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9 512e and 4Kn Disk Formats
Begin
Host issues 512-byte write
command
Drive reads an entire 4k-secto,
containing the targeted 512-byte
LBA, from the media into its
DRAM
Drive updates the 512-byte sector
in the 4K-sector in DRAM
Drive writes the updated 4kn
sector to the media/disk
End
Drive sends status to the host
indicating command completion
Figure 3 Potential write sequence for 512-byte emulation
In this instance, the hard drive must perform extra mechanical steps in the form of reading a 4K sector,
modifying the contents and then writing the data. This process is called a read-modify-write (RMW) cycle,
which is undesirable because it has a negative impact on hard drive performance. Minimizing the
probability and frequency of read-modify-write instances is the most important aspect of making the
transition to 4K sectors smooth and painless.
The causes of read-modify-write reduced performance include:
 Write requests that are misaligned because of logical to physical partition misalignment
 Write requests smaller than 4K in size
 Write requests that are not multiples of 4K in size