Mca ii os u-4 memory management
Download as PPT, PDF4 likes1,320 views
This document discusses virtual memory management. It begins with background on virtual memory and how it allows logical address spaces to be larger than physical memory. It describes demand paging, where pages are only loaded into memory when needed rather than all at once. Copy-on-write is explained as a way to share pages between processes initially. When memory runs out, page replacement algorithms are used to select pages to remove from memory. Optimizations like faster swap space I/O and demand paging from files rather than swapping are also covered.
Mca ii os u-4 memory management
- 2. Chapter 9: Virtual Memory Background Demand Paging Copy-on-Write Page Replacement Allocation of Frames Thrashing Memory-Mapped Files Allocating Kernel Memory Other Considerations Operating-System Examples
- 3. Objectives To describe the benefits of a virtual memory system To explain the concepts of demand paging, page-replacement algorithms, and allocation of page frames To discuss the principle of the working-set model To examine the relationship between shared memory and memory-mapped files To explore how kernel memory is managed
- 4. Background Code needs to be in memory to execute, but entire program rarely used Error code, unusual routines, large data structures Entire program code not needed at same time Consider ability to execute partially-loaded program Program no longer constrained by limits of physical memory Each program takes less memory while running -> more programs run at the same time Increased CPU utilization and throughput with no increase in response time or turnaround time Less I/O needed to load or swap programs into memory -> each user program runs faster
- 5. Background Virtual memory – separation of user logical memory from physical memory Only part of the program needs to be in memory for execution Logical address space can therefore be much larger than physical address space Allows address spaces to be shared by several processes Allows for more efficient process creation More programs running concurrently Less I/O needed to load or swap processes Virtual address space – logical view of how process is stored in memory Usually start at address 0, contiguous addresses until end of space Meanwhile, physical memory organized in page frames MMU must map logical to physical Virtual memory can be implemented via: Demand paging Demand segmentation
- 6. Virtual Memory That is Larger Than Physical Memory
- 7. Virtual-address Space Usually design logical address space for stack to start at Max logical address and grow “down” while heap grows “up” Maximizes address space use Unused address space between the two is hole No physical memory needed until heap or stack grows to a given new page Enables sparse address spaces with holes left for growth, dynamically linked libraries, etc System libraries shared via mapping into virtual address space Shared memory by mapping pages read-write into virtual address space Pages can be shared during fork(), speeding process creation
- 8. Shared Library Using Virtual Memory
- 9. Demand Paging Could bring entire process into memory at load time Or bring a page into memory only when it is needed Less I/O needed, no unnecessary I/O Less memory needed Faster response More users Similar to paging system with swapping (diagram on right) Page is needed ⇒ reference to it invalid reference ⇒ abort not-in-memory ⇒ bring to memory Lazy swapper – never swaps a page into memory unless page will be needed Swapper that deals with pages is a pager
- 10. Basic Concepts With swapping, pager guesses which pages will be used before swapping out again Instead, pager brings in only those pages into memory How to determine that set of pages? Need new MMU functionality to implement demand paging If pages needed are already memory resident No difference from non demand-paging If page needed and not memory resident Need to detect and load the page into memory from storage Without changing program behavior Without programmer needing to change code
- 11. Valid-Invalid Bit With each page table entry a valid–invalid bit is associated (v ⇒ in-memory – memory resident, i ⇒ not-in-memory) Initially valid–invalid bit is set to i on all entries Example of a page table snapshot: During MMU address translation, if valid–invalid bit in page table entry is i ⇒ page fault v v v v i i i …. Frame # valid-invalid bit page table
- 12. Page Table When Some Pages Are Not in Main Memory
- 13. Page Fault If there is a reference to a page, first reference to that page will trap to operating system: page fault 1. Operating system looks at another table to decide: Invalid reference ⇒ abort Just not in memory 1. Find free frame 2. Swap page into frame via scheduled disk operation 3. Reset tables to indicate page now in memory Set validation bit = v 4. Restart the instruction that caused the page fault
- 14. Steps in Handling a Page Fault
- 15. Aspects of Demand Paging Extreme case – start process with no pages in memory OS sets instruction pointer to first instruction of process, non-memory-resident -> page fault And for every other process pages on first access Pure demand paging Actually, a given instruction could access multiple pages -> multiple page faults Consider fetch and decode of instruction which adds 2 numbers from memory and stores result back to memory Pain decreased because of locality of reference Hardware support needed for demand paging Page table with valid / invalid bit Secondary memory (swap device with swap space) Instruction restart
- 16. Instruction Restart Consider an instruction that could access several different locations block move auto increment/decrement location Restart the whole operation? What if source and destination overlap?
