From mboxrd@z Thu Jan 1 00:00:00 1970 From: Peter Macko Subject: Question about back references Date: Sat, 05 Sep 2009 16:19:50 -0400 Message-ID: <4AA2C7E6.40200@eecs.harvard.edu> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed To: linux-btrfs@vger.kernel.org Return-path: List-ID: I am trying to understand how exactly the file extent back references work in btrfs. Can please someone tell me if the following is correct? - The back references are accumulated in an in-memory balanced tree (delayed-ref.c and delayed-ref.h) and pushed to disk during the transaction commit (a part of a checkpoint). They are placed into the B-tree under the key (bytenr, BTRFS_EXTENT_REF_KEY, hash of the four fields of the record), so that they are stored next to the file extent forward references. I am also wondering about the implications of copy on write: Imagine that you have an inode with four file extents and thus also four back references. COW of one of the extents then causes the COW of the inode. The new version of the inode has a different transaction ID, which is also one of the fields of back reference records. This causes the file system to add four new back reference records - one for the modified extent and three for the unmodified ones (since the transaction ID field has to be updated). Does this really happen, or is there some scheme to avoid adding these extra records? Thank you, Peter Macko