FreeSpace Management

1
Free-Space Management

• Need to protect
• Pointer to free list
• Bit map
• Must be kept on disk
• Copy in memory and disk may differ.
• Cannot allow for blocki to have a situation
  where biti 1 in memory and biti 0 on
  disk.
• Solution
• Set biti 1 in disk.
• Allocate blocki
• Set biti 1 in memory

2
Directory Implementation

• Linear list of file names with pointer to the
  data blocks.
• simple to program
• time-consuming to execute
• Hash Table linear list with hash data
  structure.
• decreases directory search time
• collisions situations where two file names hash
  to the same location
• fixed size

3
Efficiency and Performance

• Efficiency dependent on
• disk allocation and directory algorithms
• types of data kept in files directory entry
• Performance
• disk cache separate section of main memory for
  frequently sued blocks
• free-behind and read-ahead techniques to
  optimize sequential access
• improve PC performance by dedicating section of
  memory as virtual disk, or RAM disk.

4
File System Performance (1)

• The block cache data structures

5
File System Performance (2)

• I-nodes placed at the start of the disk
• Disk divided into cylinder groups
• each with its own blocks and i-nodes

6
Improving performance

• Disk is much slower than memory
• Controller has on-board memory for complete
  tracks - sectors and blocks are delivered
• Not so simple to use - multiprocessing...
• Readahead need one block ? read more
• Sequential access Readahead and Freebehind
• Prevent lost data
• periodic SYNC (write all buffers)
• write-through caches (MS-DOS)
• Free memory can be shared for cache uses of disk
  blocks and for memory pages (SUN Unix)

7
Various Disk-Caching Locations
8
Buffer Cache - Retrieval

• Five possible scenarios when the kernel retrieves
  a block
• 1. The block is found in its hash queue AND is
  free
• the buffer is marked busy
• buffer is removed from free list
• 2. No block is found in the hash queue - a free
  block is allocated from the free list
• 3. No block is found in the hash queue AND in
  searching the free list for a free block a
  delayed-write buffer is found (or more) -
  write delayed-write buffer(s) to disk, move them
  to head of list (LRU) and find a free buffer
• 4. No block is found in the hash queue AND free
  list empty block requesting process, when
  scheduled, go through hash-queue again
• 5. The block is found in its hash queue AND is
  busy process sleeps until a buffer is freed,
  then starts algorithm again..

9
Caching in Windows 2000

• The path through the cache to the hardware

10
Log-Structured File Systems

• With CPUs faster, memory larger
• disk caches can also be larger
• increasing number of read requests can come from
  cache
• thus, most disk accesses will be writes
• LFS Strategy structures entire disk as a log
• have all writes initially buffered in memory
• periodically write these to the end of the disk
  log
• when file opened, locate i-node, then find blocks

11
Recovery

• Consistency checker compares data in directory
  structure with data blocks on disk, and tries to
  fix inconsistencies.
• Use system programs to back up data from disk to
  another storage device (floppy disk, magnetic
  tape).
• Recover lost file or disk by restoring data from
  backup.

12
File System Reliability (1)
File that has not changed

• A file system to be dumped
• squares are directories, circles are files
• shaded items, modified since last dump
• each directory file labeled by i-node number

13
File System Reliability (2)

• Bit maps used by the logical dumping algorithm

14
File System Reliability (3)

• File system states
• (a) consistent
• (b) missing block
• (c) duplicate block in free list
• (d) duplicate data block