- 17. Performance of Demand Paging Stages in Demand Paging (worse case) 1. Trap to the operating system 2. Save the user registers and process state 3. Determine that the interrupt was a page fault 4. Check that the page reference was legal and determine the location of the page on the disk 5. Issue a read from the disk to a free frame: 1. Wait in a queue for this device until the read request is serviced 2. Wait for the device seek and/or latency time 3. Begin the transfer of the page to a free frame 6. While waiting, allocate the CPU to some other user 7. Receive an interrupt from the disk I/O subsystem (I/O completed) 8. Save the registers and process state for the other user 9. Determine that the interrupt was from the disk 10. Correct the page table and other tables to show page is now in memory 11. Wait for the CPU to be allocated to this process again 12. Restore the user registers, process state, and new page table, and then resume the interrupted instruction
- 18. Performance of Demand Paging (Cont.) Three major activities Service the interrupt – careful coding means just several hundred instructions needed Read the page – lots of time Restart the process – again just a small amount of time Page Fault Rate 0 ≤ p ≤ 1 if p = 0 no page faults if p = 1, every reference is a fault Effective Access Time (EAT) EAT = (1 – p) x memory access + p (page fault overhead + swap page out + swap page in )
- 19. Demand Paging Example Memory access time = 200 nanoseconds Average page-fault service time = 8 milliseconds EAT = (1 – p) x 200 + p (8 milliseconds) = (1 – p x 200 + p x 8,000,000 = 200 + p x 7,999,800 If one access out of 1,000 causes a page fault, then EAT = 8.2 microseconds. This is a slowdown by a factor of 40!! If want performance degradation < 10 percent 220 > 200 + 7,999,800 x p 20 > 7,999,800 x p p < .0000025 < one page fault in every 400,000 memory accesses
- 20. Demand Paging Optimizations Swap space I/O faster than file system I/O even if on the same device Swap allocated in larger chunks, less management needed than file system Copy entire process image to swap space at process load time Then page in and out of swap space Used in older BSD Unix Demand page in from program binary on disk, but discard rather than paging out when freeing frame Used in Solaris and current BSD Still need to write to swap space Pages not associated with a file (like stack and heap) – anonymous memory Pages modified in memory but not yet written back to the file system Mobile systems Typically don’t support swapping Instead, demand page from file system and reclaim read-only pages (such as code)
- 21. Copy-on-Write Copy-on-Write (COW) allows both parent and child processes to initially share the same pages in memory If either process modifies a shared page, only then is the page copied COW allows more efficient process creation as only modified pages are copied In general, free pages are allocated from a pool of zero-fill-on-demand pages Pool should always have free frames for fast demand page execution Don’t want to have to free a frame as well as other processing on page fault Why zero-out a page before allocating it? vfork() variation on fork() system call has parent suspend and child using copy-on-write address space of parent Designed to have child call exec() Very efficient
- 22. Before Process 1 Modifies Page C
- 23. After Process 1 Modifies Page C
- 24. What Happens if There is no Free Frame? Used up by process pages Also in demand from the kernel, I/O buffers, etc How much to allocate to each? Page replacement – find some page in memory, but not really in use, page it out Algorithm – terminate? swap out? replace the page? Performance – want an algorithm which will result in minimum number of page faults Same page may be brought into memory several times
- 25. Page Replacement Prevent over-allocation of memory by modifying page-fault service routine to include page replacement Use modify (dirty) bit to reduce overhead of page transfers – only modified pages are written to disk Page replacement completes separation between logical memory and physical memory – large virtual memory can be provided on a smaller physical memory
- 26. Need For Page Replacement
