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    SubjectRe: [RFC] writeback and cgroup

    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.


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