LBD: Bigger sectors for better disks

By Mark Ferelli

—Seven years of committee work has resulted in the International Disk Drive, Equipment and Materials Association (IDEMA) approval of a new standard expanding the basic block size of a disk drive for the first time in more than 30 years. Referred to as the Long Block Data (LBD) standard, it proposes expansion from a 512-byte sector size to 4,096 bytes.

Dr. Michael Russak, executive director at IDEMA USA, points out that the change in sector size will enable greater densities and higher capacity. It would also significantly reduce error rates and the read/write overhead per block that error correction requires.

However, switching to a new standard is a non-trivial task. Operating systems, BIOS, applications, and other software/firmware components would have to be modified. Currently, only Windows Vista is "tuned" to a 4K sector size. Until other OS makers do the same and enable "native" LBD, the industry is proposing a workaround known as "4K emulation."

Willis Whittington, senior product marketing manager at Seagate, explains it this way: "In 4K emulation, data from eight 512-byte sectors is packed into a single 4K block and protected with a single ECC field at the end. Manufacturers will also have extra fields for functions such as data integrity checks."

However, Whittington also notes: "This works really well until the system asks for a write update."

For example, an application could ask for a data update from the middle of one of the "faux" 4K sectors. Dr. William Cain, a WD Fellow at Western Digital, says, "In that case, a read/modify/write would be required. You'd have to read all the data in the sector, select the data for update, and connect it with the data from the system. Then you perform the data update. This can mean a 10% to 30% performance hit."

Drive manufacturer Hitachi Global Storage Technologies has been mischaracterized in the trade press as being hesitant to comply with the new LBD sector standard. But any hesitation is focused on the emulation workaround, not the actual standard, according to company officials. Dan Reno, strategic product market analyst at Hitachi GST, explains: "When and if we get to 4K sectors, there will be no issues. We are prototyping 4K now. But data misalignment in 4K [emulation] gives an opportunity for both data loss and performance penalties."

Seagate's Whittington concurs that the intermediate solution isn't perfect. He notes: "The question is: 'Can we live with the performance problem until the industry?is ready to transition to a true 'native' 4K sector implementation?' At the moment the answer is no, but the industry is tackling this 4K emulation problem on two fronts: finding ways to improve performance during read/modify/write, and developing utilities to realign data in legacy files and applications so that writes always begin and end on 4K sector boundaries, making read/modify/write unnecessary."

Some vendors expect implementation of native LBD in the 2010 timeframe. But this timetable will depend on managing data misalignment on the OS level, as well as demand from drive vendors' largest OEM customers.

Mark Ferelli is a freelance writer. He can be contacted at mcferelli@yahoo.com

This article was originally published on August 29, 2007