Re: Allocating inodes from a single block

[David Chinner <dgc@sgi.com>]

On Wed, Jul 18, 2007 at 12:01:30PM +1000, Nathan Scott wrote:
> On Tue, 2007-07-17 at 20:43 -0500, Eric Sandeen wrote:
> > Chris Wedgwood wrote:
> > > On Tue, Jul 17, 2007 at 11:11:50AM -0700, Michael Nishimoto wrote:
> > > 
> > >> Filesystem free space becomes fragmented over time.  It's possible
> > >> for total free space to be a decent size and still not have a chunk
> > >> large enough to allocate new inodes.
> > > 
> > > by default there is a restriction that indoes shouldn't consume more
> > > that 25% of the total space
> > > 
> > > see the mkfs.xfs man-page for details, search for 'maxpct'
> > > 
> > > for existing filesystems you can use xfs_db to rewrite this value
> 
> FWIW, xfs_growfs can be used to change this online.
> 
> > The problem is that inodes are allocated in "clusters" of blocks.
> > 
> > If your free blocks aren't such that they can form a cluster, I think
> > you're out of luck when trying to allocate new inodes if your existing
> > clusters are full.
> 
> Have you looked into this much Mike?  I've not recently, but from a
> quick peek it looks like the cluster size is set in xfs_mount.c as
> mp->m_inode_cluster_size and a different value is used depending on
> the machines memory size ... so, perhaps this can be made a mount
> option?  (XFS_INODE_SMALL_CLUSTER_SIZE is 1FSB AFAICT).  But, maybe
> I'm missing something or not remembering some details here that'd
> make that infeasible.

The issue here is not the cluster size - that is purely an in-memory
arrangement for reading/writing muliple inodes at once. The issue
here is inode *chunks* (as Eric pointed out).

Basically, each record in the AGI btree has a 64 bit but-field for
indicating whether the inodes in the chunk are used or free and a
64bit address of the first block of the inode chunk.

It is assumed that all the inodes in the chunk are contiguous as
they are addressed in a compressed form - AG #, block # of first inode,
inode number in chunk.

That means that:

	a) the inode size across the entire AG must be fixed
	b) the inodes must be allocated in contiguous chunks of
	   64 inodes regardless of their size

To change this, you need to completely change the AGI format, the
inode allocation code and the inode freeing code and all the code that
assumes that inodes appear in 64 inode chunks e.g. bulkstat. Then
repair, xfs_db, mkfs, check, etc....

The best you can do to try to avoid these sorts of problems is
use the "ikeep" option to keep empty inode chunks around. That way
if you remove a bunch of files then fragement free space you'll
still be able to create new files until you run out of pre-allocated
inodes....

> Even better than a mount option would be to degrade to smaller size
> dynamically... not sure how hard that'd be either ... probably lots
> of corner cases lurking there.

And a major on-disk format change.

Cheers,

Dave.
-- 
Dave Chinner
Principal Engineer
SGI Australian Software Group