fix(scanner): recover legacy usage floor from backup (#6964)

* fix(scanner): recover legacy usage floor from backup

Allow scanner usage-floor startup and leadership fencing to use a valid legacy backup when the legacy primary read fails with a corruption-shaped error.

Keep v2 primary read failures, stale metadata, transient I/O, and missing or invalid backups fail-closed.

Co-Authored-By: heihutu <heihutu@gmail.com>

* fix(scanner): cover legacy backup fencing gaps (#6966)

* fix(scanner): recover legacy usage from valid backup

* fix(scanner): recover legacy usage floor from backup

Allow scanner usage-floor startup and leadership fencing to use a valid legacy backup when the legacy primary read fails with a corruption-shaped error.

Keep v2 primary read failures, stale metadata, transient I/O, and missing or invalid backups fail-closed.

Co-Authored-By: heihutu <heihutu@gmail.com>

* fix(scanner): cover legacy backup fencing gaps

---------

Co-authored-by: houseme <housemecn@gmail.com>
Co-authored-by: heihutu <heihutu@gmail.com>

---------

Co-authored-by: heihutu <heihutu@gmail.com>
Co-authored-by: Henry Guo <marshawcoco@gmail.com>

crates/scanner/src/data_usage_define.rs

@@ -131,6 +131,39 @@ pub(crate) async fn read_config_with_revision<S: ScannerObjectIO>(
     }
 }
 
+pub(crate) fn usage_floor_primary_read_error_allows_backup(err: &Error) -> bool {
+    match err {
+        Error::FileCorrupt
+        | Error::CorruptedFormat
+        | Error::CorruptedBackend
+        | Error::PartMissingOrCorrupt
+        | Error::LessData
+        | Error::MoreData => true,
+        Error::Io(io_error) => {
+            matches!(io_error.kind(), std::io::ErrorKind::InvalidData | std::io::ErrorKind::UnexpectedEof)
+                || error_chain_has_usage_floor_corruption_signature(io_error)
+        }
+        _ => false,
+    }
+}
+
+fn error_chain_has_usage_floor_corruption_signature(error: &(dyn std::error::Error + 'static)) -> bool {
+    let mut current = Some(error);
+    while let Some(err) = current {
+        let message = err.to_string();
+        if message.contains("InlineData value out of range")
+            || message.contains("InlineData key out of range")
+            || message.contains("insufficient data for metadata")
+            || message.contains("insufficient data for meta length")
+            || message.contains("insufficient data for CRC")
+        {
+            return true;
+        }
+        current = err.source();
+    }
+    false
+}
+
 /// Read only the object revision without materializing its body.
 pub(crate) async fn read_config_revision<S: ScannerObjectIO>(store: Arc<S>, path: &str) -> StorageResult<DataUsageCacheRevision> {
     match store

crates/scanner/src/scanner/cycle_state.rs

@@ -14,7 +14,9 @@
 /// Scanner cycle-state codec, persisted usage floors, and cycle-state persistence.
 use super::*;
 use crate::ScannerGetObjectReader;
-use crate::data_usage_define::{DATA_USAGE_BLOOM_RECOVERY_PATH, DATA_USAGE_RECOVERY_PATH};
+use crate::data_usage_define::{
+    DATA_USAGE_BLOOM_RECOVERY_PATH, DATA_USAGE_RECOVERY_PATH, usage_floor_primary_read_error_allows_backup,
+};
 use crate::storage_api::owner::ObjectIO as _;
 use tokio::io::AsyncReadExt as _;
 
