How do I debug 'non-restorable' backups?

Using FoundationDB
1

In my past experience, I’ve only ever seen a backup be Restorable: false either:

  • before the initial snapshot is complete, or
  • when attempting to describe it without the --deep flag (sometimes).

Earlier this week we tried, for the first time, to back up only specific key prefixes within our DB. One of the backups was absolutely fine:

fdbbackup describe -d "${BLOBSTORE_ADDRESS}" --deep
URL: blobstore://${ACCESS_KEY}@s3.eu-west-1.amazonaws.com:443/2025060401-log-messages?bucket=${BUCKET_NAME}
Restorable: true
Partitioned logs: false
Snapshot:  startVersion=83046118839195 (maxLogEnd -7.26 days)  endVersion=83049708662248 (maxLogEnd -7.21 days)  totalBytes=99955861408  restorable=true  expiredPct=0.00
Snapshot:  startVersion=83049715484866 (maxLogEnd -7.21 days)  endVersion=83092893689245 (maxLogEnd -6.72 days)  totalBytes=100044154514  restorable=true  expiredPct=0.00
...
Snapshot:  startVersion=83481436029181 (maxLogEnd -2.22 days)  endVersion=83524546067316 (maxLogEnd -1.72 days)  totalBytes=103012254405  restorable=true  expiredPct=0.00
Snapshot:  startVersion=83524697109238 (maxLogEnd -1.72 days)  endVersion=83567649427321 (maxLogEnd -1.22 days)  totalBytes=103389594662  restorable=true  expiredPct=0.00
Snapshot:  startVersion=83567656346500 (maxLogEnd -1.22 days)  endVersion=83610847209961 (maxLogEnd -0.72 days)  totalBytes=103607456557  restorable=true  expiredPct=0.00
Snapshot:  startVersion=83610881406093 (maxLogEnd -0.72 days)  endVersion=83654011862021 (maxLogEnd -0.22 days)  totalBytes=103981513035  restorable=true  expiredPct=0.00
SnapshotBytes: 1528822048570
MinLogBeginVersion:      83046110266362 (maxLogEnd -7.26 days)
ContiguousLogEndVersion: 83673069959863 (maxLogEnd -0.00 days)
MaxLogEndVersion:        83673069959863 (maxLogEnd -0.00 days)
MinRestorableVersion:    83049708662248 (maxLogEnd -7.21 days)
MaxRestorableVersion:    83673069959862 (maxLogEnd -0.00 days)

However the other showed every backup as being non-restorable:

fdbbackup describe -d "${BLOBSTORE_ADDRESS}" --deep
URL: blobstore://${ACCESS_KEY}@s3.eu-west-1.amazonaws.com:443/2025060901-ledger-state?bucket=${BUCKET_NAME}
Restorable: false
Partitioned logs: false
Snapshot:  startVersion=83496714014103 (maxLogEnd -2.04 days)  endVersion=83500292277220 (maxLogEnd -2.00 days)  totalBytes=37484123161  restorable=false  expiredPct=0.00
Snapshot:  startVersion=83500355653263 (maxLogEnd -2.00 days)  endVersion=83543466420071 (maxLogEnd -1.50 days)  totalBytes=37553931052  restorable=false  expiredPct=0.00
Snapshot:  startVersion=83543568722234 (maxLogEnd -1.50 days)  endVersion=83586646722924 (maxLogEnd -1.00 days)  totalBytes=37626690619  restorable=false  expiredPct=0.00
Snapshot:  startVersion=83586726004492 (maxLogEnd -1.00 days)  endVersion=83629649376931 (maxLogEnd -0.50 days)  totalBytes=37672814077  restorable=false  expiredPct=0.00
Snapshot:  startVersion=83629703691843 (maxLogEnd -0.50 days)  endVersion=83672822730284 (maxLogEnd -0.00 days)  totalBytes=37759880815  restorable=false  expiredPct=0.00
SnapshotBytes: 188097439724
MinLogBeginVersion:      83496628287694 (maxLogEnd -2.04 days)
ContiguousLogEndVersion: 83496648287694 (maxLogEnd -2.04 days)
MaxLogEndVersion:        83673009871752 (maxLogEnd -0.00 days)

I was hoping for pointers as to why this might be, and/or how to begin debugging it.