Free Space Management

• Since disk is limited, we need to reuse it
• To keep track of free disk space, system maintains a free space list
• Free space list records all free disk blocks
• New file allocation is done amongst the free disk block
• When file is deleted, its disk space is added to free space list

Bit vector  – approach 1

• Frequently, free space list is maintained as a bit map or bit vector
• Each block is represented as 1 bit
  • If block is free, bit is 1
  • If block is allocated, bit is 0

     

• For eg. consider disk where blocks 2,3,5,7,8,10 are free and rest are allocated. The free space bit map would be 0110 1011 01
• Advantage – relatively simple and efficient to find first free block

Linked list – approach 2

• Free blocks are linked with each other
• First block contains pointer to next free block and so on
• Scheme is not efficient as list traversing needs substantial I/O time

Grouping- approach 3

• Store address of n free block in the first free block
• The first n-1 of these blocks are actually free
• The last block contains addresses of another n free blocks
• Address of large number of free blocks can now be found quickly

Counting- approach 4

• Generally several contiguous blocks are allocated or freed simultaneously
• So idea is to keep address of first block and number n of free contiguous blocks that follow first block
• Now each entry in free space list then consists of disk address and a count

Disk management

• A storage device mechanism by which the computer may store some information in such a way that this information may be retrieved at a later time
• Size of main memory is small to accommodate all data and programs and loses data when power is lost
• Operating system is responsible for:
  • free space management
  • storage allocation
  • disk scheduling
• In UNIX / Linux, every thing is consider as file
• For eg. if you have two hard disks then representation will be
  • /dev/hda  (primary master HDD)
  • /dev/hdb, (primary slave HDD)
• For floppy drive – /dev/fd0
• For CD ROM – /dev/cdrom
• For DVD-writer – /dev/dvdwriter
• USB mass storage – /dev/sda

Swapping

• Swap space management is a low level task of operating system
• The main goal for the design and implementation of swap space is to provide best throughput for the virtual memory system
• Swap device is a block device in configurable section of a disk
• Kernel allocates contiguous space on the swap device without fragmentation
• Free space of swap device is maintained in core table – map
• Kernel treats each unit of swap map as group of disk blocks
• As kernel allocates and frees resources, it updates map accordingly

Mapping process to swap device. Here, Swap out.