@@ -1717,6 +1719,7 @@ pub(super) async fn persisted_usage_floor_for_startup(
         let backup_path = format!("{primary_path}.bkp");
         let is_v2_path = primary_path == DATA_USAGE_OBJ_NAME_PATH.as_str();
         let mut recovered_primary_companion_epoch = None;
+        let mut primary_read_error = None;
         let primary_epoch = match read_config_with_revision(storeapi.clone(), primary_path).await {
             Ok((Some(data), revision)) => {
                 let usage = serde_json::from_slice::<DataUsageInfo>(&data).map_err(|err| {
@@ -1776,6 +1779,12 @@ pub(super) async fn persisted_usage_floor_for_startup(
                 }
             }
             Ok((None, _)) => None,
+            Err(err) if !is_v2_path && usage_floor_primary_read_error_allows_backup(&err) => {
+                primary_read_error = Some(format!("failed to read scanner usage epoch floor from {primary_path}: {err}"));
+                invalid_baseline_path.get_or_insert_with(|| primary_path.to_string());
+                unrecoverable_baseline_path.get_or_insert_with(|| primary_path.to_string());
+                None
+            }
             Err(err) => {
                 return Err(ScannerError::Other(format!(
                     "failed to read scanner usage epoch floor from {primary_path}: {err}"
@@ -1855,6 +1864,14 @@ pub(super) async fn persisted_usage_floor_for_startup(
                 )));
             }
         }
+        if let Some(primary_read_error) = primary_read_error
+            && !any_found
+        {
+            return Err(ScannerError::Other(format!(
+                "{}; no valid scanner usage floor backup was available at {backup_path}",
+                primary_read_error
+            )));
+        }
         if any_found {
             if bootstrap_pending {
                 return Err(ScannerError::Other(

crates/scanner/src/scanner/leadership.rs

@@ -13,6 +13,7 @@
 // limitations under the License.
 /// Leader-lock claiming, usage-epoch fencing, and lock-loss handling.
 use super::*;
+use crate::data_usage_define::usage_floor_primary_read_error_allows_backup;
 
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
 pub(super) enum ScannerLeadershipClaimReconcile {
@@ -142,20 +143,34 @@ pub(super) async fn usage_snapshot_for_epoch_fence(
         }
     }
 
-    for path in [
-        LEGACY_DATA_USAGE_OBJ_NAME_PATH.as_str().to_string(),
-        format!("{}.bkp", LEGACY_DATA_USAGE_OBJ_NAME_PATH.as_str()),
-    ] {
-        let (legacy, _) = read_config_with_revision(storeapi.clone(), &path)
-            .await
-            .map_err(|err| ScannerError::Other(format!("failed to read legacy scanner usage epoch fence: {err}")))?;
+    let legacy_primary_path = LEGACY_DATA_USAGE_OBJ_NAME_PATH.as_str().to_string();
+    let legacy_backup_path = format!("{}.bkp", LEGACY_DATA_USAGE_OBJ_NAME_PATH.as_str());
+    let mut legacy_primary_read_error = None;
+    for path in [&legacy_primary_path, &legacy_backup_path] {
+        let legacy = match read_config_with_revision(storeapi.clone(), path).await {
+            Ok((legacy, _)) => legacy,
+            Err(err) if path == &legacy_primary_path && usage_floor_primary_read_error_allows_backup(&err) => {
+                legacy_primary_read_error = Some(format!("failed to read legacy scanner usage epoch fence from {path}: {err}"));
+                continue;
+            }
+            Err(err) => {
+                return Err(ScannerError::Other(format!(
+                    "failed to read legacy scanner usage epoch fence from {path}: {err}"
+                )));
+            }
+        };
         if let Some(legacy) = legacy.as_deref() {
-            let usage = decode_usage_snapshot_for_epoch_fence(legacy, &path, false)?;
+            let usage = decode_usage_snapshot_for_epoch_fence(legacy, path, false)?;
             if invalid_primary_epoch.is_none_or(|epoch| usage.scanner_epoch.unwrap_or_default() >= epoch) {
                 return Ok(Some(usage));
             }
         }
     }
+    if let Some(legacy_primary_read_error) = legacy_primary_read_error {
+        return Err(ScannerError::Other(format!(
+            "{legacy_primary_read_error}; no valid legacy scanner usage epoch fence backup was available at {legacy_backup_path}"
+        )));
+    }
     // A missing usage snapshot is an uninitialized state, not an empty
     // snapshot. Leadership fencing may proceed without creating a plausible
     // default; the first authoritative scanner publication will create it.
@@ -258,6 +273,12 @@ pub(super) async fn fence_scanner_usage_epoch_with_expected_epoch(
             Some(epoch) if epoch == claimed_epoch => return Ok(()),
             Some(_) | None => {}
         }
+        // A validated pre-marker legacy baseline needs an explicit complete
+        // identity before acquiring an epoch. Otherwise the v2 reader would
+        // reject the fenced value on its next startup.
+        if !usage.usage_snapshot_bootstrap_pending {
+            usage.usage_snapshot_complete = true;
+        }
         usage.scanner_epoch = Some(claimed_epoch);
         let data = serde_json::to_vec(&usage)
             .map_err(|err| ScannerError::Other(format!("failed to encode scanner usage epoch fence: {err}")))?;