- 27. Basic Page Replacement 1. Find the location of the desired page on disk 2. Find a free frame: - If there is a free frame, use it - If there is no free frame, use a page replacement algorithm to select a victim frame - Write victim frame to disk if dirty 3. Bring the desired page into the (newly) free frame; update the page and frame tables 4. Continue the process by restarting the instruction that caused the trap Note now potentially 2 page transfers for page fault – increasing EAT
- 28. Page Replacement
- 29. Page and Frame Replacement Algorithms Frame-allocation algorithm determines How many frames to give each process Which frames to replace Page-replacement algorithm Want lowest page-fault rate on both first access and re-access Evaluate algorithm by running it on a particular string of memory references (reference string) and computing the number of page faults on that string String is just page numbers, not full addresses Repeated access to the same page does not cause a page fault Results depend on number of frames available In all our examples, the reference string of referenced page numbers is 7,0,1,2,0,3,0,4,2,3,0,3,0,3,2,1,2,0,1,7,0,1
- 30. Graph of Page Faults Versus The Number of Frames
- 31. First-In-First-Out (FIFO) Algorithm Reference string: 7,0,1,2,0,3,0,4,2,3,0,3,0,3,2,1,2,0,1,7,0,1 3 frames (3 pages can be in memory at a time per process) Can vary by reference string: consider 1,2,3,4,1,2,5,1,2,3,4,5 Adding more frames can cause more page faults! Belady’s Anomaly How to track ages of pages? Just use a FIFO queue 15 page faults
- 32. FIFO Illustrating Belady’s Anomaly
- 33. Optimal Algorithm Replace page that will not be used for longest period of time 9 is optimal for the example How do you know this? Can’t read the future Used for measuring how well your algorithm performs
- 34. Least Recently Used (LRU) Algorithm Use past knowledge rather than future Replace page that has not been used in the most amount of time Associate time of last use with each page 12 faults – better than FIFO but worse than OPT Generally good algorithm and frequently used But how to implement?
- 35. LRU Algorithm (Cont.) Counter implementation Every page entry has a counter; every time page is referenced through this entry, copy the clock into the counter When a page needs to be changed, look at the counters to find smallest value Search through table needed Stack implementation Keep a stack of page numbers in a double link form: Page referenced: move it to the top requires 6 pointers to be changed But each update more expensive No search for replacement LRU and OPT are cases of stack algorithms that don’t have Belady’s Anomaly
- 36. Use Of A Stack to Record Most Recent Page References
- 37. LRU Approximation Algorithms LRU needs special hardware and still slow Reference bit With each page associate a bit, initially = 0 When page is referenced bit set to 1 Replace any with reference bit = 0 (if one exists) We do not know the order, however Second-chance algorithm Generally FIFO, plus hardware-provided reference bit Clock replacement If page to be replaced has Reference bit = 0 -> replace it reference bit = 1 then: – set reference bit 0, leave page in memory – replace next page, subject to same rules
- 38. Second-Chance (clock) Page-Replacement Algorithm
- 39. Enhanced Second-Chance Algorithm Improve algorithm by using reference bit and modify bit (if available) in concert Take ordered pair (reference, modify) 1. (0, 0) neither recently used not modified – best page to replace 2. (0, 1) not recently used but modified – not quite as good, must write out before replacement 3. (1, 0) recently used but clean – probably will be used again soon 4. (1, 1) recently used and modified – probably will be used again soon and need to write out before replacement When page replacement called for, use the clock scheme but use the four classes replace page in lowest non-empty class Might need to search circular queue several times
- 40. Counting Algorithms Keep a counter of the number of references that have been made to each page Not common Lease Frequently Used (LFU) Algorithm: replaces page with smallest count Most Frequently Used (MFU) Algorithm: based on the argument that the page with the smallest count was probably just brought in and has yet to be used
