Any system call modifying the filesystem is usually split into a series of low-level operations that manipulate disk data structures.
For instance, suppose that Ext3 must satisfy a user request to append a block of data to a regular file. The filesystem layer must determine the last block of the file, locate a free block in the filesystem, update the data block bitmap inside the proper block group, store the logical number of the new block either in the file's inode or in an indirect addressing block, write the contents of the new block, and finally, update several fields of the inode. As you see, the append operation translates into many lower-level operations on the data and metadata blocks of the filesystem.
Now, just imagine what could happen if a system failure occurred in the middle of an append operation, when some of the lower-level manipulations have already been executed while others have not. Of course, the scenario could be even worse, with high-level operations affecting two or more files (for example, moving a file from one directory to another).
To prevent data corruption, the Ext3 filesystem must ensure that each system call is handled in an atomic way. An atomic operation handle is a set of low-level operations on the disk data structures that correspond to a single high-level operation. When recovering from a system failure, the filesystem ensures that either the whole high-level operation is applied or none of its low-level operations is.
Any atomic operation handle is represented by a descriptor of type handle_t. To start an atomic operation, the Ext3 filesystem invokes the journal_start( ) JBD function, which allocates, if necessary, a new atomic operation handle and inserts it into the current transactions (see the next section). Since any low-level operation on the disk might suspend the process, the address of the active handle is stored in the journal_info field of the process descriptor. To notify that an atomic operation is completed, the Ext3 filesystem invokes the journal_stop( ) function.
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