Chapter 9: The Extended Filesystem Family
The JBD layer providesjournal_dirty_metadatato write modified metadata to the journal:
fs/jbd/transaction.c
int journal_dirty_metadata(handle_t *handle, struct buffer_head *bh)The matchingjournal_dirty_datafunction writes useful data to the journal and is used indatamode.
Transactions are represented by a dedicated data structure; again a much simplified version is shown:
<jbd.h>
typedef transaction_s transaction_t;struct transaction_s
{
journal_t *t_journal;
tid_t t_tid;enum {
T_RUNNING,
...
T_FLUSH,
T_COMMIT,
T_FINISHED
} t_state;struct journal_head *t_buffers;
unsigned long t_expires;
int t_handle_count;
};❑ t_journalis a pointer to the journal to which the transaction data are written (for the sake of
simplicity, the data structure used is not discussed because it is overburdened with technical
details).
❑ Each transaction can have different states that are held int_state:❑ T_RUNNINGindicates that new atomic handles can be added to the journal.
❑ T_FLUSHindicates that log entries are being written at the moment.
❑ T_COMMITindicates when all data have been written to disk, but the metadata still need to
be processed.
❑ T_FINISHEDindicates that all log entries have been written safely to disk.❑ t_bufferspoints to the buffers associated with the transaction.
❑ t_expiresspecifies the time by which the transaction data must have been physically written
to the journal. The kernel uses a timer that expires by default 5 seconds after the transaction has
been generated.
❑ t_handle_countindicates the number of handles associated with the transaction.The Ext3 code uses ‘‘checkpoints‘‘ at which a check is made to ascertain whether the changes in the
journal have been written to the filesystem. If theyhave, the data in the journal are no longer needed