Re: basic questions regarding COW in Btrfs

[Josef Bacik <jbacik@fusionio.com>]

On Mon, Feb 25, 2013 at 08:15:40AM -0700, Aastha Mehta wrote:
> Thanks again Josef.
> 
> I understood that cow_file_range is called for a regular file. Just to
> clarify, in cow_file_range is cow done at the time of reserving
> extents in the extent btree for the io to be done in this delalloc? I
> see the following comment above find_free_extent() which is called
> while trying to reserve extents:
> 
> /*
>  * walks the btree of allocated extents and find a hole of a given size.
>  * The key ins is changed to record the hole:
>  * ins->objectid == block start
>  * ins->flags = BTRFS_EXTENT_ITEM_KEY
>  * ins->offset == number of blocks
>  * Any available blocks before search_start are skipped.
>  */
> 
> This seems to be the only place where a cow might be done, because a
> key is being inserted into an extent which modifies it.
>

The key isn't inserted at this time, it's just returned with those values for us
to do as we please.  There is no update of the btree until
insert_reserved_extent/btrfs_mark_extent_written in btrfs_finish_ordered_io.
Thanks,

Josef