Re: [PATCH] Get rid of extents mount option - try 2

[Andrew Morton <akpm@osdl.org>]

On Sat, 7 Oct 2006 13:20:27 -0400
Theodore Tso <tytso@mit.edu> wrote:

> > Also, do we update the superblock in every transaction that creates or
> > deletes a file?  Otherwise, how do we guarantee the count is accurate
> > after replaying the journal?
> 
> Yes, we do.  The number of free inodes has to be kept up-to-date,
> after all, so the superblock is marked dirty and as being part of the
> transaction.

Actually we cheat, and we don't keep the superblock free inodes counter up
to date in real time.  Done for CPU consumptions reasons, but it was
perhaps a false optimisation, given that we still have a system-wide
inode_lock.

The free inode count is already triply redundant: inode table scan, inode
bitmap scan, ext4_group_desc.bg_free_inodes_count.  Making it quadruply
redundant seemed a bit over the top.

At runtime the definitive free-inodes count is the sum of the
per-blockgroup free-inode counts.  On clean shutdown we regenerate that and
write it into the superblock.