crates/scanner/src/scanner/tests.rs

@@ -574,6 +574,7 @@ struct MemoryConfigStore {
     objects: Mutex<HashMap<String, Vec<u8>>>,
     revisions: Mutex<HashMap<String, u64>>,
     insert_after_gets: Mutex<HashMap<String, Vec<u8>>>,
+    read_errors: Mutex<HashMap<String, EcstoreError>>,
     delayed_gets: Mutex<HashMap<String, Duration>>,
     non_regular_objects: Mutex<HashSet<String>>,
     fail_put_number: Mutex<HashMap<String, usize>>,
@@ -623,6 +624,9 @@ impl crate::storage_api::scanner_io::ObjectIO for MemoryConfigStore {
         _opts: &ObjectOptions,
     ) -> EcstoreResult<GetObjectReader> {
         let key = memory_config_key(bucket, object);
+        if let Some(error) = self.read_errors.lock().await.get(&key).cloned() {
+            return Err(error);
+        }
         if let Some(delay) = self.delayed_gets.lock().await.remove(&key) {
             tokio::time::sleep(delay).await;
         }
@@ -2902,6 +2906,147 @@ async fn scanner_usage_floor_recovers_from_incomplete_v2_primary_using_fenced_ba
     );
 }
 
+async fn seed_legacy_primary_read_error_with_backup(store: &Arc<MemoryConfigStore>, error: EcstoreError, epoch: u64, cycle: u64) {
+    let legacy_primary = LEGACY_DATA_USAGE_OBJ_NAME_PATH.as_str();
+    let legacy_backup = format!("{legacy_primary}.bkp");
+    let mut backup = complete_usage_with_bucket_count(Some(std::time::SystemTime::UNIX_EPOCH), 0);
+    backup.scanner_epoch = Some(epoch);
+    backup.scanner_cycle = Some(cycle);
+
+    store
+        .read_errors
+        .lock()
+        .await
+        .insert(memory_config_key(RUSTFS_META_BUCKET, legacy_primary), error);
+    store.objects.lock().await.insert(
+        memory_config_key(RUSTFS_META_BUCKET, &legacy_backup),
+        serde_json::to_vec(&backup).expect("legacy backup usage snapshot should encode"),
+    );
+}
+
+#[tokio::test]
+async fn scanner_usage_floor_recovers_legacy_backup_after_primary_decode_error() {
+    let store = Arc::new(MemoryConfigStore::default());
+    seed_legacy_primary_read_error_with_backup(&store, EcstoreError::other("InlineData value out of range"), 19, 41).await;
+
+    let (floor, state) = persisted_usage_floor_for_startup(store.clone(), true)
+        .await
+        .expect("valid legacy backup should recover the startup floor");
+    assert_eq!(state, PersistedUsageFloorStartup::Authoritative);
+    assert_eq!(
+        floor,
+        PersistedUsageFloor {
+            next_cycle: 42,
+            leader_epoch: 19,
+        }
+    );
+    assert_eq!(
+        persisted_usage_floor(store)
+            .await
+            .expect("valid legacy backup should recover the authoritative floor"),
+        floor
+    );
+}
+
+#[tokio::test]
+async fn scanner_usage_floor_does_not_bootstrap_over_corrupt_legacy_primary_without_backup() {
+    let store = Arc::new(MemoryConfigStore::default());
+    let legacy_primary = LEGACY_DATA_USAGE_OBJ_NAME_PATH.as_str();
+    store
+        .read_errors
+        .lock()
+        .await
+        .insert(memory_config_key(RUSTFS_META_BUCKET, legacy_primary), EcstoreError::FileCorrupt);
+
+    let err = persisted_usage_floor_for_startup(store, true)
+        .await
+        .expect_err("corrupt legacy primary without a valid backup must remain fail-closed");
+    assert!(err.to_string().contains("no valid scanner usage floor backup"), "unexpected error: {err}");
+}
+
+#[tokio::test]
+async fn scanner_usage_floor_does_not_fallback_to_legacy_after_corrupt_v2_primary() {
+    let store = Arc::new(MemoryConfigStore::default());
