Introduction

Hitachi's No-ID™ sector format allows disk drives to be formatted more efficiently, improving the capacity, reliability and performance. The ID (or header) information is stored in solid state memory instead of on the disk surface. This results in the increased capacity of each track without impacting the linear density. When combined with a magneto-resistive head, the No-ID sector format can be used to dramatically increase the track density as well. Further benefits are increased data throughput, improved access time, defect management and power management.

Format efficiency

Definition
The term "Format Efficiency" refers to the amount of each track in a disk drive devoted to storing user data. The format efficiency may be improved by reducing the overhead. There are a number of contributors to overhead in the format of fixed block architecture disk drives. Some of these, such as synchronization fields, are required for reading the data. Others, such as error correcting codes (ECC) and sector servo, offset their overhead by allowing the areal density to be increased. One contributor to the overhead that does not increase the areal density is the header or ID field. Hitachi's new No-ID sector format eliminates the ID fields and all the information they contain from the track format, providing a marked improvement in the format efficiency.

ID functions (drive operation)
Figure 1 illustrates the track layout of a typical fixed block disk drive using embedded servo. Each track is divided into a number of data sectors and servo sectors. The servo fields contain the positioning information used to locate the head over a given track. The user data is stored in the data fields, each with an associated ID field. The ID fields contain information which identifies the data sector and other information, such as flags to indicate defective sectors (see figure 2).

The majority of disk drives manufactured today use an addressing scheme where the data sectors are identified to the host system by a logical block number (LBN). In operation, the host computer sends a list of logical block numbers to be written or read. The disk drive converts these values into zone, cylinder, head and sector (ZCHS) values. The servo system seeks to the desired zone, cylinder and head, and the disk drive begins reading ID fields until a match is found. Once the appropriate ID field has been read, the drive may then read or write the following data field.

ID field impact
The use of ID fields allows great flexibility in the format, and provides a simple mechanism for handling defects. However, there are substantial costs associated with the use of ID fields. The ID fields themselves can occupy up to 10% of a track -- space that would otherwise be used to store data. Further, since the disk drive must read the ID field for each sector prior to a read or write operation, additional space is required to allow for write-to-read recovery (w-r) prior to each ID field. Such w-r fields can occupy over 5% of a track.

Defect management
Defect management is typically accomplished by reserving a fixed number of spare sectors at some chosen interval (see figure 1). If a sector is determined to be defective, the data is relocated to one of the spare sectors. The relocation process ranges from shifting all the sectors between the defect and the spare, to using a specific spare sector to replace the defective sector. A performance degradation can occur if the sectors are not at their expected locations, requiring an additional seek operation. To reduce the likelihood of sector relocation, a large number of sectors are typically reserved as spares, which reduces the format efficiency.

The effect of digital channels
The use of PRML (partial response, maximum likelihood) digital data detection channels may result in increasing the size of ID fields. PRML channels allow disk drives to operate at higher linear densities. But, PRML digital channels may require replacing the cyclical redundancy checking (CRC) bytes typically used to detect ID field errors with a greater number of ECC bytes capable of correcting errors. Eliminating the ID fields from the format would solve all these problems.

No-ID sector format
Hitachi's No-ID sector format uses the servo control system to locate physical sectors, and a defect map stored in RAM to identify logical sectors. This allows the disk data controller to perform a new operation -- converting logical block numbers (LBN) to physical block numbers (PBN). Figure 3 illustrates the sequence of operation. The LBN is just a number from 0 to the number of addressable blocks on the disk drive. The PBN is a number from 0 to the number of physical blocks on the disk drive, but with the defective and spare sectors mapped out. Once the PBN is computed, it may be converted to the exact Zone/Cylinder/Head/Sector (ZCHS) value for the sector. Since the defect information is known in advance, the proper logical block is guaranteed to be located at the computed ZHCS. The defect map is stored in a compressed format, optimized for small size and rapid lookup. The servo system is used to locate the physical sector, based upon knowledge of the track formats in each zone. This information includes the locations of any data field splits due to embedded servo, which are also stored in RAM.

The No-ID sector format enhances disk drive reliability since the header and data field split information are stored in RAM, not on the disk. Current disk drives rely on CRC or ECC to reduce the vulnerability to errors in the ID fields. However, if the drive is unable to read an ID field, it may not be possible for it to recover the associated data sector.

The MR head connection

While the No-ID sector format provides a significant capacity improvement for disk drives employing metal-in-gap ferrite (MIG) or thin film inductive (TFI) heads, it provides an even greater advantage with magneto-resistive (MR) heads. MR heads provide higher areal density than MIG or thin film inductive heads. The areal density is achieved through increased linear density (bits per inch along a track -- BPI) and increased track density (number of tracks per inch -- TPI). Figure 4 shows the basic geometry of an MR head, as seen from the disk surface. The head consists of a thin film inductive write element (shown in red), and an MR read element (shown in green). The read element is typically narrower than the write element to improve the off-track performance. In practice, there is an offset between the center of the read and write elements due to the longitudinal separation of the elements. When used with a rotary actuator, the head is skewed with respect to the tracks as the actuator moves across the disk. The result is a lateral offset between the read and write head centerlines. Optimum performance is achieved by centering the read head over the data track for read operations, and centering the write head over the data track for write operations. This operation will cause the read head to be partially off-track during a write operation.

This offset presents a problem when ID fields are present, since they must be accurately read for both read and write operations. ID fields may be written partially off-track, requiring increased write width to ensure the read head can reliably read the ID field. The increased write width imposes limits on the track density. By removing the ID field, No-ID effectively eliminates the limit on track density. Since all the header and data field split information is stored in RAM, there is no information to be read from the disk relating to the data sector identities or locations. The servo system need only know the write-to-read element offset to center the write head on the track during write operations. Therefore, the lateral location of the read head is not important during a write operation.

Summary

Hitachi's No-ID sector format increases the capacity of disk drives by reducing the overhead and by allowing the MR head to be utilized to the fullest extent possible. Figure 5 shows a comparison between a typical sector format and a No-ID sector format. Since the format no longer imposes any constraints on the read and write head offsets, manufacturing yields for MR heads will be improved. The removal of offset ID fields allows the tracks to be placed closer together than in a traditional environment, resulting in greater capacity in the same space. Disk drive manufacturing yield is further enhanced by the advanced defect management capabilities. The performance is enhanced by the increased throughput (reduced overhead) and by the knowledge of the absolute sector locations. Power management is enhanced since there is no need to turn on the read electronics to read ID fields when searching for a sector. In this environment, No-ID with MR heads, a capacity increase of up to 30% can be achieved.

Dr. Steven R. Hetzler

Hitachi Research Division
Almaden Research Center
San Jose, California