Dell PowerEdge T630 - Manual (Page 8)

8 512e and 4Kn Disk Formats
4.1.2 Aligned versus unaligned hard drive partitions
Up to now we have not discussed how host systems and hard drives communicate the location of sectors
on the media. It’s time to introduce the Logical Block Address (LBA).
Each 512-byte sector is assigned a unique LBA, from zero (0) to the number required based on the size of
the disk. The host requests a specific block of data using the assigned LBA. When the host requests to
write data, an LBA address is returned at the end of the write request telling the host where the data is
located. This becomes important in the transition to 4K sectors since for every 4K sector there are eight
different possibilities for where the host LBA starts.
When LBA 0 is aligned to the first virtual 512-byte block in the 4K physical sector, the logical to physical
alignment condition for 512-byte emulation is termed Alignment 0. Another possible alignment is when
LBA 0 is aligned to the second virtual 512-byte block in the 4K physical sector. This situation is termed
Alignment 1 and is shown in comparison to the Alignment 0 condition in Figure 4. There are six additional
possibilities for unaligned partitions that can result in read-modify-write events similar to the Alignment 1
condition.
Figure 4 Alignment conditions
Alignment 0 conditions work very well with the new 4K sectors in the Advanced Format standard. This is
because a hard drive can easily map eight contiguous 512-byte sectors into a single 4K sector by storing
512-byte write requests in the hard drive’s cache until enough contiguous 512-byte blocks are received to
form a 4K sector. Since modern computing applications deal with chunks of data that are typically larger
than 4K, runts (transfers smaller than 4K) are extremely rare. However, the Alignment 1 situation is another
matter.
When hard drive partitions are created that result in an unaligned condition as shown in Figure 4, read-
modify-write cycles occur that can slow hard drive performance. See section 5.5 for information on
avoiding these cycles when implementing Advanced Format hard drives.
4.2 Small or runt writes
In modern computing applications, data such as documents, images and video streams are much larger
than 512 bytes. Therefore, hard drives can store these write requests in cache until there are enough
sequential 512-byte blocks to build a 4K sector. As long as hard drive partitions are aligned, the hard drive
can easily map 512-byte sectors into 4K sectors without any performance penalties. There are, however,
certain low-level processes that can force a hard drive to handle runt writes that are not associated with
unaligned partitions. When I/O size is either smaller than 4K or not a multiple of 4K-sector size, the drive
must handle the RMW and small writes in the same way. Dell recommends modifying these processes to
maximize performance for the 4K transition.