+    let mut v2_backup = complete_usage_with_bucket_count(Some(std::time::SystemTime::UNIX_EPOCH), 0);
+    v2_backup.scanner_epoch = Some(8);
+    v2_backup.scanner_cycle = Some(11);
+    let mut legacy = complete_usage_with_bucket_count(Some(std::time::SystemTime::UNIX_EPOCH), 0);
+    legacy.scanner_epoch = Some(3);
+    legacy.scanner_cycle = Some(7);
+    store.read_errors.lock().await.insert(
+        memory_config_key(RUSTFS_META_BUCKET, DATA_USAGE_OBJ_NAME_PATH.as_str()),
+        EcstoreError::FileCorrupt,
+    );
+    store.objects.lock().await.insert(
+        memory_config_key(RUSTFS_META_BUCKET, &format!("{}.bkp", DATA_USAGE_OBJ_NAME_PATH.as_str())),
+        serde_json::to_vec(&v2_backup).expect("v2 backup usage snapshot should encode"),
+    );
+    store.objects.lock().await.insert(
+        memory_config_key(RUSTFS_META_BUCKET, LEGACY_DATA_USAGE_OBJ_NAME_PATH.as_str()),
+        serde_json::to_vec(&legacy).expect("legacy usage snapshot should encode"),
+    );
+
+    let err = persisted_usage_floor_for_startup(store, true)
+        .await
+        .expect_err("corrupt v2 primary must not recover without a primary revision");
+    assert!(
+        err.to_string().contains(&format!(
+            "failed to read scanner usage epoch floor from {}",
+            DATA_USAGE_OBJ_NAME_PATH.as_str()
+        )),
+        "unexpected error: {err}"
+    );
+}
+
+#[tokio::test]
+async fn scanner_usage_floor_keeps_transient_primary_read_error_fail_closed() {
+    let store = Arc::new(MemoryConfigStore::default());
+    let legacy_primary = LEGACY_DATA_USAGE_OBJ_NAME_PATH.as_str();
+    seed_legacy_primary_read_error_with_backup(
+        &store,
+        EcstoreError::Io(std::io::Error::new(
+            std::io::ErrorKind::ConnectionReset,
+            "connection reset while reading usage primary",
+        )),
+        19,
+        41,
+    )
+    .await;
+
+    let err = persisted_usage_floor_for_startup(store, true)
+        .await
+        .expect_err("transient primary errors must not be converted into backup recovery");
+    assert!(
+        err.to_string()
+            .contains(&format!("failed to read scanner usage epoch floor from {legacy_primary}")),
+        "unexpected error: {err}"
+    );
+    assert!(
+        !err.to_string().contains("no valid scanner usage floor backup"),
+        "transient error should not enter corrupt-primary fallback: {err}"
+    );
+}
+
+#[tokio::test]
+async fn scanner_usage_floor_keeps_outdated_primary_metadata_fail_closed() {
+    let store = Arc::new(MemoryConfigStore::default());
+    let legacy_primary = LEGACY_DATA_USAGE_OBJ_NAME_PATH.as_str();
+    seed_legacy_primary_read_error_with_backup(&store, EcstoreError::OutdatedXLMeta, 19, 41).await;
+
+    let err = persisted_usage_floor_for_startup(store, true)
+        .await
+        .expect_err("outdated primary metadata must not be converted into backup recovery");
+    assert!(
+        err.to_string()
+            .contains(&format!("failed to read scanner usage epoch floor from {legacy_primary}")),
+        "unexpected error: {err}"
+    );
+    assert!(
+        !err.to_string().contains("no valid scanner usage floor backup"),
+        "outdated metadata should not enter corrupt-primary fallback: {err}"
+    );
+}
+
 #[tokio::test]
 async fn scanner_usage_floor_does_not_bootstrap_over_incomplete_v2_primary() {
     let store = Arc::new(MemoryConfigStore::default());
@@ -3004,6 +3149,19 @@ async fn scanner_leadership_fencing_recovers_incomplete_v2_primary_from_backup()
     assert_eq!(recovered.scanner_cycle, Some(103));
 }
 
