No subject

in IO patterns and performance under heavy load here with this
patch set. it doesn't however, reduce the buffer cache lookups all
that much on such workloads - about 10% at most - as most of the
lookups are common from the directory and inode buffer
modifications. Here's a sample profile:

-  10.09%  [kernel]  [k] _xfs_buf_find
   - _xfs_buf_find
      - 99.57% xfs_buf_get
         - 99.35% xfs_buf_read
            - 99.87% xfs_trans_read_buf
               + 50.36% xfs_da_do_buf
               + 26.12% xfs_btree_read_buf_block.constprop.24
               + 12.36% xfs_imap_to_bp.isra.9
               + 10.73% xfs_read_agi

This shows that 50% of the lookups from the directory code, 25% from
the inode btree lookups, 12% from mapping inodes, and 10% from
reading the AGI buffer during inode allocation. 

You know, I suspect that we could avoid almost all those AGI buffer
lookups by moving to a similar in-core log and flush technique that
the inodes use. We've already got all the information in the struct
xfs_perag - rearranging it to have a "in-core on disk" structures
for the AGI, AGF and AGFL would make a lot of the "select an AG"
code much simpler than having to read and modify the AG buffers
directly. It might even be possible to do such a change without
needing to change the on-disk journal format for them...

I think I'll put that on my list of stuff to do - right next to
in-core unlinked inode lists....

Cheers,

Dave.
-- 
Dave Chinner
david at fromorbit.com