512e and 4Kn Disk Formats This whitepaper provides the context for 512e and 4Kn disk format migration, as well as pointing out the long-term benefits to customers and potential pitfalls to avoid when moving from 512-byte to 4K sector formats.
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Contents 1 Overview...................................................................................................................................................................................... 5 2 Background ................................................................................................................................................................................. 6 3 Long-term benefits of 4K sectors ...........................................................................
1 Overview A change is coming in the hard drive industry. As storage densities dramatically increase, one of the most elemental aspects of hard drive design — the logical block format size known as a sector has remained constant. The storage industry has transitioned over to a new type of format for media, known as Advanced Format, which has a 4 KB physical sector size.
2 Background The legacy sector format contains a Gap section, a Sync section, an Address Mark section, a Data section and Error Correction Code (ECC) section as shown in Figure 1. Figure 1 Legacy sector format The sector layout is structured as follows: Gap: Separates sectors. Sync: Sync marks the beginning of the sector and also provides timing alignment. Address Mark: Not only stores information to identify the sector’s number and location but also provides sector status.
correction make the transition to 4K sectors well worth the effort. Managing this transition properly to capture the long-term benefits with minimal side effects is a key focus for the hard drive industry. 4 Understanding the impacts of the 4K transition As noted earlier, there are many aspects of modern computing systems that continue to assume that sectors are always 512 bytes.
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.
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 Drive sends status to the host indicating command completion End 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
4.1.2 Aligned versus unaligned hard drive partitions Both 512e/4Kn HDDs use 4KB sectors but the read-write operations depends on the transfer size request and alignment with Logical Block Address (LBA). Each 512-byte sector on the drive is assigned a unique LBA, from zero (0) to the number required based on the drive capacity. The host requests a specific block of data using the assigned LBA.
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.
Windows Server 2008 with MS KB 2553708 2. Advanced Format, AF, 4K Native 4096 bytes 4096 bytes Windows Server 2012 (4K data disks are supported and as boot disks in UEFI mode) Note that Windows Server 2003 and Windows Server 2003 R2 do not support 512e or 4Kn media. While the system may boot up and operate minimally, there may be functionality issues, data loss, or suboptimal performance. Dell does not recommend using 512e media with Windows Server 2003.
5.4 VMware support The key strategies in managing the transition to 4K sectors in a Windows environment also apply to VMware. VMware is yet to show concrete plans to support 512e and 4Kn drives. Table 4 5.5 VMware support Operating system 512e 4Kn VMware ESXi TBD TBD Dealing with unaligned conditions Using a 4K-aware version of an operating system to create hard drive partitions is a simple, straightforward method for avoiding unaligned conditions.
logos may be seen populating hard drive product labels, product pages, and various literatures to indicate the usage of AF technologies versus legacy sector size architectures that were used in earlier drives. The AF emulation logo has one rounded corner and is used on any client or enterprise hard drive that is equipped with industry-standard emulation techniques. AF 512e is the current standard by which downward compatibility with legacy sector formats is achieved.
http://blogs.technet.com/b/exchange/archive/2013/04/24/exchange-2010-database-availability-groupsand-disk-sector-sizes.aspx http://www.idema.
