Re: basic questions regarding COW in Btrfs

[Josef Bacik <jbacik@fusionio.com>]

On Wed, Feb 20, 2013 at 10:28:10AM -0700, Aastha Mehta wrote:
> Hello,
> 
> I am trying to understand the COW mechanism in Btrfs. Is it correct to
> say that unless nodatacow option is specified, Btrfs always performs
> COW for all the data+metadata extents used in the system?
> 

So we always cow the metadata, but yes nodatacow means we don't cow the actual
data in the data extents.

> I saw that COWing is implemented in btrfs_cow_block() function, which
> is called at the time of searching a slot for a particular item, while
> inserting into a new slot, committing transactions, while creating
> pending snapshots and few other places.
> 
> However, while tracing through the complete write path, I could not
> quite figure out when extents actually get COWed. Could you please
> point me to the place where COWing takes place? Is there any time
> when, for performance or any other reasons, the extents are not COWed
> but overwritten in place (apart from the explicit nodatacow flag being
> set during mount)?

You'll want to look at the tree operation ->fill_delalloc().  Thats where we do
cow_file_range().  We allocate new space and write.  When we finish the ordered
io we do btrfs_drop_extents() on the range we just wrote which will free up any
existing extents that exist, and then insert our new file extent.  Thanks,

Josef