- 41. Page-Buffering Algorithms Keep a pool of free frames, always Then frame available when needed, not found at fault time Read page into free frame and select victim to evict and add to free pool When convenient, evict victim Possibly, keep list of modified pages When backing store otherwise idle, write pages there and set to non-dirty Possibly, keep free frame contents intact and note what is in them If referenced again before reused, no need to load contents again from disk Generally useful to reduce penalty if wrong victim frame selected
- 42. Applications and Page Replacement All of these algorithms have OS guessing about future page access Some applications have better knowledge – i.e. databases Memory intensive applications can cause double buffering OS keeps copy of page in memory as I/O buffer Application keeps page in memory for its own work Operating system can given direct access to the disk, getting out of the way of the applications Raw disk mode Bypasses buffering, locking, etc
- 43. Allocation of Frames Each process needs minimum number of frames Example: IBM 370 – 6 pages to handle SS MOVE instruction: instruction is 6 bytes, might span 2 pages 2 pages to handle from 2 pages to handle to Maximum of course is total frames in the system Two major allocation schemes fixed allocation priority allocation Many variations
- 44. Fixed Allocation Equal allocation – For example, if there are 100 frames (after allocating frames for the OS) and 5 processes, give each process 20 frames Keep some as free frame buffer pool Proportional allocation – Allocate according to the size of process Dynamic as degree of multiprogramming, process sizes change m S s pa m sS ps i ii i ii ×== = ∑= = forallocation framesofnumbertotal processofsize m =64 s1 =10 s2 =127 a1 = 10 137 ×62 ≈ 4 a2 = 127 137 ×62 ≈ 57
- 45. Priority Allocation Use a proportional allocation scheme using priorities rather than size If process Pi generates a page fault, select for replacement one of its frames select for replacement a frame from a process with lower priority number
- 46. Global vs. Local Allocation Global replacement – process selects a replacement frame from the set of all frames; one process can take a frame from another But then process execution time can vary greatly But greater throughput so more common Local replacement – each process selects from only its own set of allocated frames More consistent per-process performance But possibly underutilized memory
- 47. Non-Uniform Memory Access So far all memory accessed equally Many systems are NUMA – speed of access to memory varies Consider system boards containing CPUs and memory, interconnected over a system bus Optimal performance comes from allocating memory “close to” the CPU on which the thread is scheduled And modifying the scheduler to schedule the thread on the same system board when possible Solved by Solaris by creating lgroups Structure to track CPU / Memory low latency groups Used my schedule and pager When possible schedule all threads of a process and allocate all memory for that process within the lgroup
- 48. Thrashing If a process does not have “enough” pages, the page-fault rate is very high Page fault to get page Replace existing frame But quickly need replaced frame back This leads to: Low CPU utilization Operating system thinking that it needs to increase the degree of multiprogramming Another process added to the system Thrashing ≡ a process is busy swapping pages in and out
- 50. Demand Paging and Thrashing Why does demand paging work? Locality model Process migrates from one locality to another Localities may overlap Why does thrashing occur? Σ size of locality > total memory size Limit effects by using local or priority page replacement
- 51. Selecting a Disk-Scheduling Algorithm SSTF is common and has a natural appeal SCAN and C-SCAN perform better for systems that place a heavy load on the disk Less starvation Performance depends on the number and types of requests Requests for disk service can be influenced by the file-allocation method And metadata layout The disk-scheduling algorithm should be written as a separate module of the operating system, allowing it to be replaced with a different algorithm if necessary Either SSTF or LOOK is a reasonable choice for the default algorithm What about rotational latency? Difficult for OS to calculate How does disk-based queueing effect OS queue ordering efforts?
