[PATCH 1/2] xfs: swap extents operations for CRC filesystems

[Mark Tinguely]

On 08/29/13 19:23, Dave Chinner wrote:
> From: Dave Chinner<dchinner at redhat.com>
>
> For CRC enabled filesystems, we can't just swap inode forks from one
> inode to another when defragmenting a file - the blocks in the inode
> fork bmap btree contain pointers back to the owner inode. Hence if
> we are to swap the inode forks we have to atomically modify every
> block in the btree during the transaction.
>
> We are doing an entire fork swap here, so we could create a new
> transaction item type that indicates we are changing the owner of a
> certain structure from one value to another. If we combine this with
> ordered buffer logging to modify all the buffers in the tree, then
> we can change the buffers in the tree without needing log space for
> the operation. However, this then requires log recovery to perform
> the modification of the owner information of the objects/structures
> in question.
>
> This does introduce some interesting ordering details into recovery:
> we have to make sure that the owner change replay occurs after the
> change that moves the objects is made, not before. Hence we can't
> use a separate log item for this as we have no guarantee of strict
> ordering between multiple items in the log due to the relogging
> action of asynchronous transaction commits. Hence there is no
> "generic" method we can use for changing the ownership of arbitrary
> metadata structures.
>
> For inode forks, however, there is a simple method of communicating
> that the fork contents need the owner rewritten - we can pass a
> inode log format flag for the fork for the transaction that does a
> fork swap. This flag will then follow the inode fork through
> relogging actions so when the swap actually gets replayed the
> ownership can be changed immediately by log recovery.  So that gives
> us a simple method of "whole fork" exchange between two inodes.
>
> This is relatively simple to implement, so it makes sense to do this
> as an initial implementation to support xfs_fsr on CRC enabled
> filesytems in the same manner as we do on existing filesystems. This
> commit introduces the swapext driven functionality, the recovery
> functionality will be in a separate patch.
>
> Signed-off-by: Dave Chinner<dchinner at redhat.com>
> ---

Makes sense. Change owner in btree entry, Don't log the modified btree 
buffers but recreate in recovery when necessary.

Reviewed-by: Mark Tinguely <tinguely at sgi.com>