lab5.md

lab key points

• key files

fs/fs.c	Code that mainipulates the file system's on-disk structure.
fs/bc.c	A simple block cache built on top of our user-level page fault handling facility.
fs/ide.c	Minimal PIO-based (non-interrupt-driven) IDE driver code.
fs/serv.c	The file system server that interacts with client environments using file system IPCs.
lib/fd.c	Code that implements the general UNIX-like file descriptor interface.
lib/file.c	The driver for on-disk file type, implemented as a file system IPC client.
lib/console.c	The driver for console input/output file type.
lib/spawn.c	Code skeleton of the spawn library call.

• file system (on-disk structure)

  • sector and block: 512 -> 4096bytes

  • superblocks

    struct Super {
    	uint32_t s_magic;		// Magic number: FS_MAGIC
    	uint32_t s_nblocks;		// Total number of blocks on disk
    	struct File s_root;		// Root directory node
    };
    

    

  • file meta-data: struct File

    struct File {
    	char f_name[MAXNAMELEN];	// filename
    	off_t f_size;			// file size in bytes
    	uint32_t f_type;		// file type
    
    	// Block pointers.
    	// A block is allocated iff its value is != 0.
    	uint32_t f_direct[NDIRECT];	// direct blocks
    	uint32_t f_indirect;		// indirect block
    
    	// Pad out to 256 bytes; must do arithmetic in case we're compiling
    	// fsformat on a 64-bit machine.
    	uint8_t f_pad[256 - MAXNAMELEN - 8 - 4*NDIRECT - 4];
    } __attribute__((packed));	// required only on some 64-bit machines
    

    

  • directory vs regular file

    unlike xv6, no special inode blocks; the tree-like hierarchy has to be deduced from cache in memory

    

• disk access priviledge: ENV_TYPE_FS; IOPL_3

• block cache

  • #define DISKMAP 0x10000000: disk block n mapped in memory

  • bc_pgfault: demand paging

    void
    bc_init(void)
    {
    	struct Super super;
    	set_pgfault_handler(bc_pgfault); // file server
    	...
    }
    

  • diskaddr(uint32_t blockno)

  • flush_block

• file operations

  workhorses of the file system

  • file_block_walk: maps from a block offset within a file to the pointer for that block in the struct File
  • file_get_block: maps to the actual disk block, allocating a new one if necessary

• file system interface

  • c/s mode

  • ipc mechanism

  • various structures & definitions: union Fsipc, FDTABLE, struct Dev, struct OpenFile

    Regular env           FS env
       +---------------+   +---------------+
       |      read     |   |   file_read   |
       |   (lib/fd.c)  |   |   (fs/fs.c)   |
    ...|.......|.......|...|.......^.......|...............
       |       v       |   |       |       | RPC mechanism
       |  devfile_read |   |  serve_read   |
       |  (lib/file.c) |   |  (fs/serv.c)  |
       |       |       |   |       ^       |
       |       v       |   |       |       |
       |     fsipc     |   |     serve     |
       |  (lib/file.c) |   |  (fs/serv.c)  |
       |       |       |   |       ^       |
       |       v       |   |       |       |
       |   ipc_send    |   |   ipc_recv    |
       |       |       |   |       ^       |
       +-------|-------+   +-------|-------+
               |                   |
               +-------------------+
    

• spawn process: spawn vs exec

  • state sharing between env through fork and spawn
    • PTE_SHARE: serve_open: *perm_store = PTE_P|PTE_U|PTE_W|PTE_SHARE;
    • file descriptor sharing: different env write to same file

• shell redirect: dup usage and its implementation tricks

jos file ipc

Questions

q1: in xv6, how does filewrite & writei ensure atomicity and durability or not?

Ans: from xv6 implementation, if filewrite was broken into several parts, then writei will only ensure consistency of each part

q2: xv6 file system

ubuntu version issue

excercise 2 check_super won't pass on ubuntu20, changed to ubuntu14-i386 it reported mmap /fs/clean-fs.img invalid argument error. Finally passed on ubuntu-16.04.6-desktop-i386

reference

ATA PIO Mode

Parallel ATA

knowledge

• storage dev