On Wed, Mar 10, 2021 at 06:00:45PM -0800, Darrick J. Wong wrote: > On Fri, Mar 05, 2021 at 04:11:42PM +1100, Dave Chinner wrote: > > From: Dave Chinner <dchinner@xxxxxxxxxx> > > > > When xlog_sync() rounds off the tail the iclog that is being > > flushed, it manually subtracts that space from the grant heads. This > > space is actually reserved by the transaction ticket that covers > > the xlog_sync() call from xlog_write(), but we don't plumb the > > ticket down far enough for it to account for the space consumed in > > the current log ticket. > > > > The grant heads are hot, so we really should be accounting this to > > the ticket is we can, rather than adding thousands of extra grant > > head updates every CIL commit. > > > > Interestingly, this actually indicates a potential log space overrun > > can occur when we force the log. By the time that xfs_log_force() > > pushes out an active iclog and consumes the roundoff space, the > > Ok I was wondering about that when I was trying to figure out what all > this ticket space stealing code was doing. > > So in addition to fixing the theoretical overrun, I guess the > performance fix here is that every time we write an iclog we might have > to move the grant heads forward so that we always write a full log > sector / log stripe unit? And since a CIL context might write a lot of > iclogs, it's cheaper to make those grant adjustments to the CIL ticket > (which already asked for enough space to handle the roundoffs) since the > ticket only jumps in the hot path once when the ticket is ungranted? > > If I got that right, > Reviewed-by: Darrick J. Wong <djwong@xxxxxxxxxx> You got it right. :) Thanks! -Dave. -- Dave Chinner david@xxxxxxxxxxxxx
