CS 537 Notes, Section #24: Unix and DEMOS Disk Allocation

Storage Management: For a given file, how the does OS find the blocks contained in that file? The data structure that decribes the contents of file is generically called a file descriptor. We will see several other names, as we study about file systems.

The file descriptor information has to be stored on disk, so it will stay around even when the OS does not.

• In Unix, all the descriptors are stored in a fixed size array on disk. The descriptors also contain protection and accounting information.
• A special area of disk is used for this (disk contains two parts: the fixed-size descriptor array, and the remainder, which is allocated for data and indirect blocks).
• The size of the descriptor array is determined when the disk is initialized, and cannot be changed. In Unix, the descriptor is called an i-node, and its index in the array is called its i-number. Internally, the OS uses the i-number to refer to the file.
• When a file is open, its descriptor is kept in main memory. When the file is closed, the descriptor is stored back to disk.

• File descriptors: 13 block pointers. The first 10 point to data blocks, the next three to indirect, doubly-indirect, and triply-indirect blocks (256 pointers in each indirect block). Maximum file length is fixed, but large. Descriptor space is not allocated until needed.
• Examples: block 23, block 5 block 340
• Free blocks: stored on a free list in no particular order.
• Go through examples of allocation and freeing.
• Advantages: simple, easy to implement, incremental expansion, easy access to small files.
• Drawbacks:
  • Indirect mechanism does not provide very efficient access to large files: 3 descriptor ops for each real operation. A cache is used, but this takes up main memory space.
  • Block-by-block organization of free list means that that file data gets spread around the disk.

The Demos solution: allocates files contiguously, has more compact file descriptors, uses more CPU time. (refer to contiguous allocation picture in section 26).

• File descriptors: select sequences of physical blocks, called block groups, rather than single blocks. Block groups were called extents by IBM.
• A block group has three fields:
  • Starting disk block: the starting address on disk of this block group,
  • Starting logical block: the starting block number within the file for the block group,
  • Count: the number of blocks in the group.
• There are 10 block groups in file descriptor; if files become large, then these become pointers to groups of indirect blocks. The resulting structure is like a B-tree.
• Free blocks: described with a bit map. Just an array of bits, one per block. 1 means block free, 0 means block allocated. For a 300 Mbyte drive there are about 300000 1kbyte blocks, so bit map takes up 40000 bytes. Keep only a small part of the bit map in memory at once. In allocation, scan bit map for adjacent free blocks.
• Advantages:
  • It is easy to allocate block groups, since the bit map automatically merges adjacent free blocks.
  • File descriptors take up less space on disk, require fewer accesses in random access to large files.
• Disadvantages:
  • Slightly more complex than Unix scheme: trades CPU time for disk access time (OK for CRAY-1).
  • When disk becomes full, this becomes VERY expensive, and does not get much in the way of adjacency.

Even if it is possible to allocate in groups, how do you know when to do it? Use past history: if file is already big, it will probably get bigger.