Logical Disk Manager - Compatibility Problems - Advantages of Using A 1-MiB Alignment Boundary

Advantages of Using A 1-MiB Alignment Boundary

• Microsoft states that on large-sector drives, also known as advanced format, there is a "performance issue" (i.e., problem) when the starting offset of the first partition is an "odd" number. This partition alignment offset also affects newer generation drives not necessarily specified as advanced format such as SSD drives. Misalignment causes severe degradation not only with random write performance, but also with sequential write performance (normally to a far lesser degree).

(Quoting Microsoft directly): "In earlier versions of Windows, the default starting offset for the first partition on a hard disk drive was sector 0x3F. Because this starting offset was an odd number, it could cause performance issues on large-sector drives because of misalignment between the partition and the physical sectors. In Windows Vista, the default starting offset will generally be sector 0x800."

The "starting offset" refers to the sectors before the partition. "3F" is a hexadecimal value; in decimal that is "63". This is the size of one head (or "track") in terms of a standard CHS geometry. The hex value "800", in decimal form is 2048, so this is referring to the first MiB of space, from the start of the disk. Because a 1-MiB alignment boundary is used by the Vista partitioner(s), 1 MiB is "reserved for" the Master boot record (MBR), instead of the traditional 63 sectors (31.5 KiB). Note that the MBR is only one sector in size, but many softwares use these 63 sectors of "free space" (or any "free space" preceding the partition) for their purposes. 63 sectors is the minimum space that should be reserved for the MBR or an Extended Boot Record(EBR).

What this means is that, if there were no partitions on a disk when it was booted, a partition was created using standard CHS geometry (as would be used by most partition editors such as MS DOS fdisk, Linux fdisk, Ranish Partition Manager, or an XP installation disk), and this partition was created at the start of the disk, then the partition will start on the 2nd head. The partition could not begin at the first sector because the MBR located there, so instead it starts the partition on the second head. When primary partitions are created anywhere else (following typical CHS geometry alignment) they start at the beginning of a cylinder.

If a computer is booted with no partitions defined, and the first partition is created with the Vista installation disk, it will instead use the 1-MiB alignment. (The partitioner on the Vista installation disk follows the 1-MiB alignment just as Vista Disk Management does.) By following a 1-MiB alignment, all partitions will have an even numbered offset (so long as no other partitioner is used on the drive).

• Using a 1-MiB alignment boundary allows safer editing of the partition table with Vista Disk Management.

Partition tables using a CHS geometry (or some mix of alignments) may prevent Vista from resizing partitions or it may produce unconventional partition tables. For example, the endings of the "EBR extended partitions" (type 0x05) may overlap other partitions, and it will leave unprecedented amounts of space between an extended boot record and its logical drive. Ranish Partition Manager does not show an error if there are more than 63 sectors between the EBR and its logical volume, so long as the EBRs and VBRs are on the first sector of any track (head). If either the VBR or the EBR are not on the first sector of a head, Ranish Partition Manager shows the "partition" in red and when the partition is highlighted, this error message appears: "Boot sector doesn't have valid information". Using the 1-MiB alignment boundary will not put VBRs or EBRs on the first sector of a track (according to any standard CHS geometry) and Vista Disk Management will not use the CHS geometry of a partition table created with a different partition editor.

• A disk that is solely partitioned with Vista Disk Management can create partitions in exact MiB sizes and never leave leftover bits of space between partitions.

With Vista DM, all the logical drives besides the one nearest the beginning of the extended partition (type 0x0F), usually reserve exactly one additional MiB, for an extended boot record, instead of the 63 sectors (31.5 KiB) that is most commonly reserved according to alignments based on a CHS geometry. If other partition editors are used on the disk then Vista DM may reserve different amounts of space for the EBRs.