Virtual File System -Uniform interface for the OS to see different file systems.

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1 Virtual File System -Uniform interface for the OS to see different file systems. Temporary File Systems -Disks built in volatile storage NFS -file system addressed over network File Allocation -Contiguous Allocation -Linked Allocation -Indexed Allocation Contiguous Allocation -Files are laid out in neighboring blocks on the disk. -start block & Length Create a new file find a big enough contiguous chunk. Problem Need to know file size - External Fragmentation Linked Allocation Add a reference to the next block in each file block. -Last block has a Null Pointer. -Want to hand out block clusters File Allocation Table (FAT) DOS & Windows

2 Indexed Allocation Index Block -File Meta Data -Pointers to file blocks. References: -Direct: points to file block -Indirect: points to index block. Recovery: -What do we do if we allocate a directory entry but not a FCB. Consistency Checking -Check all directory entries (fsck) are linked to file control blocks and vice versa. -Ensure all blocks are free or attached to an FCB. If things are wrong -put them back if you can If you can t -Panic -log the errors -Let the operator fix it. Log Based Transactions (JFS) -Write changes to the file system out to a log. -Then execute the changes on disk

3 -Then Update the log. If there is a crash -Check which log entries are updated and do them. Disks -Non volatile storage -We have electronic/digital Data -Find some thing we can record this onto that doesn t degrade quickly. Disk System -Floppy media on a thin slice of plastic -Hard = media on a thin piece of glass. -Solid State DRAM with battery or flash. Disks attach to hosts: -Directly through a bus -Indirectly -Network attached storage -Storage area network Disk Organization -Partitioning -Logically break up the disk -Swap space -Disk Grouping -Make several disks act as one. RAID Redundant Array of inexpensive disks -Mirror our data -Stripe our data.

4 RAID2 Memory style error correction 3- bit inter leaved parity 4- Block interleaved parity 5- Block interleaved distributed parity 6- P + Q parity Traversal Time -How long to move the head to the cylinder Rotational Latency -how long for the disk to move under the head. Disk Scheduling -OS is responsible for scheduling read & Writes -keeps a queue of requests wants to serve them as efficiently as possible. Read/Write Queue:

5 SCAN Algorithm Says: Once the head is moving in a direction, keep moving that direction until you reach the end of the disk. C-Scan (circular): Only move the head in one direction when it reaches the edge of the disk, return to the beginning. Look and C-Look Like SCAN and C-SCAN except that you stop at the last job and reverse/ start over. Final Exam: 40 Questions 16 pages 2 hours 8am Monday Multiple choice 26 questions Code/psudeocode 1 Algorithm application 12 questions (deadlock, disk scheduling,process scheduling, paging). Discussion 1 Describe Operation system etc. There might be a question where you need to do binary calculation quickly. OS Concepts /Processes- 13 questions. Process Scheduling -6 (Algorithm and Theoretical) Memory Management 9 Questions. Concurrency/Deadlocks 8 questions File System/Disk Stuff 4 questions. Code question is going to be on concurrency question. Dining question is not coding question. Kernel Mode Protect Access -User =/= User mode. -Root =/= Kernel Hyper visor Type 1 Hypervisor directly runs hardware 2- a host OS supports the hardware.

6 Threads Execution =/= resources. resources: Address space + process stuff Execution: Stack, program counter, registers. Kernel mode vs. User mode threads User mode threads: No OS support. process is scheduled as normal. - one thread blocks, whole process blocks. - Don t need to syscall between threads. Kernel Mode: -OS support -OS schedules Threads -Syscall to work with threads -Doesn t block all threads in a process. Preemptive running process can be stopped. Non-preemptive running process cannot be stopped. Ideal Memory: -Infinitely fast -Infinitely Large -private -Nonvolatile. Can you find physical addresses from logical addresses when given page tables. Paging only copy when needed -Frames -Consistent sized memory chunks of physical memory. Pages chunks of logical memory. -fit into frames -load from disk or make new. Memory Manager Translates -Translation look at side buffer. Hardware solution: Memory cache on chip. Virtual Memory: -Address space -we can create a reference table space of a given size. -That doesn t relate to the available memory. We get as many addresses as we can point to. 4 bit address space MMU Maps to actual physical address