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# Free Space Management, Efficiency & Performance, Recovery and NFS

A short presentation on Free Space Management, Efficiency & Performance, Recovery and Network File System in operating system.

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### Free Space Management, Efficiency & Performance, Recovery and NFS

1. 1. Free Space Management, Efficiency & Performance, Recovery and NFS Department of Computer Science & Engineering Hamdard University Bangladesh
2. 2. Free Space Management To keep track of disk space : o The system maintains a free space list o The free space list records all disk blocks that are free
3. 3. Various Implementations of Free Space List  Bit Vector  Linked List  Grouping  Counting
4. 4. Bit Vector • Free space list is implemented as a bit map or bit vector • If the block is free the bit is set to 1 • If the block is allocated the bit is set to 0
5. 5. Bit vector (n blocks) … 0 1 2 n-1 bit[i] = 0  block[i] free 1  block[i] occupied Block number calculation (number of bits per word) * (number of 0-value words) + offset of first 1 bit Bit Vector
6. 6. Bit map requires extra space. Example: block size = 212 bytes (4 K) disk size = 230 bytes (1 gigabyte) n = 230/212 = 218 bits (or 32K bytes) Easy to get contiguous files Linked list (free list) Cannot get contiguous space easily No waste of space Grouping Large free blocks can be quickly found Counting Bit Vector
7. 7. Linked List  Link all free disk blocks together  Keep a pointer to the first free block  This block contains a pointer to next free block, and so on
8. 8. Linked Free Space List on Disk Example: Pointer to block 2, as the first free block Block 2 would contain a pointer to block 3, Which would point to block 4, Which would point 5,8,9,10,11,12,13,17,18,25,26,27
9. 9. Grouping  Store addresses of n free blocks in the first free block  The first n-1 of these are actually free  The last block contains the address as of another free blocks. Block No . Block No . . Block No Block No . Block No Block No Block No . . . Block No Block No . . . First Free block
10. 10. Counting  Based on the fact that several contiguous blocks may be allocated and freed simultaneously  Holds the address of the first free block and number “n” of free contiguous blocks that follow the first block  Each entry is the free space list consist of a disk address and a count
11. 11. Counting • Entry in a free space list consists of – Disk address – Count
12. 12. Efficiency and Performance  Efficiency dependent on:  disk allocation and directory algorithms  types of data kept in file’s directory entry  Performance  disk cache – separate section of main memory for frequently used 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.
13. 13. Recovery Techniques • Consistency checking – 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, other magnetic disk, optical) • Recover lost file or disk by restoring data from backup
14. 14.  An implementation and a specification of a software system for accessing remote files across LANs (or WANs).  The implementation is part of the Solaris and SunOS operating systems running on Sun workstations using an unreliable datagram protocol (UDP/IP protocol and Ethernet. Network File System (NFS)