Hello, On Tue 17-04-12 15:01:06, Tejun Heo wrote: > On Wed, Apr 11, 2012 at 09:22:31PM +0200, Jan Kara wrote: > > > So all the metadata IO will happen thorough journaling thread and that > > > will be in root group which should remain unthrottled. So any journal > > > IO going to disk should remain unthrottled. > > > > Yes, that is true at least for ext3/ext4 or btrfs. In principle we don't > > have to have the journal thread (as is the case of reiserfs where random > > writer may end up doing commit) but let's not complicate things > > unnecessarily. > > Why can't journal entries keep track of the originator so that bios > can be attributed to the originator while committing? That shouldn't > be too difficult to implement, no? I think I was just describing the current state but yes, in future we can track which cgroup first attached a buffer to a transaction. > > > Now, IIRC, fsync problem with throttling was that we had opened a > > > transaction but could not write it back to disk because we had to > > > wait for all the cached data to go to disk (which is throttled). So > > > my question is, can't we first wait for all the data to be flushed > > > to disk and then open a transaction for metadata. metadata will be > > > unthrottled so filesystem will not have to do any tricks like bdi is > > > congested or not. > > > > Actually that's what's happening. We first do filemap_write_and_wait() > > which syncs all the data and then we go and force transaction commit to > > make sure all metadata got to stable storage. The problem is that writeout > > of data may need to allocate new blocks and that starts a transaction and > > while the transaction is started we may need to do some reads (e.g. of > > bitmaps etc.) which may be throttled and at that moment the whole > > filesystem is blocked. I don't remember the stack traces you showed me so > > I'm not sure it this is what your observed but it's certainly one possible > > scenario. The reason why fsync triggers problems is simply that it's the > > only place where process normally does significant amount of writing. In > > most cases flusher thread / journal thread do it so this effect is not > > visible. And to precede your question, it would be rather hard to avoid IO > > while the transaction is started due to locking. > > Probably we should mark all IOs issued inside transaction as META (or > whatever which tells blkcg to avoid throttling it). We're gonna need > overcharging for metadata writes anyway, so I don't think this will > make too much of a difference. Agreed. Honza -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
