When a machine crashes under Unix, files in a directory can become detached from that directory. When this happens, those orphaned files are saved in a directory called lost+found. Because filenames are saved only in directories, orphaned files are nameless. Consequently, Unix stores them in lost+found using their inode numbers as their names.

To illustrate, consider finding these four files in lost+found after a crash:

#1528 #1200 #3124 #3125

Two of these are qf files, and two are df files. Beginning with V8.7 sendmail, the qf files contain a record of the inode numbers for their corresponding df files. That information is stored in the I line:

Imajor/minor/ino

Here, the major and minor are the major and minor device numbers for the disk device that the df file was stored on. The ino is the inode number for the df file. In our lost+found example the following command could be run to pair up the orphaned files:

% grep "^I.*/.*/" *
#1200:I123/45/3124
#1325:I123/45/1528

This shows that the qf file #1200 has the df file #3124 and that the qf file #1325 has the df file #1528.

The sendmail program does not check the inode number in the I line against the actual inode number of the df file. Instead, the I line is generated afresh each time the qf file is processed.

When df, qf, and xf subdirectories are used, and when those subdirectories are on separate disks, a crash of one disk can leave the df or qf file intact, and the other in lost+found.