Opening a device file is also slightly more efficient because dentry objects of devfs files include pointers to the proper file operations (see Section 13.3.4 later in this chapter).
There are, however, some problems with devfs. The most important one is that major and minor numbers are somewhat indispensable for Unix systems. First, some User Mode applications like the NFS server or the find command rely on the major and minor numbers to identify the physical disk partition containing a given file. Second, device numbers are required even by the POSIX standard. Thus, the devfs layer lets the kernel define major and minor numbers for each device driver, like the old-style device files. Currently, almost all device drivers associate the devfs device file with the same major and minor numbers of the corresponding old-style device file. For this reason, we mainly focus on old-style device files in the rest of this chapter.
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