XFS Adds Shared Data Extents For Linux 4.9
Written by Michael Larabel in Linux Storage on 12 October 2016 at 09:03 AM EDT. 11 Comments
LINUX STORAGE --
Building off last week's XFS updates for Linux 4.9 is now a specific feature merge for this file-system: shared data extents.

Dave Chinner sent in the pull request today and he does expect that there will be some follow-up bug-fixes and clean-ups as a result of the big code change.

Chinner explained in the pull request what shared data extents mean for XFS:
At the most basic level we are simply adding shared data extents to XFS - i.e. a single extent on disk can now have multiple owners. To do this we have to add new on-disk features to both track the shared extents and the number of times they've been shared. This is done by the new "refcount" btree that sits in every allocation group. When we share or unshare an extent, this tree gets updated.

Along with this new tree, the reverse mapping tree needs to be updated to track each owner or a shared extent. This also needs to be updated ever share/unshare operation. These interactions at extent allocation and freeing time have complex ordering and recovery constraints, so there's a significant amount of new intent-based transaction code to ensure that operations are performed atomically from both the runtime and integrity/crash recovery perspectives.
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Of course, being XFS, nothing is simple - we use delayed allocation for CoW similar to how we use it for normal writes. ENOSPC is a significant issue here - we build on the reservation code added in 4.8-rc1 with the reverse mapping feature to ensure we don't get spurious ENOSPC issues part way through a CoW operation. These mechanisms also help minimise fragmentation due to repeated CoW operations. To further reduce fragmentation overhead, we've also introduced a CoW extent size hint, which indicates how large a region we should allocate when we execute a CoW operation.
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As such, it's a huge chunk of new functionality with new on-disk format features and internal infrastructure. It warns at mount time as an experimental feature and that it may eat data (as we do with all new on-disk features until they stabilise). We have not released userspace suport for it yet - userspace support currently requires download from Darrick's xfsprogs repo and build from source, so the access to this feature is really developer/tester only at this point. Initial userspace support will be released at the same time the kernel with this code in it is released.
This experimental feature is coming to Linux 4.9 while as mentioned in the text, the user-space xfsprogs support isn't yet officially released.
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