pSOSystem System Calls - Read

verify_vol pHILE+ System Calls
manufacturer provides a list of such blocks with each device. They can also be
detected by testing the device prior to its initialization.
Failed Blocks — These are blocks that fail some time after the volume has been
initialized. They are normally detected in the course of reading or writing the
affected block.
Dead blocks are much simpler to handle than failed blocks, because they are
detected before the pHILE+ file system manager has allocated them. To handle these
blocks, perform the following steps:
1. Initialize the volume, taking care not to place the bitmap or FLIST onto any dead
blocks (Note: blocks 2 and 3 must not be dead.)
2. Mount the volume and call verify_vol(), providing a bad block list
containing all dead blocks.
3. Have faultp() always return status = 1 (correct fault and continue) for
fdb_code == VF_BBFRE so that verify_vol() will mark the blocks as in use
(note that the “bad block is marked free” error is correctable).
Failed blocks are harder to handle because the block was already allocated by
pHILE+ file system manager before it failed. The block may or may not contain valid
data, depending on exactly when the failure occurred. However, since the block is
allocated, its corresponding bit is already set. To eliminate such bad blocks,
perform the following steps:
1. Add the failed block to the existing list of bad blocks.
2. Invoke verify_vol(), which will report VF_BBUSE, bad block is in use
by a particular file.
3. By whatever means, salvage as much of the file as possible, and then delete the
file. This returns the bad block to the free block pool and clears its
corresponding bit.
4. Invoke verify_vol() again. verify_vol() now reports VF_BBFRE, “bad
block is marked free”. Now have faultp() use return status = 1, to mark
the bad block as unavailable for allocation.
Step 3 may be complicated. For example, if a bad block occurs within a directory
page, then the entire directory must be deleted after saving as much of its contents
as possible.
2-140 pSOSystem System Calls