Lecture 24 - Filesystem Consistency

Every "reasonable" filesystem has some method of monitoring its own self-consistency (even chkdisk).

No matter what allocation policy is used for allocating disk blocks, it is necessary to keep track of which ones have been allocated. You can use:

• Linked list: when you need one block, search the list for an appropriate amount of space
• bitmap: do the same thing with a bitmap; uses less memory but may require more searching as the disk fills up

We can do the same thing for inodes if your filesystem has them.

To check them for consistency:

1. Create an array of counters for allocated and free blocks:
   
2. Sweep through the filesystem and free list and increment the aprpopriate counter each time a block is found.

If all goes well each block will be found in one of the lists:

If not there's a problem:

You may have the following problems:

• Missing block: the solution is to place it in the free list
• Allocated block in free list: remove it from the free list
• Block allocated twice: copy the block to a free one and hope things work out ok

3. Do the same thing for references counts from directories to inodes.

Possible problems:

• Referenced inode has incorrect link count: correct the count if this is the case
• unreferenced inode has nonzero link count: put the inode in lost and found (yes, really) and anything that links to it can check there as a last resort.