- 52. Disk Management Low-level formatting, or physical formatting — Dividing a disk into sectors that the disk controller can read and write Each sector can hold header information, plus data, plus error correction code (ECC) Usually 512 bytes of data but can be selectable To use a disk to hold files, the operating system still needs to record its own data structures on the disk Partition the disk into one or more groups of cylinders, each treated as a logical disk Logical formatting or “making a file system” To increase efficiency most file systems group blocks into clusters Disk I/O done in blocks File I/O done in clusters Raw disk access for apps that want to do their own block management, keep OS out of the way (databases for example) Boot block initializes system The bootstrap is stored in ROM Bootstrap loader program stored in boot blocks of boot partition Methods such as sector sparing used to handle bad blocks
- 53. Booting from a Disk in Windows
- 54. Swap-Space Management Swap-space — Virtual memory uses disk space as an extension of main memory Less common now due to memory capacity increases Swap-space can be carved out of the normal file system, or, more commonly, it can be in a separate disk partition (raw) Swap-space management 4.3BSD allocates swap space when process starts; holds text segment (the program) and data segment Kernel uses swap maps to track swap-space use Solaris 2 allocates swap space only when a dirty page is forced out of physical memory, not when the virtual memory page is first created File data written to swap space until write to file system requested Other dirty pages go to swap space due to no other home Text segment pages thrown out and reread from the file system as needed What if a system runs out of swap space? Some systems allow multiple swap spaces
- 55. Data Structures for Swapping on Linux Systems
- 56. RAID Structure RAID – redundant array of inexpensive disks multiple disk drives provides reliability via redundancy Increases the mean time to failure Mean time to repair – exposure time when another failure could cause data loss Mean time to data loss based on above factors If mirrored disks fail independently, consider disk with 1300,000 mean time to failure and 10 hour mean time to repair Mean time to data loss is 100, 0002 / (2 10) = 500 10∗ ∗ 6 hours, or 57,000 years! Frequently combined with NVRAM to improve write performance RAID is arranged into six different levels
- 57. RAID (Cont.) Several improvements in disk-use techniques involve the use of multiple disks working cooperatively Disk striping uses a group of disks as one storage unit RAID schemes improve performance and improve the reliability of the storage system by storing redundant data Mirroring or shadowing (RAID 1) keeps duplicate of each disk Striped mirrors (RAID 1+0) or mirrored stripes (RAID 0+1) provides high performance and high reliability Block interleaved parity (RAID 4, 5, 6) uses much less redundancy RAID within a storage array can still fail if the array fails, so automatic replication of the data between arrays is common Frequently, a small number of hot-spare disks are left unallocated, automatically replacing a failed disk and having data rebuilt onto them
- 58. RAID Levels
- 59. RAID (0 + 1) and (1 + 0)
- 60. Other Features Regardless of where RAID implemented, other useful features can be added Snapshot is a view of file system before a set of changes take place (i.e. at a point in time) More in Ch 12 Replication is automatic duplication of writes between separate sites For redundancy and disaster recovery Can be synchronous or asynchronous Hot spare disk is unused, automatically used by RAID production if a disk fails to replace the failed disk and rebuild the RAID set if possible Decreases mean time to repair
- 61. Extensions RAID alone does not prevent or detect data corruption or other errors, just disk failures Solaris ZFS adds checksums of all data and metadata Checksums kept with pointer to object, to detect if object is the right one and whether it changed Can detect and correct data and metadata corruption ZFS also removes volumes, partitions Disks allocated in pools Filesystems with a pool share that pool, use and release space like malloc() and free() memory allocate / release calls
- 62. ZFS Checksums All Metadata and Data
- 63. Traditional and Pooled Storage
- 64. General Graph Directory (Cont.) How do we guarantee no cycles? Allow only links to file not subdirectories Garbage collection Every time a new link is added use a cycle detection algorithm to determine whether it is OK
