Disk Management

12.5 Disk Management

12.5.1 Disk Formatting

  • Before a disk can be used, it has to be low-level formatted, which means laying down all of the headers and trailers demarking the beginning and ends of each sector. Included in the header and trailer are the linear sector numbers, and error-correcting codes, ECC, which allow damaged sectors to not only be detected, but in many cases for the damaged data to be recovered ( depending on the extent of the damage. ) Sector sizes are traditionally 512 bytes, but may be larger, particularly in larger drives.
  • ECC calculation is performed with every disk read or write, and if damage is detected but the data is recoverable, then a soft error has occurred. Soft errors are generally handled by the on-board disk controller, and never seen by the OS. ( See below. )
  • Once the disk is low-level formatted, the next step is to partition the drive into one or more separate partitions. This step must be completed even if the disk is to be used as a single large partition, so that the partition table can be written to the beginning of the disk.
  • After partitioning, then the filesystems must be logically formatted, which involves laying down the master directory information ( FAT table or inode structure ), initializing free lists, and creating at least the root directory of the filesystem. ( Disk partitions which are to be used as raw devices are not logically formatted. This saves the overhead and disk space of the filesystem structure, but requires that the application program manage its own disk storage requirements. )

12.5.2 Boot Block

  • Computer ROM contains a bootstrap program ( OS independent ) with just enough code to find the first sector on the first hard drive on the first controller, load that sector into memory, and transfer control over to it. ( The ROM bootstrap program may look in floppy and/or CD drives before accessing the hard drive, and is smart enough to recognize whether it has found valid boot code or not. )
  • The first sector on the hard drive is known as the Master Boot Record, MBR, and contains a very small amount of code in addition to the partition table. The partition table documents how the disk is partitioned into logical disks, and indicates specifically which partition is the active or boot partition.
  • The boot program then looks to the active partition to find an operating system, possibly loading up a slightly larger / more advanced boot program along the way.
  • In a dual-boot ( or larger multi-boot ) system, the user may be given a choice of which operating system to boot, with a default action to be taken in the event of no response within some time frame.
  • Once the kernel is found by the boot program, it is loaded into memory and then control is transferred over to the OS. The kernel will normally continue the boot process by initializing all important kernel data structures, launching important system services ( e.g. network daemons, sched, init, etc. ), and finally providing one or more login prompts. Boot options at this stage may include single-user a.k.a. maintenance or safe modes, in which very few system services are started - These modes are designed for system administrators to repair problems or otherwise maintain the system.

12.5.3 Bad Blocks

  • No disk can be manufactured to 100% perfection, and all physical objects wear out over time. For these reasons all disks are shipped with a few bad blocks, and additional blocks can be expected to go bad slowly over time. If a large number of blocks go bad then the entire disk will need to be replaced, but a few here and there can be handled through other means.
  • In the old days, bad blocks had to be checked for manually. Formatting of the disk or running certain disk-analysis tools would identify bad blocks, and attempt to read the data off of them one last time through repeated tries. Then the bad blocks would be mapped out and taken out of future service. Sometimes the data could be recovered, and sometimes it was lost forever. ( Disk analysis tools could be either destructive or non-destructive. )
  • Modern disk controllers make much better use of the error-correcting codes, so that bad blocks can be detected earlier and the data usually recovered. ( Recall that blocks are tested with every write as well as with every read, so often errors can be detected before the write operation is complete, and the data simply written to a different sector instead. )
  • Note that re-mapping of sectors from their normal linear progression can throw off the disk scheduling optimization of the OS, especially if the replacement sector is physically far away from the sector it is replacing. For this reason most disks normally keep a few spare sectors on each cylinder, as well as at least one spare cylinder. Whenever possible a bad sector will be mapped to another sector on the same cylinder, or at least a cylinder as close as possible. Sector slipping may also be performed, in which all sectors between the bad sector and the replacement sector are moved down by one, so that the linear progression of sector numbers can be maintained.
  • If the data on a bad block cannot be recovered, then a hard error has occurred., which requires replacing the file(s) from backups, or rebuilding them from scratch.
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