The structure of the Reiser file system
__u32 j_realblock[JOURNAL_TRANS_HALF];
where JOURNAL_TRANS_HALF is a constant set to 1018. This means that the blocksize has to be 4096 for journaling to work with reiserfs under Linux!
The actual block mapping is done as follows: The "Real blocks" field is seen as an array that contains for each block in the transaction the actual block number of the block in the file system. If we number every four bytes in the field as r0 through rn, then block 0 of the transaction is how block number r0 needs to look like after flushing the journal. Block 1 of the transaction is block r1, and so on. If the "Real blocks" field of the description block is not large enough, the field in the commit block is used in addition. This limits the maximum number of blocks in one transaction to 2*(blocksize-24)/4. (2036 for a block size of 4K), but the actual limit is set in the superblock.
Commit block The commit block terminates a transaction. It contains a copy of the transaction ID and the transaction length. There is also a 16 byte field reserved for a digest value at the end of the block, but this is not used currently.
| Name |
Size |
Description |
| Transaction ID |
4 |
The transaction ID |
| Len |
4 |
Length (in blocks) of the transaction |
| Real blocks |
Block size - 24 |
Mapping for blocks in transaction |
| Digest |
16 |
Digest of all blocks in transaction. Not used. |
Example:
The following example describes an old transaction in our example partition. The transaction starts in block 7243 (the description block), spans 4 data blocks (7244-7247) and has its commit block at block number 7248. Only the description block is shown, as the other blocks are not relevant for the example. 00000000 1b 6e 02 00 04 00 00 00 1b 01 00 00 90 22 00 00 .n..........."..
00000010 07 f7 00 00 aa 22 00 00 10 00 00 00 00 00 00 00 .÷..ª"..........
00000020 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
...
00000ff0 00 00 00 00 52 65 49 73 45 72 4c 42 00 00 00 00 ....ReIsErLB....
Transaction ID: 159259
Length: 4 blocks
Mount ID: 283
Real blocks[0]: 8848
Real blocks[1]: 63239
Real blocks[2]: 8874
Real blocks[3]: 16
Magic: ReIsErLB
This transaction therefore describes the following mapping: when the transaction is committed/flushed, block 7244 is written to block 8848, block 7245 to block 63239, block 7246 to block 8874, and block 7247 to block 16 (the superblock).
Navigating reiserfs In addition to the file system tree itself, in order to access files, one needs to navigate through the directory tree, as well. The root directory of a Reiser file system always has the key {1, 2, 0, 0}. The keys for subsequent directories and files within the directory hierarchy can then be found in the headers of the directory items. Since the keys in reiserfs are sorted by parent directory ID first, items that are in the same directory are grouped together in the file system tree. This allows for searching for keys locally instead of always having to go through the root node of the file system.
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