- 65. File-System Structure File structure Logical storage unit Collection of related information File system resides on secondary storage (disks) Provided user interface to storage, mapping logical to physical Provides efficient and convenient access to disk by allowing data to be stored, located retrieved easily Disk provides in-place rewrite and random access I/O transfers performed in blocks of sectors (usually 512 bytes) File control block – storage structure consisting of information about a file Device driver controls the physical device File system organized into layers
- 67. File System Layers Device drivers manage I/O devices at the I/O control layer Given commands like “read drive1, cylinder 72, track 2, sector 10, into memory location 1060” outputs low- level hardware specific commands to hardware controller Basic file system given command like “retrieve block 123” translates to device driver Also manages memory buffers and caches (allocation, freeing, replacement) Buffers hold data in transit Caches hold frequently used data File organization module understands files, logical address, and physical blocks Translates logical block # to physical block # Manages free space, disk allocation
- 68. File System Layers (Cont.) Logical file system manages metadata information Translates file name into file number, file handle, location by maintaining file control blocks (inodes in Unix) Directory management Protection Layering useful for reducing complexity and redundancy, but adds overhead and can decrease performance Logical layers can be implemented by any coding method according to OS designer Many file systems, sometimes many within an operating system Each with its own format (CD-ROM is ISO 9660; Unix has UFS, FFS; Windows has FAT, FAT32, NTFS as well as floppy, CD, DVD Blu-ray, Linux has more than 40 types, with extended file system ext2 and ext3 leading; plus distributed file systems, etc) New ones still arriving – ZFS, GoogleFS, Oracle ASM, FUSE
- 69. File-System Implementation We have system calls at the API level, but how do we implement their functions? On-disk and in-memory structures Boot control block contains info needed by system to boot OS from that volume Needed if volume contains OS, usually first block of volume Volume control block (superblock, master file table) contains volume details Total # of blocks, # of free blocks, block size, free block pointers or array Directory structure organizes the files Names and inode numbers, master file table Per-file File Control Block (FCB) contains many details about the file inode number, permissions, size, dates NFTS stores into in master file table using relational DB structures
- 70. In-Memory File System Structures Mount table storing file system mounts, mount points, file system types The following figure illustrates the necessary file system structures provided by the operating systems Figure 12-3(a) refers to opening a file Figure 12-3(b) refers to reading a file Plus buffers hold data blocks from secondary storage Open returns a file handle for subsequent use Data from read eventually copied to specified user process memory address
- 71. In-Memory File System Structures
- 72. Partitions and Mounting Partition can be a volume containing a file system (“cooked”) or raw – just a sequence of blocks with no file system Boot block can point to boot volume or boot loader set of blocks that contain enough code to know how to load the kernel from the file system Or a boot management program for multi-os booting Root partition contains the OS, other partitions can hold other Oses, other file systems, or be raw Mounted at boot time Other partitions can mount automatically or manually At mount time, file system consistency checked Is all metadata correct? If not, fix it, try again If yes, add to mount table, allow access
- 73. Virtual File Systems Virtual File Systems (VFS) on Unix provide an object-oriented way of implementing file systems VFS allows the same system call interface (the API) to be used for different types of file systems Separates file-system generic operations from implementation details Implementation can be one of many file systems types, or network file system Implements vnodes which hold inodes or network file details Then dispatches operation to appropriate file system implementation routines The API is to the VFS interface, rather than any specific type of file system
- 74. Virtual File System Implementation For example, Linux has four object types: inode, file, superblock, dentry VFS defines set of operations on the objects that must be implemented Every object has a pointer to a function table Function table has addresses of routines to implement that function on that object For example: • int open(. . .)—Open a file • int close(. . .)—Close an already-open file • ssize t read(. . .)—Read from a file • ssize t write(. . .)—Write to a file • int mmap(. . .)—Memory-map a file
