[深刻理解文件系统之五] 从SVR3 到SVR4

为什么有必要从SVR3到SRV4?为了吸取AT&T当时开发的操做系统的一些特性，集成VM的更新。

• 从SVR3到SVR4主要改动的地方包括：

1.用读写锁替代了以前的锁策略；

2.在IO路径上，用pages cache替换了以前的buffer caches, 用来提供meta data传输的吞吐率和效率

• 具体的变化以下：

• 文件描述符方面：

SVR3:文件描述符就是u_ofile[]数组的Index;

SVR4： 文件描述符是动态分配且可调节的, u_ofile[]被去掉，用u_nofiles[]和u_flist, a structure of type ufchunkthat contains an array of NFPCHUNK(which is 24) pointers to file table entries

替代了。每一个进程最大文件描述符的个数是由rlimit数据结构来限制的。

There are a number of per-process limits within the u_rlimit[]array. The u_rlimit[RLIMIT_NOFILE]entry defines both a soft and hard file descriptor limit. Allocation of file descriptors will fail once the soft limit is reached. Thesetrlimit()system call can be invoked to increase the soft limit up to that of the hard limit, but not beyond. The hard limit can be raised, but only by root

SVR4中文件描述符的分配图以下：

• Virtual Filesystem Switch Table方面的改变

内核编译的时候动态构造，由vfssw[]数组指定的file system switch table , 每个成员的构造以下：

struct vfssw {

char *vsw_name;

int (*vsw_init)();

struct vfsops *vsw_vfsops;

}

Thevfsstructure with SVR4 contained all of the original Sun vfsfields and introduced a few others including vfs_dev, which allowed a quick and easy scan to see if a filesystem

was already mounted, and the vfs_fstypefield, which is used to index the

vfssw[]array to specify the filesystem type

• vnode和VOP层的改变

vnode数据结构中去掉了v_shlockc, v_exlockc;

加入了：

v_stream指向vstream设备；

v_filocks指向当前文件所指向的全部文件和锁

v_pages基于SVR4以后，全部的读写操做都是基于page cache，而非以前的buffer cache (buffer cache如今只是在像inodes/directories等meta-data中用到）

对应的vnode operations vector 数组中经历了更多的改动：

从中去掉的函数包括：vop_bmap()/vop_bread()/vop_brelse()/vop_strategy()/vop_rdwr()/vop_select()

新引入的函数包括：

vop_read()/vop_write()/

vop_setfl() : in response to an fcntl() system call

where the F_SETFL (set file status flags) flag is specified. This allows the

filesystem to validate any flags passed.

vop_fid():用来生成惟一的文件句柄

vop_rwlock(): 经过引入了LOCK_SHARED or LOCK_EXCL 标示符，支持了单写者多读者模型

vop_rwunlock():释放上面申请使用的锁

vop_seek(): When specifying an offset to lseek(), this function is called to determine whether the filesystem deems the offset to be appropriate.

vop_cmp():比较两个指定的vnode

vop_frlock(): implement file and record locking

vop_space(): fcntl() system call has an option, F_FREESP, which  allows the caller to free space within a file

vop_realvp():A call  toVOP_REALVP()is made by filesystems when performing a link()system call to ensure that the link goes to the underlying file and not the specfs file, that has no physical representation on disk.

vop_getpage():read pages of data from the file in response to a page fault.

vop_putpage():flush a modified page of file data to disk

vop_map():implementing memory mapped files

vop_addmap():adds a mapping

vop_delmap(): deletes a mapping

vop_poll():implementing the poll()system call.

vop_pathconf():implement the pathconf()and

fpathconf()system calls. Filesystem-specific information can be returned, such as the maximum number of links to a file and the maximum file size

vnode的操做最后都是用宏实现的：

#define VOP_LOOKUP(vp,cp,vpp,pnp,f,rdir,cr) \ (*(vp)->v_op->vop_lookup)(vp,cp,vpp,pnp,f,rdir,cr)

这样一来：The filesystem-independent layer of the kernel will only access the filesystem

through macros. Obtaining a vnode is performed as part of an open()or

creat()system call or by the kernel invoking one of the veneer layer functions

when kernel subsystems wish to access files directly.

基于上述第二步，不少以前的操做，好比bread/bwrite/都被去掉，用没使用buffer的函数替代。