19
The data on a 512e drive must be aligned to accommodate the following scenarios:
Short write accesses
(less than 4 KB)
Most accesses to a drive are 4 KB or greater. However, when the host writes less than 4 KB, all 4
KB on the media must be read, the old data modified with new data, and then written back to the
media as a 4-KB physical sector.
For the typical user, accesses shorter than 4 KB are rare.
Misalignment
In some cases, a logical address may not coincide with the beginning of a physical address.
For example, a data write may extend beyond the end of one physical sector. In this case, the drive
must read both physical sectors to drive memory, modify both data sets, and then rewrite the new
data to the media.
In both scenarios, the drive reads the data from the media, modifies the old data with the new data
and then rewrites the modified data to the media. Depending on the rotational speed of the drive, this
could add 16 – 22 milliseconds to a write. Read performance is not impacted; the drive reads the
whole 4 KB of data into drive memory and only sends out the data sector(s) needed.
Why alignment helps
Alignment issues with older operating systems are based on the starting point of partitions. In
Windows XP, for example, the partition boot sector is located at logical block address 63, which is
not divisible by eight. Thus, the remaining information in the partition information (directories and
files) is not aligned to physical addresses on the disk drive.
However, if logical writes are aligned to physical sectors and write lengths are in multiples of 4 KB,
then new data can completely replace old data; the drive does not have to perform any extra steps
for a write operation.
Newer operating systems like Windows Vista SP1 or later and Windows 7 start the partition on
logical block address 2048, which is divisible by eight. In addition, changes have been made in the
OS to reduce the number of writes less than 4 KB in length.
