[RFC] Delayed logging

[Christoph Hellwig]

Some comments on the patches:

 - "xfs: cluster fsync transaction" seems like something that can stand
   on it's own and go into the tree now.  Despite the comments it
   currently only clusters in fsync and not write_inode which might
   lead to higher benefits, btw.

 - the busy extent tracking might be worth to be reordered before the
   delayed logging series.  In itself it might also want some reordering
   as there's a lot churn in the patches there.  Making
   "XFS: Simplify transaction busy extent tracking" first in that
   subseries might help quite a bit to reduce that churn

 - The actual CIL implementation seems to be split into too small
   patches IMHO.  E.g. "xfs: extend the log item to support delayed
   logging" and "xfs: Introduce the Committed Item List" are two
   sides of the same coin and splitting it might not make too much
   sense.