19).
4. Create volumes—see Creating a volume (p. 25).
If Windows 7 or Windows Vista is installed in addition to Windows XP, select the disk layout of either
of those operating systems.
