Page 253 - DCAP103_Principle of operating system

Principles of Operating Systems

Notes
Figure 7.6: MS-DOS Disk Layout

Sector 0 Boot block
Sector 1
FAT
Root directory

Data blocks
(subdirectories)

More sophisticated disks, such as the SCSI disks used in high-end PCs and most workstations
and servers, are smarter about bad-block recovery. The controller maintains a list of bad blocks
on the disk. The list is initialized during the low-level format at the factory, and is updated over
the life of the disk. Low level formatting also sets aside spare sectors not visible to the operating
system. The controller can be told to replace each bad sector logically with one of the spare
sectors. This scheme is known as sector sparing or forwarding. A typical bad-sector transaction
might be as follows: The operating system tries to read logical block 87. The controller calculates
the ECC and finds that the sector is bad. It reports this finding to the operating system. The
next time that the system is rebooted, a special command is run to tell the SCSI controller to
replace the bad sector with a spare. After that, whenever the system requests logical block 87, the
request is translated into the replacement sector’s address by the controller. Such a redirection
by the controller could invalidate any optimization by the operating system’s disk-scheduling
algorithm! For this reason, most disks are formatted to provide a few spare sectors in each
cylinder, and a spare cylinder as well. When a bad block is remapped, the controller uses a spare
sector from the same cylinder, if possible. As an alternative to sector sparing, some controllers
can be instructed to replace a bad block by sector slipping. Here is an example: Suppose that
logical block 17 becomes defective, and the first available spare follows sector 202. Then, sector
slipping would remap all the sectors from 17 to 202, moving them all down one spot. That is,
sector 202 would be copied into the spare, then sector 201 into 202, and then 200 into 201, and
so on, until sector 18 is copied into sector 19. Slipping the sectors in this way frees up the space
of sector 18, so sector 17 can be mapped to it. The replacement of a bad block generally is not a
totally automatic process because the data in the bad block are usually lost. Thus, whatever file
was using that block must be repaired (for instance, by restoration from a backup tape), and
that requires manual intervention.

The operating system tries to read logical block 87. The controller calculates
the ECC and finds that the sector is bad. It reports this finding to the operating
system.

7.6 Swap Space Management

Swap space is an area on a high-speed storage device (almost always a disk drive), reserved
for use by the virtual memory system for deactivation and paging processes. At least one swap

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