- 75. Directory Implementation Linear list of file names with pointer to the data blocks Simple to program Time-consuming to execute Linear search time Could keep ordered alphabetically via linked list or use B+ tree Hash Table – linear list with hash data structure Decreases directory search time Collisions – situations where two file names hash to the same location Only good if entries are fixed size, or use chained-overflow method
- 76. Allocation Methods - Contiguous An allocation method refers to how disk blocks are allocated for files: Contiguous allocation – each file occupies set of contiguous blocks Best performance in most cases Simple – only starting location (block #) and length (number of blocks) are required Problems include finding space for file, knowing file size, external fragmentation, need for compaction off-line (downtime) or on-line
- 77. Contiguous Allocation Mapping from logical to physical LA/512 Q R Block to be accessed = Q + starting address Displacement into block = R
- 78. Extent-Based Systems Many newer file systems (i.e., Veritas File System) use a modified contiguous allocation scheme Extent-based file systems allocate disk blocks in extents An extent is a contiguous block of disks Extents are allocated for file allocation A file consists of one or more extents
- 79. Allocation Methods - Linked Linked allocation – each file a linked list of blocks File ends at nil pointer No external fragmentation Each block contains pointer to next block No compaction, external fragmentation Free space management system called when new block needed Improve efficiency by clustering blocks into groups but increases internal fragmentation Reliability can be a problem Locating a block can take many I/Os and disk seeks FAT (File Allocation Table) variation Beginning of volume has table, indexed by block number Much like a linked list, but faster on disk and cacheable New block allocation simple
- 80. Linked Allocation Each file is a linked list of disk blocks: blocks may be scattered anywhere on the disk pointerblock =
- 81. Linked Allocation Mapping Block to be accessed is the Qth block in the linked chain of blocks representing the file. Displacement into block = R + 1 LA/511 Q R
- 84. Allocation Methods - Indexed Indexed allocation Each file has its own index block(s) of pointers to its data blocks Logical view index table
- 85. Example of Indexed Allocation
- 86. Indexed Allocation (Cont.) Need index table Random access Dynamic access without external fragmentation, but have overhead of index block Mapping from logical to physical in a file of maximum size of 256K bytes and block size of 512 bytes. We need only 1 block for index table LA/512 Q R Q = displacement into index table R = displacement into block
- 87. Indexed Allocation – Mapping (Cont.) Mapping from logical to physical in a file of unbounded length (block size of 512 words) Linked scheme – Link blocks of index table (no limit on size) LA / (512 x 511) Q1 R1 Q1 = block of index table R1 is used as follows: R1 / 512 Q2 R2 Q2 = displacement into block of index table R2 displacement into block of file:
- 88. Indexed Allocation – Mapping (Cont.) Two-level index (4K blocks could store 1,024 four-byte pointers in outer index -> 1,048,567 data blocks and file size of up to 4GB) LA / (512 x 512) Q1 R1 Q1 = displacement into outer-index R1 is used as follows: R1 / 512 Q2 R2 Q2 = displacement into block of index table R2 displacement into block of file:
- 89. Combined Scheme: UNIX UFS (4K bytes per block, 32-bit addresses) Note: More index blocks than can be addressed with 32-bit file pointer
- 90. I/O Without a Unified Buffer Cache
- 91. Unified Buffer Cache A unified buffer cache uses the same page cache to cache both memory-mapped pages and ordinary file system I/O to avoid double caching But which caches get priority, and what replacement algorithms to use?
- 92. I/O Using a Unified Buffer Cache
- 93. Recovery Consistency checking – compares data in directory structure with data blocks on disk, and tries to fix inconsistencies Can be slow and sometimes fails Use system programs to back up data from disk to another storage device (magnetic tape, other magnetic disk, optical) Recover lost file or disk by restoring data from backup
- 94. Log Structured File Systems Log structured (or journaling) file systems record each metadata update to the file system as a transaction All transactions are written to a log A transaction is considered committed once it is written to the log (sequentially) Sometimes to a separate device or section of disk However, the file system may not yet be updated The transactions in the log are asynchronously written to the file system structures When the file system structures are modified, the transaction is removed from the log If the file system crashes, all remaining transactions in the log must still be performed Faster recovery from crash, removes chance of inconsistency of metadata
- 95. REFRENCES Operating System Concepts – 9th Edition By Silberchatz, Galvin and Gagne