Re: Question about back references

["Yan, Zheng " <yanzheng@21cn.com>]

2009/9/6 Peter Macko <pmacko@eecs.harvard.edu>:
> 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.
>
This was correct for btrfs in 2.6.30 and earlier version. We introduced a new
back references format in 2.6.31. For more information about the new format,
please read the comments in extent-tree.c

> 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?
>
It's avoid by using the new back references format.

Yan, Zheng