1. 11.1 Silberschatz, Galvin and Gagne ©2005 Operating System Principles 11.5 Free-Space Management Bit vector (n blocks) … 012n-1 bit[i] =  1  block[i] free 0  block[i] occupied The first non-0 word (one word may be 8, 16, or 32 bits) is scanned to find the first 1 bit, which is the location of the first free block. Its block number calculation is: (number of bits per word) *(number of 0-value words) +offset of first 1 bit

2. 11.2 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Free-Space Management Bit map requires extra space Example: block size = 2 9 bytes = 512 bytes disk size = 2 30 bytes (1 gigabyte) n = 2 30 /2 9 =2 21 bits (or 2 18 bytes = 2 8 K bytes = 256K bytes) Example: block size = 1024 bytes = 2 10 bytes disk size = 40 GB =10 * 2 32 bytes n =10*2 32 / 2 10 = 10* 2 22 bits (or 10*2 19 bytes = 5*2*2 19 bytes = 5 * 2 20 bytes = 5M bytes) Clustering the blocks in groups of four reduces this number to 64KB Easy to get contiguous files

3. 11.3 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Free-Space Management Linked list (free list) Linking together all the free disk blocks, keeping a pointer to the first free block in a special location on the disk and cache it in memory. The first block contains a pointer to the next free disk block, and so on. Cannot get contiguous space easily No waste of space

4. 11.4 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Free-Space Management Grouping A modification of the linked free-list Stores the address of n free blocks in the first free block. The first n-1 of these blocks are actually free The last block contains the address of another n free blocks, and so on. The addresses of a large number of free blocks can be found quickly now. Counting A modification of the linked free-list Keeps the address of the first free block and the number n of free contiguous blocks that follow the first block. Each entry in the free space list then consists of a disk address and a count. Useful when space is allocated with the contiguous-allocation algorithm or through clustering

5. 11.5 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Free-Space Management (extra material) Need to protect: Pointer to free list Bit map  Must be kept on disk  Copy in memory and disk may differ  Cannot allow for block[i] to have a situation where bit[i] = 1 in memory and bit[i] = 0 on disk Solution:  Set bit[i] = 1 in disk  Allocate block[i]  Set bit[i] = 1 in memory

6. 11.6 Silberschatz, Galvin and Gagne ©2005 Operating System Principles 11.6 Efficiency and Performance Efficiency dependent on: disk allocation and directory algorithms types of data kept in file’s directory entry In UNIX, the file system’s performance is improved by pre-allocating the inodes and spreading them across the volume, and by keeping a file’s data blocks near that file’s inode block to reduce seek time. BSD UNIX varies the cluster size as a file grows to reduce internal fragmentation. Consideration of kept data in a file’s directory entry Last write date, last access date Size of pointers in the allocation list will limit the length of a file Need to plan for changing technology (FAT from 12 to16 to 32) Early Solaris needs to reboot the system to change system table sizes

7. 11.7 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Efficiency and Performance Performance Most disk controllers include local memory to form on-board cache that is large enough to store entire tracks at a time Buffer cache – separate section of main memory for blocks that will be used shortly A page cache caches pages rather than disk blocks using virtual memory techniques free-behind and read-ahead – techniques to optimize sequential access  Free-behind removes a page from the buffer as soon as the next page is requested  With read-ahead, a requested page and several subsequent pages are read and cached improve PC performance by dedicating section of memory as virtual disk, or RAM disk

8. 11.8 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Page Cache Memory-mapped I/O uses a page cache Routine I/O through the file system uses the buffer (disk) cache unified virtual memory: Many OS’s use page caching to cache both process pages and file data This leads to the following figure

9. 11.9 Silberschatz, Galvin and Gagne ©2005 Operating System Principles I/O Without a Unified Buffer Cache A unified buffer cache uses the same page cache to cache both memory-mapped pages and ordinary file system I/O I/O using a Unified Buffer Cache double caching

10. 11.10 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Issues Whether writes to the file system occur synchronously or asynchronously Synchronous writes: the calling routine must wait for the data to reach the disk drive before it can proceed Asynchronous writes: done the majority of the time. Data are stored in the cache, and control return to the caller. Metadata writes can be synchronous. Interactions among the page cache, the file system, and the disk drivers When data are written to a disk file, the pages are buffered in the cache, and the disk driver sorts its output queue according to disk address, to minimize disk-head seeks and to write data at times optimized for disk rotation Thus, output to the disk through the file system is often faster than input for large transfers Skip: last paragraph of p. 417

11. 11.11 Silberschatz, Galvin and Gagne ©2005 Operating System Principles 10.7 Recovery Consistency checking – compares data in directory structure with data blocks on disk, and tries to fix inconsistencies fsck in UNIX, chkdsk in MS-DOS A special program is run at reboot time to check for and correct disk inconsistencies The allocation and free-space management algorithms dictate what type of problems the checker can find and how successful it will fix them Use system programs to back up data from disk to another storage device (floppy disk, magnetic tape, other magnetic disk, optical) Full back and incremental backup Full back and each day back up all files that have changed since the full backup Full back may have to saved forever Recover lost file or disk by restoring data from backup

12. 11.12 Silberschatz, Galvin and Gagne ©2005 Operating System Principles 10.8 Log Structured File Systems Log-based transaction-oriented (or journaling) file systems record each update to the file system as a transaction To avoid irreparable inconsistency, all metadata changes are written sequentially to a log. Each set of operations for performing a specific task is a transaction. A transaction is considered committed once it is written to the log, and the system call can return to the user process However, the file system may not yet be updated. These log entries are replayed across the actual file system structures.

13. 11.13 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Log Structured File Systems The transactions in the log are asynchronously written to the file system When the file system is modified, the transaction is removed from the log file, which is actually a circular buffer If the file system crashes, all remaining transactions in the log must still be performed When a transaction was aborted, not committed before the system crashed, any changes from such a transaction to the file system must be undone. The costly synchronous random metadata writes are turned into much less costly synchronous sequential writes to the logging area. They are replayed asynchronously via random writes to the appropriate structures. So, file creation and deletion are performed much faster.