UFS2 Superblock
The UFS 2 superblock stores the same basic information as the UFS1 version, but it has removed many of the unused fields and is a little simpler. It has many of the same fields but has replaced the essential fields that were only 32 bits with 64 bit versions. It is typically located in sector 128, and the version that is used by FreeBSD and NetBSD has the fields shown in Table 17.3.
|
Byte Range |
Description |
Essential |
|---|---|---|
|
07 |
Unused |
No |
|
811 |
Offset to backup superblock in cylinder group relative to a "base" |
Yes |
|
1215 |
Offset to group descriptor in cylinder group relative to a "base" |
Yes |
|
1619 |
Offset to inode table in cylinder group relative to a "base" |
Yes |
|
2023 |
Offset to first data block in cylinder group relative to a "base" |
No |
|
2443 |
Unused |
No |
|
4447 |
Number of cylinder groups in file system |
Yes |
|
4851 |
Size of a block in bytes |
Yes |
|
5255 |
Size of a fragment in bytes |
Yes |
|
5659 |
Size of a block in fragments |
No |
|
6063 |
Minimum % of free blocks |
No |
|
6471 |
Unused |
No |
|
7275 |
Mask used to calculate the address for a block |
No |
|
7679 |
Mask used to calculate the address for a fragment |
No |
|
8083 |
Shift used to calculate the byte address for a block |
No |
|
8487 |
Shift used to calculate the byte address for a fragment |
No |
|
8891 |
Maximum number of contiguous blocks to allocate |
No |
|
9295 |
Maximum number of blocks per cylinder group |
No |
|
9699 |
Number of bits to convert between a block address and a fragment address |
No |
|
100103 |
Number of bits to convert between a fragment address and a sector address |
No |
|
104107 |
Size of superblock |
No |
|
108115 |
Unused |
No |
|
116119 |
Number of indirect addresses per fragment |
No |
|
120123 |
Number of inodes per block in inode table |
No |
|
124127 |
Unused |
No |
|
128131 |
Optimization technique |
No |
|
132143 |
Unused |
No |
|
144151 |
File System Id |
No |
|
152155 |
Unused |
No |
|
156159 |
Size of cylinder group summary area in bytes |
No |
|
160163 |
Size of cylinder group descriptor in bytes |
No |
|
164183 |
Unused |
No |
|
184187 |
Inodes per cylinder group |
Yes |
|
188191 |
Fragments per cylinder group |
Yes |
|
192207 |
Unused |
No |
|
208208 |
Super block modified flag |
No |
|
209209 |
FS was clean when it was mounted |
No |
|
210210 |
Mounted read only flag (set to 1 if read only) |
No |
|
211211 |
Unused |
No |
|
212679 |
Last mount point |
No |
|
680711 |
Volume name |
No |
|
712719 |
System UID |
No |
|
720723 |
Unused |
No |
|
724727 |
Last cylinder group searched |
No |
|
728999 |
Unused |
No |
|
10001007 |
Location of superblock |
No |
|
10081015 |
Number of directories |
No |
|
10161023 |
Number of free blocks |
No |
|
10241031 |
Number of free inodes |
No |
|
10321039 |
Number of free fragments |
No |
|
10401047 |
Number of free clusters |
No |
|
10481071 |
Unused |
No |
|
10721079 |
Last written time |
No |
|
10801087 |
Number of fragments in file system |
Yes |
|
10881095 |
Number of fragments that can store file data |
No |
|
10961103 |
Fragment address of cylinder group summary area |
No |
|
11041111 |
Blocks in process of being freed |
No |
|
11121115 |
Inodes in process of being freed |
No |
|
11161195 |
Array inode addresses for snap inodes |
No |
|
11961199 |
Expected average file size |
No |
|
12001203 |
Expected number of files per directory |
No |
|
12041311 |
Unused |
No |
|
13121315 |
Flags (see Table 17.2) |
No |
|
1316 1319 |
Size of cluster summary array in group descriptors |
No |
|
13201323 |
Maximum length of internal symbolic link |
Yes |
|
13241327 |
Format of inodes |
Yes |
|
13281335 |
Maximum file size |
No |
|
13361343 |
Mask used to calculate the offset in a block for an address |
No |
|
13441351 |
Mask used to calculate the offset in a fragment for an address |
No |
|
13521355 |
File system state |
No |
|
13561371 |
Unused |
No |
|
13721375 |
Signature value (0x19540119) |
Yes |
You might notice that fields have moved around. The only changes that could be of interest are that the mount point is shorter and there is now a volume label field. The flags field is four bytes instead of only one, but the same flag values given in Table 17.2 are used. Also note that the magic value is different, which is how we can differentiate between UFS1 and UFS2.