+#[tokio::test]
+async fn scanner_usage_floor_leadership_fencing_recovers_legacy_backup_after_primary_decode_error() {
+    let store = Arc::new(MemoryConfigStore::default());
+    seed_legacy_primary_read_error_with_backup(&store, EcstoreError::other("InlineData value out of range"), 19, 41).await;
+
+    let recovered = usage_snapshot_for_epoch_fence(store, None, false)
+        .await
+        .expect("a valid legacy backup should provide the fencing baseline")
+        .expect("the fencing baseline should be present");
+    assert_eq!(recovered.scanner_epoch, Some(19));
+    assert_eq!(recovered.scanner_cycle, Some(41));
+}
+
 #[tokio::test]
 async fn scanner_usage_floor_ignores_older_backup_after_primary_epoch_fence() {
     let store = Arc::new(MemoryConfigStore::default());
@@ -3955,6 +4113,162 @@ async fn scanner_defers_leadership_when_usage_snapshots_are_stably_absent() {
     assert!(read_config(store, DATA_USAGE_OBJ_NAME_PATH.as_str()).await.is_err());
 }
 
+#[tokio::test]
+async fn scanner_usage_floor_leadership_claim_recovers_legacy_backup_after_primary_decode_error() {
+    let store = Arc::new(MemoryConfigStore::default());
+    let ctx = CancellationToken::new();
+    seed_legacy_primary_read_error_with_backup(&store, EcstoreError::other("InlineData value out of range"), 19, 41).await;
+
+    let mut revision = DataUsageCacheRevision::Missing;
+    let mut cycle = CurrentCycle::default();
+    let mut persisted_epoch = 19;
+    assert!(
+        claim_scanner_leadership(
+            &ctx,
+            store.clone(),
+            &mut cycle,
+            &mut revision,
+            &mut persisted_epoch,
+            false,
+            ScannerCycleResetPolicy::None,
+        )
+        .await
+    );
+
+    let state = read_config(store.clone(), DATA_USAGE_BLOOM_NAME_PATH.as_str())
+        .await
+        .expect("leadership claim should persist after legacy backup recovery");
+    let (_, claimed_epoch) = decode_scanner_cycle_state(&state).expect("leadership claim should decode");
+    assert_eq!(claimed_epoch, 20);
+    assert_eq!(persisted_epoch, 20);
+
+    let usage = read_config(store, DATA_USAGE_OBJ_NAME_PATH.as_str())
+        .await
+        .expect("legacy backup recovery should publish a fenced v2 usage primary");
+    let usage = serde_json::from_slice::<DataUsageInfo>(&usage).expect("fenced v2 usage primary should decode");
+    assert_eq!(usage.scanner_epoch, Some(20));
+    assert_eq!(usage.scanner_cycle, Some(41));
+}
+
+#[tokio::test]
+#[serial_test::serial]
+async fn scanner_legacy_usage_backup_survives_fencing_and_restart_after_real_metadata_truncation() {
+    crate::scanner_io::clear_dirty_usage_buckets_for_tests();
+    let (temp_dir, store) = setup_scanner_cycle_store_with_usage_baseline(false).await;
+    let mut usage = complete_usage_with_bucket_count(Some(std::time::SystemTime::UNIX_EPOCH), 0);
+    usage.usage_snapshot_complete = false;
+    usage.scanner_cycle = Some(41);
+    let mut data = serde_json::to_vec(&usage).expect("legacy usage should encode");
+    data.resize(data.len() + 16 * 1024, b' ');
+    let legacy_path = LEGACY_DATA_USAGE_OBJ_NAME_PATH.as_str();
+    let backup_path = format!("{legacy_path}.bkp");
+    for path in [legacy_path, backup_path.as_str()] {
+        save_config(store.clone(), path, data.clone())
+            .await
+            .expect("legacy usage fixture should persist");
+    }
+    let mut truncated_files = Vec::new();
+    for disk_index in 0..4 {
+        let path = temp_dir
+            .path()
+            .join(format!("pool0/disk{disk_index}"))
+            .join(RUSTFS_META_BUCKET)
+            .join(legacy_path)
+            .join("xl.meta");
+        let file = tokio::fs::OpenOptions::new()
