Messages in this thread | | | Date | Wed, 14 Jan 2009 15:05:32 +0100 | From | Jan Kara <> | Subject | Re: ext2 + -osync: not as easy as it seems |
| |
On Wed 14-01-09 08:21:46, Theodore Tso wrote: > On Wed, Jan 14, 2009 at 11:35:33AM +0100, Jan Kara wrote: > > Yes, I noticed that yesterday as well. But then I was puzzled why ext4 > > would need the flush where it has it... sync_inode() has started and > > committed a transaction which issued a barrier when the commit was done. > > You're right; I'm not convinced we need the flush in ext4 (or ext3) at > all. We write the data blocks, *then* we call ext4_write_inode(), > which will force a commit. Now, if we apply that patch which > optimizes out commits if there are no dirty blocks, then we'll be > trouble, because we won't know for sure whether or not > ext4_write_inode() will have forced a journal commit. > > If we optimize out the journal commit when there are no blocks > attached to the transaction, we could change the patch to only force a > flush if inode->i_state did not have I_DIRTY before the call to > sync_inode(). Does that sound sane? Yes. And also add a flush in case of fdatasync().
> > The only reason I could imagine is that barrier (although it is usually > > translated to flushing writeback caches) actually means just an ordering > > requirement and hence does not necessarily mean that the caches are > > properly flushed. Is that so Eric? > > I'm not sure what you mean; if the barrier operation isn't flushing > all of the caches all the way out to the iron oxide, it's not going to > be working properly no matter where it is being called, whether it's > in ext4_sync_file() or in jbd2's journal_submit_commit_record(). Well, I thought that a barrier, as an abstraction, only guarantees that any IO which happened before the barrier hits the iron before any IO which has been submitted after a barrier. This is actually enough for a journalling to work correctly but it's not enough for fsync() guarantees. But I might be wrong...
Honza -- Jan Kara <jack@suse.cz> SUSE Labs, CR
| |