1. 1 Project: File System Textbook: pages 501-506 Lubomir Bic

2. 2 Assignment Design and implement a simple file system using ldisk (a file to emulate a physical disk) Overall organization: File System I/O System (ldisk) user driver Input cr foo op foo wr 1 y 10 sk 1 0 rd 1 3 Output foo created foo opened 1 10 bytes written position is 0 yyy

3. 3 I/O System I/O system presents disk as a linear sequence of blocks: –ldisk[L][B] L is the number of logical blocks on ldisk B is the block length (in bytes) –implement as byte array I/O system interface: read_block(int i, char *p) write_block(int i, char *p) –each command reads or writes an entire block (B bytes) –memory area (*p) is also a byte array use type casting or conversion to r/w integers FS can access ldisk using only these functions (no direct access to ldisk is allowed)

4. 4 File System -- User Interface create(symbolic_file_name) destroy(symbolic_file_name) open(symbolic_file_name): return OFT index close(index) read(index, mem_area, count): return bytes read write(index, mem_area, count): return #bytes written lseek(index, pos) directory: return list of files init(file.txt): restore ldisk from file.txt or create new (if no file) save(file.txt): save ldisk to file.txt

5. 5 Organization of the file system directory –single flat list of all files –implemented as one regular file use regular file operations: read, write, lseek –organized as unsorted array of fixed-size slots –each slot contains: symbolic name (4 bytes max) index of descriptor (int)

6. 6 Organization of the file system file descriptors –kept in dedicated k disk blocks (ldisk[1..k]) –each contains: length (bytes), disk map –disk map: fixed list of max 3 disk blocks –descriptor 0 is reserved for directory free storage management –bit map –kept in dedicated disk block 0

7. 7 Organization of the file system … bit map file descriptors Descriptor for FS directory ldisk[0]...[k] ldisk[k+1]... Data blocks … length Each descriptor Block numbers File length in bytes

8. 8 Create a file cr abc –find a free file descriptor –find a free directory entry –fill both entries … free … lenbit map 0 …… lenbit map … free … abc … i … i0

9. 9 Destroy a file search directory to find file descriptor remove directory entry update bit map (if file was not empty) free file descriptor return status 0 …… lenbit map abc … i …

10. 10 Open a file search directory to find index of file descriptor (i) allocate a free OFT entry (reuse deleted entries) fill in current position (0) and file descriptor index (i) read block 0 of file into the r/w buffer (read-ahead) return OFT index (j) (or return error) consider adding a file length field (to simplify checking) OFT: current position index... j:  r/w buffer i0block 0... j:

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12. 12 Close a file write buffer to disk update file length in descriptor free OFT entry return status

13. 13 Read an (open) file compute position in the r/w buffer copy from buffer to memory until 1.desired count or end of file is reached: update current position, return status 2.end of buffer is reached write the buffer to disk read the next block continue copying

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15. 15 Write a file compute position in the r/w buffer copy from memory into buffer until 1.desired count or end of file is reached: update current pos, return status 2.end of buffer is reached if block does not exist yet (file is expanding): –allocate new block (search and update bit map) –update file descriptor with new block number write the buffer to disk block continue copying –update file length in descriptor

16. 16 Seek in a file if the new position is not within the current block –write the buffer to disk –read the new block set the current position to the new position return status

17. 17 List the directory read directory file for each non-empty entry print file name

18. 18 The Bit Map (pg 217) BM size: # of bits needed = # of ldisk blocks represent bit map as an array of int (32 bits each): BM[n] How to set, reset, and search for bits in BM? prepare a mask array: MASK[32] –diagonal contains “1”, all other fields are “0” –use bit operations (bitwise or/and) to manipulate bits

19. 19 The Bit Map MASK (assume 16 bits only) 010… 1010… 20010… 300010… …… 150 … 01 to set bit i of BM[j] to “1”: BM[j] = BM[j] | MASK[i]

20. 20 The Bit Map how to create MASK? MASK[0] = 0x8000 (1000 0000 0000 0000) MASK[1] = 0x4000 (0100 0000 0000 0000) MASK[2] = 0x2000 (0010 0000 0000 0000) MASK[3] = 0x1000 (0001 0000 0000 0000) MASK[4] = 0x0800 (0000 1000 0000 0000) … MASK[15] = 0x0001 (0000 0000 0000 0001) another approach: MASK[15] = 1; MASK[i] = MASK[i+1] <<

21. 21 The Bit Map to set a bit to “0”: –create MASK2, where MASK2[i] = ~MASK[i] e.g., 0010 0000 0000 0000  1101 1111 1111 1111 set bit i of BM[j] to “0”: BM[j] = BM[j] & MASK2[i]

22. 22 The Bit Map to search for a bit equal to “0” in BM: for (i=0; … /* search BM from the beginning for (j=0; … /* check each bit in BM[i] for “0” test = BM[i] & MASK[j]) if (test == 0) then bit j of BM[i] is “0”; stop search

23. Disk and FS Specifications ldisk: 64 blocks block = 64 B =16 integer block 0 holds bitmap: 64 bits (one per block) = 2 integers Q: how many blocks to reserve for descriptors? descriptor: 4 integers (file length plus 3 block #s) number of descriptors depends on directory size –each directory entry: 2 integers file name: maximum 4 chars, no extension (=1 int) descriptor index: 1 integer –directory size = 3 blocks = 3*64 B = 48 integers = 24 entries 24 descriptors = 24*4 = 96 integers = 6 blocks 23

24. Disk and FS Specifications ldisk can be saved into a text file at any point with the sv command ldisk can be restored from a previously saved text file a new empty ldisk is created if no saved file is given –it consists of 64 blocks –block 0 contains the initial bitmap –next 6 blocks contain the descriptor slots –slot 0 describes the empty directory directory is opened automatically with init (OFT index = 0) OFT has 4 entries: directory plus up to 3 other open files all files (including directory) must close with sv command 24

25. 25 Testing shell (driver) develop testing shell: –repeatedly accept command (e.g. cr abc ) from a file –invoke corresponding FS function (e.g. create(abc) ) –write status/data to an output file (e.g. abc created or error ) project will be tested using an input file containing multiple test sequences, each starting with the command in (initialize or restore disk)

26. 26 Shell commands and Output cr –Output : created de –Output : destroyed op –Output : opened cl –Output : closed rd –Output : wr –Output : bytes written

27. 27 Shell commands and Output sk –Output : position is dr –Output : … in –if file does not exist, output: disk initialized –if file does exist, output: disk restored sv –Output : disk saved If any command fails, output: error

28. Sample Interaction Input in cr foo op foo wr 1 x 60 wr 1 y 10 sk 1 55 rd 1 10 dr sv dsk.txt in dsk.txt op foo rd 1 3 cr foo Output disk initialized foo created foo opened 1 60 bytes written 10 bytes written position is 55 xxxxxyyyyy foo disk saved disk restored foo opened 1 xxx error 28

29. 29 Summary of tasks design and implement I/O interface (ldisk array plus read/write operations) design and implement FS using only read/write on ldisk develop test/presentation shell error checks on all commands submit documentation schedule testing