Rbd-638 //top\\ Info

rbd export-diff poolA/image1@snap1 poolB/image2@snap2 - > /tmp/diff.bin Result : The command aborts with pointing at the destination image. 3️⃣ What We’ve Tried So Far | Attempt | Outcome | |---------|---------| | Run rbd info on the destination image (both via CLI and via the mount) | Works – image and snapshot are visible. | | Use the full rbd path ( /mnt/remote-rbd/poolB/image2@snap2 ) | Same error. | | Copy the image locally first ( rbd cp ) then run export-diff | Works – confirms the issue is with cross‑pool handling. | | Set RBD_FEATURE_LAYERING to 0 (disable layering) | No effect – error still occurs. | | Upgrade to Octopus 15.2.11 (latest stable) | Error persists; no regression fix noted in release notes. | | Enable debug_rbd=20 and capture logs | Logs show a failure in rbd::Image::Open() with ENOENT after a remote‑pool lookup. No more details. | 4️⃣ Likely Root Causes (hypotheses) | # | Hypothesis | Evidence / Reasoning | |---|------------|----------------------| | 1 | Path translation bug – the CLI builds a local object name but does not translate the remote mount point correctly when the destination image is not in the default namespace. | The error occurs after the CLI has resolved the source image (which works) and before any data is read/written. | | 2 | Missing feature flag – export-diff expects the destination image to have the RBD_FEATURE_EXCLUSIVE_LOCK flag when the pool is accessed via a remote mount. | The command works when the image is local (feature automatically present). | | 3 | CephFS permission mismatch – the client user (e.g., client.admin ) may not have the rbd_map capability on the remote pool, causing a silent failure at open time. | rbd info works because it uses the monitor’s auth, but export-diff opens a data connection that hits the OSDs. | | 4 | Bug in the rbd CLI’s export-diff implementation that doesn’t pass the pool argument correctly when the destination image name contains a slash ( / ). | The CLI parses arguments with parse_image_spec() ; known edge‑cases exist for images with @ and / . | 5️⃣ Immediate Work‑Arounds | Work‑Around | Pros | Cons | |------------|------|------| | Copy the destination image locally first ( rbd cp poolB/image2 /tmp/tmpimg ) then run export-diff against the temporary local copy. | Simple; no code changes required. | Requires extra storage; not suitable for large images or production pipelines. | | Use rbd diff + manual dd : generate the diff with rbd diff on the source, then apply it manually on the remote side via ssh . | Bypasses the buggy CLI path. | More steps; higher chance of human error. | | Mount the remote pool via rbd map (i.e., expose the RBD device locally) and then run export-diff with the device path ( /dev/rbdX ). | Keeps the workflow entirely in the RBD layer. | Requires extra kernel module handling; may need root. | | Upgrade to Ceph Pacific (16.x) – a newer codebase includes a fix for export-diff cross‑pool look‑ups (see commit c3f5d9b in octopus → pacific backport). | Long‑term solution if upgrade is feasible. | Migration effort, possible downstream impacts. | 6️⃣ Next Steps / Action Items | Owner | Action | Due | |-------|--------|-----| | @alice | Attach the full debug_rbd=30 log (both client and OSD side) for the failing run. | 2024‑11‑07 | | @bob (Ceph QA) | Re‑run the test on a single‑cluster setup using rbd export-diff --pool poolA --dest-pool poolB (no remote mount) to see if the issue is mount‑related. | 2024‑11‑08 | | @carol (Dev) | Search the upstream Git history for any recent changes to rbd export-diff that touched image‑spec parsing. | 2024‑11‑09 | | @dave (Ops) | Verify the client caps on the remote pool ( ceph auth get client.admin ) – ensure rbd pool=poolB is present. | 2024‑11‑07 | | Team | Decide whether to open a new upstream bug (if we confirm it’s a bug) or file a feature request for a more explicit --dest-pool flag. | 2024‑11‑12 | 7️⃣ Suggested Draft for the Bugzilla Comment Comment (Draft) – 2024‑11‑06

We have reproduced RBD‑638 consistently on a test environment (Octopus 15.2.7). The failure occurs only when the destination image lives on a remote pool (accessed via CephFS or a separate cluster). `rbd info` works, but `rbd export‑diff` aborts with “No such file or directory”. rbd-638

The most likely cause appears to be a path‑translation bug in the CLI when handling a remote pool spec. We have the following work‑arounds in place: • Copy the destination image locally first, then run export‑diff. • Use `rbd diff` + manual transfer. • Map the remote image as a block device and operate on `/dev/rbdX`. | | Copy the image locally first (

| Item | Details | |------|---------| | Bug ID | RBD‑638 | | Title | “ rbd export‑diff fails with “No such file or directory” when using a remote pool” | | Reported By | alice@example.com (2024‑11‑03) | | Component | Ceph → RBD → CLI | | Severity | High (blocks backup automation) | | Environment | - Ceph Octopus 15.2.7 (cluster ID: ceph-qa-01 ) - RBD client version: rbd‑tool 15.2.7‑0 - OS: Ubuntu 22.04 LTS (kernel 6.5) - Exporter node: backup‑01 (connected via CephFS) | | Reproducibility | Consistent (≈100 % on the test cluster) | | Current Status | Open – triaged, awaiting more logs | 1️⃣ Summary of the Issue The rbd export-diff command works fine when the source and destination pools reside on the same Ceph cluster, but fails with the following error when the destination pool is on a different Ceph cluster (or a remote CephFS mount): | | Enable debug_rbd=20 and capture logs |