為了管理磁盤空間，文件系統需要知道哪些block是空閒的。Omfs使用bitmap來達到這個目的。Bitmap的每一個bit對應磁盤上的一個block，當對應的block被分配後，bitmap中的相應bit會被設置為1。這個是很經典也很清晰的一個設計，下面我們看看omfs的具體做法：
相關閱讀：Linux文件系統omfs的目錄創建和刪除 http://www.linuxidc.com/Linux/2012-02/54025.htm
在上面新建的文件系統中新建一個文件aaaa：
Touch aaaa
ls -ali
total 4
3 drwxr-xr-x 2 root root 2048 2012-02-08 10:52 .
1048578 drwxr-xr-x 4 root root 4096 2012-02-08 13:44 ..
6 -rw-r--r-- 1 root root 0 2012-02-08 13:45 aaaa
再touch bbbb和touch cccc
3 drwxr-xr-x 2 root root 2048 2012-02-08 10:52 .
1048578 drwxr-xr-x 4 root root 4096 2012-02-08 13:44 ..
6 -rw-r--r-- 1 root root 0 2012-02-08 13:45 aaaa
8 -rw-r--r-- 1 root root 0 2012-02-08 13:54 bbbb
10 -rw-r--r-- 1 root root 0 2012-02-08 13:54 cccc
hexdump -C -s 0xa000 -n 8192 /dev/loop0
0000a000 ff 0f 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
0000a010 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
*
0000a050 00 00 00 00 11 00 00 00 ff ff ff ff ff ff ff ff |................|
0000a060 ff ff 00 00 c9 0c 02 00 00 00 00 00 00 00 00 00 |................|
0000a070 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
*
0000a190 00 00 00 00 00 00 00 00 00 00 00 00 00 00 08 00 |................|
0000a1a0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
0000a1b0 00 00 00 00 00 00 00 00 ff ff ff ff ff ff ff ff |................|
0000a1c0 ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff |................|
*
0000a800 00 00 00 00 29 05 02 00 00 00 00 00 00 00 00 00 |....)...........|
0000a810 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
最初是3f 現在是ff0f 。
3f =0011 1111 這六位表示block 0到block 5 block0是super block，block1和block2是root block，block3 和block4是root directory inode， block5是bitmap
因為aaaa的i_no是6，所以占用block6，block7是備份。
Bbbb的i_no是8，所以占用block8，block9是備份。
Cccc的i_no是10，所以占用block10，block11是備份。
所以是ff0f = 1111 1111 0000 1111 block0至block11都是占用的