+            .write(true)
+            .open(&path)
+            .await
+            .expect("legacy inline metadata should exist");
+        assert!(file.metadata().await.expect("metadata should be readable").len() > 4096);
+        file.set_len(4096).await.expect("fixture should truncate at a page boundary");
+        truncated_files.push((
+            path.clone(),
+            tokio::fs::read(&path)
+                .await
+                .expect("truncated evidence should remain readable"),
+        ));
+    }
+
+    let store = restart_scanner_cycle_store_from(&store).await;
+    let error = read_config_with_revision(store.clone(), legacy_path)
+        .await
+        .expect_err("truncated primary must fail in the real object reader");
+    assert!(
+        error.to_string().contains("InlineData value out of range"),
+        "unexpected truncated-primary error: {error}"
+    );
+    assert_eq!(
+        read_config_with_revision(store.clone(), &backup_path)
+            .await
+            .expect("backup should remain readable")
+            .0,
+        Some(data.clone()),
+    );
+    let (floor, state) = persisted_usage_floor_for_startup(store.clone(), true)
+        .await
+        .expect("intact legacy backup must recover startup despite truncated primary");
+    assert_eq!(
+        floor,
+        PersistedUsageFloor {
+            next_cycle: 42,
+            leader_epoch: 0
+        }
+    );
+    assert_eq!(state, PersistedUsageFloorStartup::Authoritative);
+
+    let baseline = read_data_usage_persist_baseline(store.clone())
+        .await
+        .expect("publication must also read the intact backup");
+    assert_eq!(baseline.data.as_deref(), Some(data.as_slice()));
+    assert_eq!(baseline.revision, DataUsageCacheRevision::Missing);
+    fence_scanner_usage_epoch_with_expected_epoch(&CancellationToken::new(), store.clone(), 7, None, false)
+        .await
+        .expect("legacy backup must be fenced into v2");
+    let fenced = read_config(store.clone(), DATA_USAGE_OBJ_NAME_PATH.as_str())
+        .await
+        .expect("fencing must publish a v2 usage primary");
+    let fenced = serde_json::from_slice::<DataUsageInfo>(&fenced).expect("fenced v2 usage primary should decode");
+    assert!(
+        fenced.usage_snapshot_complete,
+        "the fenced pre-marker baseline must become a complete v2 identity"
+    );
+
+    let store = restart_scanner_cycle_store_from(&store).await;
+    let (floor, state) = persisted_usage_floor_for_startup(store.clone(), true)
+        .await
+        .expect("a restart after fencing must preserve the recovered floor");
+    assert_eq!(
+        floor,
+        PersistedUsageFloor {
+            next_cycle: 42,
+            leader_epoch: 7
+        }
+    );
+    assert_eq!(state, PersistedUsageFloorStartup::Authoritative);
+    let restarted = restart_scanner_cycle_store_from(&store).await;
+    assert_eq!(
+        persisted_usage_floor(restarted)
+            .await
+            .expect("fenced floor must survive another restart"),
+        PersistedUsageFloor {
+            next_cycle: 42,
+            leader_epoch: 7
+        }
+    );
+    for (path, bytes) in truncated_files {
+        assert_eq!(tokio::fs::read(path).await.expect("legacy evidence must not be removed"), bytes);
+    }
+    assert_eq!(
+        read_config(store, &backup_path)
+            .await
+            .expect("legacy backup must remain intact"),
+        data
+    );
+    global_metrics().set_cycle(None).await;
+    crate::scanner_io::clear_dirty_usage_buckets_for_tests();
+}
+
 #[tokio::test]
 async fn usage_bootstrap_pending_unblocks_first_leadership_claim() {
     let store = Arc::new(MemoryConfigStore::default());

