hard disk defect management

The prevention of data errors in a hard disk by invalidating bad sectors. Built into the drive itself, defect tables are used to identify the bad sectors that are encountered when the drive is first made and in operation thereafter. The drive tables are stored in "system area tracks" that are not accessible by the user. Each drive vendor has its own proprietary table format; however SCSI drives have standard commands for reading defect tables.

First Test - The P-List
The P-list is a "permanent" or "primary" defect table that contains all the bad or marginal sectors found on each platter after testing by the manufacturer. P-list sectors are automatically bypassed by the drive electronics, and P-list sectors do not slow down drive access.

In Use - The G-List Table
A G-list is a "growth" defect table. As sectors become bad over time, they are added to the G-list, and reads and writes are automatically redirected (remapped) to spare sectors. G-list sectors do slow down drive access, and if the G-list is filling up, it is time to replace the drive.

Operating System Detection
The NetWare operating system, as well as ScanDisk and other Windows utilities, can detect and remap bad sectors on the fly that have not yet been invalidated by the drive hardware. Such remapping is stored in the drive's file system, providing an additional level of defect management.

If a sector goes bad between the last write and the next read, all operating systems will report the read error, which may halt the application.