18.6 The Journaled File System

   

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JFS is the HP-UX version of the Veritas journaled File System (VxFS). This file system exhibits fast recovery features. HP OnLineJFS (Advanced JFS) is an extension to the base JFS and can be managed online (without unmounting). For example, you can extend or shrink the file system without unmounting it. Advanced JFS can also be defragmented and reorganized while it is in use. HP OnlineJFS is not included with the standard HP-UX distribution and must be purchased separately. A disadvantage of JFS is that the HP-UX kernel can't reside on it.

Like HFS, JFS also maintains multiple copies of the superblock, but these are not stored in any file. JFS keeps a record of these copies automatically. JFS keeps a record of all transactions to the file system metadata area in an intent log. The intent log is used for system recovery in case of a system crash. If a file system update is completed successfully, a "done record" is written to the intent log showing that this update request was successful. In case of a system crash, the intent log is consulted and the file system brought to the stable state by removing all unsuccessful transactions with the help of the intent log.

Another big advantage of JFS over HFS is that it creates inodes dynamically. So if the inode table is full but there is still space on the file system, JFS can create new inodes automatically.

Like HFS, JFS also divides the file system in JFS blocks. By default, the size of each block is 1 KByte. JFS allocates a group of blocks when you create a new file. This group of blocks is called an extent. If the file size grows, JFS tries to allocate contiguous blocks to the file, thus increasing the size of the file extent. However, if the contiguous blocks are not available, JFS creates a new extent and allocates it to the file.

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