crates/scanner/src/scanner/usage_store.rs

@@ -13,6 +13,7 @@
 // limitations under the License.
 /// Data-usage snapshot persistence: CAS store pipeline, epoch baselines, and observed-snapshot cleanup.
 use super::*;
+use crate::data_usage_define::usage_floor_primary_read_error_allows_backup;
 use crate::storage_api::owner::ScannerPublicationCommitState;
 use std::collections::HashMap;
 use std::sync::atomic::AtomicBool;
@@ -53,6 +54,30 @@ pub(super) struct DataUsagePersistBaseline {
     pub(super) revision: DataUsageCacheRevision,
 }
 
+async fn read_usage_persist_candidate(
+    storeapi: Arc<impl ScannerObjectIO>,
+    path: &str,
+) -> Result<(Option<Vec<u8>>, DataUsageCacheRevision), EcstoreError> {
+    let primary = read_config_with_revision(storeapi.clone(), path).await;
+    if path != LEGACY_DATA_USAGE_OBJ_NAME_PATH.as_str() {
+        return primary;
+    }
+    let Err(primary_error) = &primary else {
+        return primary;
+    };
+    if !usage_floor_primary_read_error_allows_backup(primary_error) {
+        return primary;
+    }
+    let backup_path = format!("{path}.bkp");
+    let backup = read_config_with_revision(storeapi, &backup_path).await?;
+    if backup.0.as_deref().is_some_and(|data| {
+        serde_json::from_slice::<DataUsageInfo>(data).is_ok_and(|usage| data_usage_info_has_persisted_baseline_identity(&usage))
+    }) {
+        return Ok(backup);
+    }
+    primary
+}
+
 /// Read the bytes used as the baseline for a usage publication while keeping
 /// the v2 primary revision as the CAS fence. During an interrupted upgrade the
 /// primary can be valid JSON without a baseline identity; in that case a
@@ -68,7 +93,7 @@ pub(super) async fn read_data_usage_persist_baseline(
             LEGACY_DATA_USAGE_OBJ_NAME_PATH.as_str().to_string(),
             format!("{}.bkp", LEGACY_DATA_USAGE_OBJ_NAME_PATH.as_str()),
         ] {
-            let (candidate, _) = read_config_with_revision(storeapi.clone(), &path).await?;
+            let (candidate, _) = read_usage_persist_candidate(storeapi.clone(), &path).await?;
             let Some(candidate) = candidate else {
                 continue;
             };
@@ -107,7 +132,7 @@ pub(super) async fn read_data_usage_persist_baseline(
         LEGACY_DATA_USAGE_OBJ_NAME_PATH.as_str().to_string(),
         format!("{}.bkp", LEGACY_DATA_USAGE_OBJ_NAME_PATH.as_str()),
     ] {
-        let (candidate, _) = read_config_with_revision(storeapi.clone(), &path).await?;
+        let (candidate, _) = read_usage_persist_candidate(storeapi.clone(), &path).await?;
         let Some(candidate) = candidate else {
             continue;
         };