pSOSystem System Calls - Read

verify_vol pHILE+ System Calls
Extent Map Faults
The faults listed in Table 2-3 involve errors in the extent map of a particular file.
Recall that the extent map consists of:
■ Up to 10 extent descriptors within the file's FD.
■ Within the file's FD, an indirect block descriptor that describes an indirect block
containing additional extent descriptors.
■ Within the file's FD, an index block descriptor that describes an index block
containing additional indirect block descriptors.
verify_vol() checks for illegal blocks both within an extent and within an
indirect or index block descriptor. A block is illegal if its block number is equal to or
greater than the number of blocks on the volume. For example, on a volume
containing 1000 blocks, any block number greater than 999 is illegal.
A file can be viewed as a sequence of logical blocks numbered from 0. For example,
on a volume with 1K blocks, a 4.3 Kbyte file would consist of logical blocks 0
through 4 (logical block 4 being only partly filled.)
Every FD, every indirect block descriptor, and every index block descriptor contains
a last logical block (LLB) field that indicates the largest logical block number
addressed by the associated structure. For example, an indirect block descriptor
may have LLB = 200, meaning that the last block in the last extent in the indirect
block is the 200th block in the file. verify_vol() checks the LLB of every FD,
indirect block descriptor, and index block descriptor and reports any
inconsistencies.
TABLE 2-3 Extent Map Faults
VF_EXTFD An FD contains an extent containing an illegal block.
VF_INFD An FD contains an illegal indirect block number.
VF_IXFD An FD contains an illegal index block number.
VF_TBCFD The block count within the FD conflicts with the actual
number of blocks in the file.
VF_LLBFD The LLB in the FD (for the first 10 extents) is incorrect.
VF_LLBIN The LLB within an indirect block descriptor within an FD is
incorrect.
2-138 pSOSystem System Calls