6.828 笔记11 —— File System

date: 2019-03-25
tags: OS 6.828

这里会记录阅读6.828课程lecture note的我的个人笔记。可能会中英混杂，不是很适合外人阅读，也请见谅。

Lecture 12: File System

我们为什么需要file system

durability across restart
naming and organization
sharing among programs and users

file system有哪些有趣的点？

crash memory
performance
sharing
security
各种抽象

xv6实现

之后我们来看一下xv6的实现。因为lecture note里头的东西太乱了，不妨我们直接看xv6 book中对应的内容。

xv6 file system architecture

xv6的fs实现分为7层。从下向上其顺序和功能如下：

disk layer: 读写IDE hard drive。
buffer cache layer: 做cache存储经常访问的模块，并把并行访问处理成串行访问（感觉有点像database?）
logging layer: 提供上述layer的wrapper，从而将多个block的更新分成transaction，并确保crash recovery
inode layer: 每个individual files都被表示为一个inode，并有唯一标识符i-number，以及一些block来存储数据。
directory layer: 地址，也有i-number和file name
pathname layer: 提供像/usr/rtm/xv6/fs.c/usr/rtm/xv6/fs.c这样的地址接口
file descriptor layer: 抽象一些unix资源，让他们也能使用file system interface，如pipes, devices...

Buffer layer

buffer layer的代码位于bio.c其主要接口为bwrite和bread。

// Return a locked buf with the contents of the indicated block.
struct buf*
bread(uint dev, uint blockno) {
  struct buf *b;

  b = bget(dev, blockno);
  if((b->flags & B_VALID) == 0) {
    iderw(b);
  }
  return b;
}

// Write b's contents to disk.  Must be locked.
void
bwrite(struct buf *b)
{
  if(!holdingsleep(&b->lock))
    panic("bwrite");
  b->flags |= B_DIRTY;
  iderw(b);
}

更深入一点的话，buf的结构和bget的实现如下：

// buf.h
struct buf {
  int flags;
  uint dev;
  uint blockno;
  struct sleeplock lock;
  uint refcnt;
  struct buf *prev; // LRU cache list
  struct buf *next;
  struct buf *qnext; // disk queue
  uchar data[BSIZE];
};
#define B_VALID 0x2  // buffer has been read from disk
#define B_DIRTY 0x4  // buffer needs to be written to disk

// bio.c
// Look through buffer cache for block on device dev.
// If not found, allocate a buffer.
// In either case, return locked buffer.
static struct buf*
bget(uint dev, uint blockno) {
  struct buf *b;

  acquire(&bcache.lock);

  // Is the block already cached?
  for(b = bcache.head.next; b != &bcache.head; b = b->next){
    if(b->dev == dev && b->blockno == blockno){
      b->refcnt++;
      release(&bcache.lock);
      acquiresleep(&b->lock);
      return b;
    }
  }

  // Not cached; recycle an unused buffer.
  // Even if refcnt==0, B_DIRTY indicates a buffer is in use
  // because log.c has modified it but not yet committed it.
  for(b = bcache.head.prev; b != &bcache.head; b = b->prev){
    if(b->refcnt == 0 && (b->flags & B_DIRTY) == 0) {
      b->dev = dev;
      b->blockno = blockno;
      b->flags = 0;
      b->refcnt = 1;
      release(&bcache.lock);
      acquiresleep(&b->lock);
      return b;
    }
  }
  panic("bget: no buffers");
}

注意这里bget就是从链表里头取一个空的，并不会做MRU (most recent used)的删除操作，如果没有空的就报错。而这个释放的操作是用brelse来进行的。

// Release a locked buffer.
// Move to the head of the MRU list.
void
brelse(struct buf *b) {
  if(!holdingsleep(&b->lock))
    panic("brelse");

  releasesleep(&b->lock);

  acquire(&bcache.lock);
  b->refcnt--;
  if (b->refcnt == 0) {
    // no one is waiting for it.
    b->next->prev = b->prev;
    b->prev->next = b->next;
    b->next = bcache.head.next;
    b->prev = &bcache.head;
    bcache.head.next->prev = b;
    bcache.head.next = b;
  }
  
  release(&bcache.lock);
}

大致就是把引用计数清空，并把这个buf移到链表最前面。

Lecture 13: Crash Recovery, Logging

常见的一个crash recovery的方法是写logging。我们先来看一下xv6的logging，再来了解linux的EXT3