Dell PowerEdge T630 - Manual (Page 7)

7 512e and 4Kn Disk Formats
4.1.1 Emulated read and write processes
To read data from a 4K sector formatted drive in 512 emulation mode, the process is very straightforward,
as shown in Figure 7.
Figure 2 Potential read sequence for 512-byte emulation
The process of reading the 4K block of data and reformatting to the specific 512-byte emulated sector
requested by the host computer is performed in the drive’s DRAM memory and does not measurably
impact performance.
A write process will be more complicated, particularly when data the host computer attempts to write is a
subset of a physical 4K sector. In these cases, the hard drive must first read the entire 4K sector containing
the targeted location of the host write request, merge the existing data with the new data, and then
rewrite the entire 4K sector as shown in Figure 3. The fundamental reason for this is that the drive cannot
write just a portion of the 4K sector, but can only write the entire sector at once.
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"multiple"of"4K"in"size