pSOSystem System Calls - Read

pHILE+ System Calls verify_vol
TABLE 2-3 Extent Map Faults
VF_EXTIN An indirect block contains an extent containing an illegal
block.
VF_INIX An index block contains an illegal indirect block number.
VF_LLBIX Within an index block, the LLB associated with an indirect
block is incorrect.
VF_DBDA A directory block resides in the data area of the volume.
VF_INDA An indirect block resides in the data area of the volume.
VF_IXDA An index block resides in the data area of the volume.
Bad Blocks
A bad block is a block that cannot be read and/or written and therefore cannot be
used by pHILE+ file system manager. There are a number of possible strategies for
handling bad blocks. One strategy is to mask or redirect them at the driver level so
that pHILE+ file system manager never sees them. Another method, which is
described here in detail, involves “mapping out” those blocks in the volume’s
bitmap, so that they are never allocated by pHILE+ file system manager.
verify_vol() facilitates such modifications to the bitmap.
Recall that each volume contains a bitmap describing which blocks on the volume
are in use, and which are free. If the corresponding bit in the map is set to 1, the
block is considered to be in use; otherwise, it is considered to be available for
allocation by the pHILE+ file system manager when needed. If the bit corresponding
to a bad block can be set to 1 before the block is allocated, then the pHILE+ file
system manager will never allocate the block, and hence will never read or write it.
To facilitate bad block handling, verify_vol() accepts as an input parameter a
list of bad blocks. When examining the volume’s bitmap, verify_vol() expects
bits corresponding to these bad blocks to be set to 1, while at the same time
expecting the block to be unused. If a bad block is in use, or its corresponding bit is
not set, a fault is generated.
The remainder of this section gives a brief outline of a recommended method for
handling bad blocks.
There are two types of bad blocks:
Dead Blocks — These blocks are known to be bad prior to volume initialization. They
are normally the result of manufacturing defects. Typically, the device
pSOSystem System Calls 2-139
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