Here are the contents of a UFS2 file system from a FreeBSD 5 system:
# dd if=freebsd5.dd skip=128 count=4 | xxd 0000000: 0000 0000 0000 0000 2800 0000 3000 0000 ........(...0... 0000016: 3800 0000 d800 0000 0000 0000 0000 0000 8............... 0000032: 0000 0000 0000 0000 0000 0000 0400 0000 ................ 0000048: 0040 0000 0008 0000 0800 0000 0800 0000 .@.............. 0000064: 0000 0000 0000 0000 00c0 ffff 00f8 ffff ................ 0000080: 0e00 0000 0b00 0000 0800 0000 0008 0000 ................ 0000096: 0300 0000 0200 0000 0008 0000 0000 0000 ................ 0000112: 0000 0000 0008 0000 4000 0000 0000 0000 ........@....... 0000128: 0000 0000 0000 0000 0000 0000 0000 0000 ................ 0000144: adb2 0f41 fd01 4a17 0000 0000 0008 0000 ...A..J......... 0000160: 0008 0000 0000 0000 0000 0000 0000 0000 ................ 0000176: 0000 0000 0000 0000 0005 0000 b813 0000 ................ 0000192: 0000 0000 0000 0000 0000 0000 0000 0000 ................ 0000208: 0000 0080 2f6d 6e74 0000 0000 0000 0000 ..../mnt........ [REMOVED] 0000672: 0000 0000 0000 0000 5546 5332 0000 0000 ........UFS2.... [REMOVED] 0000832: 0000 0000 0000 0000 1038 66c3 0030 66c3 .........8f..0f. 0000848: 0038 66c3 0000 0000 0000 0000 0040 0000 .8f..........@.. [REMOVED] 0000992: 0000 0000 0000 0000 0000 0100 0000 0000 ................ 0001008: 0400 0000 0000 0000 f308 0000 0000 0000 ................ 0001024: e213 0000 0000 0000 1800 0000 0000 0000 ................ [REMOVED] 0001072: bdb4 0f41 0000 0000 c04e 0000 0000 0000 ...A.....N...... 0001088: d74b 0000 0000 0000 d800 0000 0000 0000 .K.............. [REMOVED] 0001184: 0000 0000 0000 0000 0000 0000 0040 0000 .............@.. 0001200: 4000 0000 0000 0000 0000 0000 0000 0000 @............... [REMOVED] 0001312: 0000 0000 0800 0000 7800 0000 0000 0000 ........x....... 0001328: ffff 0202 1080 0000 ff3f 0000 0000 0000 .........?...... 0001344: ff07 0000 0000 0000 0000 0000 0000 0000 ................ 0001360: 0000 0000 0000 0000 0000 0000 1901 5419 ..............T.
We see in bytes 8 to 11, 12 to 15, and 16 to 19 that the superblock is located 40 fragments (0x28) from the start of each cylinder group, the group descriptor is 48 fragments (0x30) from the start, and the inode table is 56 fragments (0x38) from the start. Bytes 44 to 47 show that there are four cylinder groups.
The size of a block is given in bytes 48 to 51, and we see that it is 16,384 bytes (0x4000), and the size of each fragment is in bytes 52 to 55, which is 2,048 bytes (0x0800). Bytes 184 to 187 show that there are 1,280 (0x0500) inodes per cylinder group, and bytes 188 to 191 show that there are 5,048 (0x13b8) fragments per group. The total number of fragments is given in bytes 1080 to 1087, and this small file system has only 20,160.
Here is the relevant output from running fsstat on the UFS2 image:
# fsstat f freebsd freebsd5.dd FILE SYSTEM INFORMATION -------------------------------------------- File System Type: UFS 2 Last Written: Tue Aug 3 10:52:29 2004 Last Mount Point: /mnt Volume Name: UFS2 System UID: 0 METADATA INFORMATION -------------------------------------------- Inode Range: 0 - 5119 Root Directory: 2 Num of Avail Inodes: 5090 Num of Directories: 4 CONTENT INFORMATION -------------------------------------------- Fragment Range: 0 - 20159 Block Size: 16384 Fragment Size: 2048 Num of Avail Full Blocks: 2291 Num of Avail Fragments: 24 [REMOVED]