"Theodore Ts'o" <tytso@xxxxxxx> writes: > Because no metadata changed, jbd2 will not even *start* a (jbd2) > transaction as a result of that write (overwrite) to an already > allocated data block.. Since it didn't start a jbd2 transaction for > this file system operation, there's no reason to force a jbd2 > transaction to close. > > (Note: this could because there *is* no currently existing open > transaction, or there might be a currently open transaction, but it's > not relevent to the activity associated with the file descriptor being > fsync'ed.) > > This is a critical performance optimization, because for many database > workloads, which are overwriting existing blocks and using > fdatasync(2), there is no reason to force a jbd2 transaction commit > for every single fdatasync(2) issued by the database. However, we > still need to send a cache flush operation so that the data block is > safely persistend onto stable storage. So maybe what ext4's sync_fs needs to know is whether ANY writes have been done since the last transaction committed? Is there a way to know that? As long as NOTHING has been written since the last commit, then there is no need to issue a flush.
