ETCD learner members cannot be defragmented, causing persistent storage bloat.

### Bug report criteria - [x] This bug report is not security related, security issues should be disclosed privately via security@etcd.io. - [x] This is not a support request or question, support requests or questions should be raised in the etcd [discussion forums](https://github.com/etcd-io/etcd/discussions). - [x] You have read the etcd [bug reporting guidelines](https://github.com/etcd-io/etcd/blob/main/Documentation/contributor-guide/reporting_bugs.md). - [x] Existing open issues along with etcd [frequently asked questions](https://etcd.io/docs/latest/faq) have been checked and this is not a duplicate. ### What happened? **Observed Behavior:** - Learner member DB size remains at 30MB while voting members are successfully defragmented to ~7MB - `etcdctl defrag` command silently skips learner endpoints - Direct defragmentation of learner endpoint fails with: `rpc error: code = Unavailable desc = etcdserver: rpc not supported for learner` - Learner database size grows unbounded over time without maintenance capability **Error Message:** ``` etcdserver: rpc not supported for learner Failed to defragment etcd member[https://181140267029446:25687] (etcdserver: rpc not supported for learner) ``` ### What did you expect to happen? Learner members should support defragmentation operations to maintain database size, either: - `etcdctl defrag` should include learner endpoints by default, OR - `etcdctl defrag --endpoints=<learner>` should work without errors, OR - Provide a supported method to defragment learner members (e.g., `--include-learners` flag) ### How can we reproduce it (as minimally and precisely as possible)? **Minimal Reproduction Steps:** ```bash # 1. Set up 3-node cluster + 1 learner member # (cluster already running with learner) # 2. Check initial status - note learner DB size etcdctl endpoint status --write-out=table # Result: Learner at 30MB, voting members at 8.5MB # 3. Perform compaction etcdctl compact 2885950 # 4. Defragment cluster etcdctl defrag # Result: Only voting members defragmented # 5. Verify learner still has large DB etcdctl endpoint status --write-out=table # Result: Voting members ~7MB, learner still 30MB # 6. Try to defrag learner directly etcdctl --endpoints=https://181140267029446:25687 defrag # Result: Fails with "rpc not supported for learner" ``` **Cluster Topology:** - 3 voting members (fb2f8c3838629cdb, 2228c0c31b9ff622, 63d10718366c821d) - 1 learner member (cf584302b1c47a59) ### Anything else we need to know? **Impact:** - Production issue causing learner storage to grow unbounded - No supported workaround for learner maintenance - Affects cluster operations in learner-promotion scenarios **Observations:** - The CLI shows `--includeLearner` flag but it doesn't resolve the underlying RPC limitation - This appears to be an intentional restriction but creates operational problems ### Etcd version (please run commands below) ```bash etcd version: 3.5.25 etcdctl version: 3.5.25 ``` ### Etcd configuration (command line flags or environment variables) **Cluster Configuration:** - 4-member cluster (3 voting + 1 learner) - HTTPS endpoints on port 25687 - Standard production setup **Relevant Settings:** ```bash --auto-compaction-retention (if applicable) --quota-backend-bytes (if applicable) --initial-cluster (4-member setup) ``` ### Etcd debug information (please run commands below, feel free to obfuscate the IP address or FQDN in the output) **Member List:** ```bash etcdctl member list -w table ``` **Endpoint Status (Before Defrag):** ``` +-------------------------------+------------------+---------+---------+-----------+------------+-----------+------------+--------------------+--------+ | ENDPOINT | ID | VERSION | DB SIZE | IS LEADER | IS LEARNER | RAFT TERM | RAFT INDEX | RAFT APPLIED INDEX | ERRORS | +-------------------------------+------------------+---------+---------+-----------+------------+-----------+------------+--------------------+--------+ | https://181140264581536:25687 | fb2f8c3838629cdb | 3.5.25 | 8.5 MB | false | false | 9 | 3276978 | 3276978 | | | https://181140266914826:25687 | 2228c0c31b9ff622 | 3.5.25 | 8.5 MB | false | false | 9 | 3276978 | 3276978 | | | https://181140266590820:25687 | 63d10718366c821d | 3.5.25 | 8.5 MB | true | false | 9 | 3276978 | 3276978 | | | https://181140267029446:25687 | cf584302b1c47a59 | 3.5.25 | 30 MB | false | true | 9 | 3276980 | 3276980 | | +-------------------------------+------------------+---------+---------+-----------+------------+-----------+------------+--------------------+--------+ ``` **Endpoint Status (After Defrag - Issue Persists):** ``` +-------------------------------+------------------+---------+---------+-----------+------------+-----------+------------+--------------------+--------+ | ENDPOINT | ID | VERSION | DB SIZE | IS LEADER | IS LEARNER | RAFT TERM | RAFT INDEX | RAFT APPLIED INDEX | ERRORS | +-------------------------------+------------------+---------+---------+-----------+------------+-----------+------------+--------------------+--------+ | https://181140264581536:25687 | fb2f8c3838629cdb | 3.5.25 | 7.0 MB | false | false | 9 | 3278633 | 3278633 | | | https://181140266914826:25687 | 2228c0c31b9ff622 | 3.5.25 | 7.0 MB | false | false | 9 | 3278633 | 3278633 | | | https://181140266590820:25687 | 63d10718366c821d | 3.5.25 | 7.0 MB | true | false | 9 | 3278633 | 3278633 | | | https://181140267029446:25687 | cf584302b1c47a59 | 3.5.25 | 30 MB | false | true | 9 | 3278633 | 3278633 | | +-------------------------------+------------------+---------+---------+-----------+------------+-----------+------------+--------------------+--------+ ``` ### Relevant log output ```Shell **Defragmentation Failure Logs:** {"level":"warn","ts":"2026-05-11T08:07:05.721070-0700","logger":"etcd-client","caller":"v3@v3.5.25/retry_interceptor.go:63","msg":"retrying of unary invoker failed","target":"etcd-endpoints://0xc0000f61e0/181140264581536:25687","attempt":0,"error":"rpc error: code = Unavailable desc = etcdserver: rpc not supported for learner"} ... {"level":"warn","ts":"2026-05-11T08:07:06.622187-0700","logger":"etcd-client","caller":"v3@v3.5.25/retry_interceptor.go:63","msg":"retrying of unary invoker failed","target":"etcd-endpoints://0xc0000f61e0/181140264581536:25687","attempt":99,"error":"rpc error: code = Unavailable desc = etcdserver: rpc not supported for learner"} Failed to defragment etcd member[https://181140267029446:25687] (etcdserver: rpc not supported for learner) ```