| Summary: | The subvolume behaviour of the default BTRFS scheme prevents reflink (COW) copies of files between subvolumes | ||
|---|---|---|---|
| Product: | [Fedora] Fedora | Reporter: | James Hogarth <james.hogarth> |
| Component: | python-blivet | Assignee: | David Lehman <dlehman> |
| Status: | CLOSED WONTFIX | QA Contact: | Fedora Extras Quality Assurance <extras-qa> |
| Severity: | unspecified | Docs Contact: | |
| Priority: | unspecified | ||
| Version: | 20 | CC: | amulhern, bcl, dlehman, g.kaviyarasu, james.hogarth, jonathan, vanmeeuwen+fedora |
| Target Milestone: | --- | ||
| Target Release: | --- | ||
| Hardware: | Unspecified | ||
| OS: | Unspecified | ||
| Whiteboard: | |||
| Fixed In Version: | Doc Type: | Bug Fix | |
| Doc Text: | Story Points: | --- | |
| Clone Of: | Environment: | ||
| Last Closed: | 2015-01-20 14:54:49 UTC | Type: | Bug |
| Regression: | --- | Mount Type: | --- |
| Documentation: | --- | CRM: | |
| Verified Versions: | Category: | --- | |
| oVirt Team: | --- | RHEL 7.3 requirements from Atomic Host: | |
| Cloudforms Team: | --- | Target Upstream Version: | |
Your suggested approach precludes removing an old root subvolume while preserving the home subvol, no? In short, this is an expected consequence of the layout we chose. The main feature of this layout is the ability to remove/replace subvolumes that occcupy certain mountpoint (eg: /) while preserving other subvolumes (eg: /home). If your personal preference is the ability to do reflink copies between subvolumes you are free to create a layout to accommodate this. Clearing needs info flag Agreed layout is needed to preserve /home and if desired user can change afterwards. |
Description of problem: The kernel in Fedora 20 supports cross-subvolume COW so that cp --reflinks=always will use the same extents to avoid using extra disk space and save time on copies. If a BTRFS filesystem has subvolumes created within /home or within / via btrfs sub create then COW will work within these. If multiple mounts are used targeting specific subvolumes (via the subvol= option) then cp is unable to carry out COW between these and has to do a full duplication of extents taking up space and time. Anaconda should not create multiple mount points but rather provide a /home subvolume within the root subvolume so that it's possible cp --reflink=always from /home to other locations (think downloaded isos in ~/Downloads to /var/lib/libvirt/images for VMs). Version-Release number of selected component (if applicable): Fedora 20 install from the current installer tree How reproducible: Always Steps to Reproduce: 1. Do an install of Fedora 20 picking the default BTRFS layout 2. Try to copy a file via cp --reflink=always form somewhere in ${HOME} to anywhere in the root filesystem.... watch it fail. 3. Use btrfs sub create testsubvol within the home or root subvolumes and cp --reflink=always within root (or home) subvolume that it exists in and watch COW work. Actual results: cp: failed to clone ‘/tmptest/filename.tar.gz’ from ‘filename.tar.gz’: Invalid cross-device link Expected results: A reflink copy to work between the subvolumes Additional info: This kernel commit was found when asking on the mailing lists about the behaviour: http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=362a20c5e27614739c4 As such it is working as (currently) designed from the kernel point of view which is why as an anaconda filesystem layout it would be nicer having /home (and any other subvolumes) not directly being mounted in fstab with the subvol= option but rather just being there naturally...