dgraph-io/badger · error

Invalid read: Len: %d read at:[%d:%d]

Error message

Invalid read: Len: %d read at:[%d:%d]

What it means

Read parses a key-value entry from the value log at the given value pointer: it reads the entry header (h.klen + h.vlen) and slices the value out of the fetched byte slice kv. If kv is shorter than the declared key+value length, the entry extends beyond the readable data — a truncated or corrupt vlog entry — so badger logs details and returns this error instead of garbage.

Source

Thrown at value.go:998

		// Fetch checksum from the end of the buffer.
		checksum := buf[len(buf)-crc32.Size:]
		if hash.Sum32() != y.BytesToU32(checksum) {
			runCallback(cb)
			return nil, nil, y.Wrapf(y.ErrChecksumMismatch, "value corrupted for vp: %+v", vp)
		}
	}
	var h header
	headerLen := h.Decode(buf)
	kv := buf[headerLen:]
	if lf.encryptionEnabled() {
		kv, err = lf.decryptKV(kv, vp.Offset)
		if err != nil {
			return nil, cb, err
		}
	}
	if uint32(len(kv)) < h.klen+h.vlen {
		vlog.db.opt.Errorf("Invalid read: vp: %+v", vp)
		return nil, nil, fmt.Errorf("Invalid read: Len: %d read at:[%d:%d]",
			len(kv), h.klen, h.klen+h.vlen)
	}
	return kv[h.klen : h.klen+h.vlen], cb, nil
}

// getUnlockCallback will returns a function which unlock the logfile if the logfile is mmaped.
// otherwise, it unlock the logfile and return nil.
func (vlog *valueLog) getUnlockCallback(lf *logFile) func() {
	if lf == nil {
		return nil
	}
	return lf.lock.RUnlock
}

// readValueBytes return vlog entry slice and read locked log file. Caller should take care of
// logFile unlocking.
func (vlog *valueLog) readValueBytes(vp valuePointer) ([]byte, *logFile, error) {
	lf, err := vlog.getFileRLocked(vp)

View on GitHub (pinned to 2a001d466f)

Solutions

1. Restore the vlog files from a known-good backup; the truncated entry is unrecoverable from the log itself.
2. Re-put affected keys from the application's source of truth so fresh entries are written at valid offsets.
3. Rely on badger's vlog replay/recovery (it truncates to the last valid entry on open); ensure a version that does this and reopen cleanly.
4. If caused by version mismatch, migrate with Stream/Backup into a fresh DB instead of opening old files in-place.
5. Check disk health (SMART/fsck) and stop copying live DB directories; use DB.Backup or stopped-DB snapshots.

Example fix

// before: reading after a crash with a truncated vlog
val, err := txn.Get(key) // Invalid read: Len: 100 read at:[10:150]
// after: guard the read and rewrite the entry when corrupt
val, err := txn.Get(key)
if err != nil && strings.Contains(err.Error(), "Invalid read") {
    regenerateAndPut(key) // rebuild value from app source of truth
}

// Go: verify vlog files are non-truncated before open (size sanity)
func checkVlogIntegrity(dir string) error {
    return filepath.Walk(dir, func(p string, fi os.FileInfo, err error) error {
        if err == nil && strings.HasSuffix(p, ".vlog") && fi.Size() < 20 {
            return fmt.Errorf("vlog %s suspiciously small/truncated", p)
        }
        return nil
    })
}

func isInvalidReadErr(err error) bool {
    return err != nil && strings.Contains(err.Error(), "Invalid read: Len:")
}

item, err := txn.Get(key)
if err != nil && isInvalidReadErr(err) {
    // corrupt/truncated vlog entry: fall back to app source of truth
    log.Printf("corrupt vlog entry for %s: %v", key, err)
    return regenerateValue(key)
}

When it happens

Trigger: Reading a key whose vlog entry is cut short: vlog truncated by a crash before fsync, disk corruption, an entry overwritten via a bad valuePointer, or a vlog written by an incompatible badger version with a different header layout.

Common situations: Power loss with SyncWrites=false leaving a partial entry; backing up a vlog mid-write; bit rot or failing disk; opening vlogs from a different badger version.

Related errors

• Empty value: %+v
• Invalid value pointer offset: %d greater than current offset
• ErrNoRewrite
• ErrTruncateNeeded
• While setting banned keys: %w

AI-assisted analysis of dgraph-io/badger@2a001d466f (2026-09-05). Data as JSON: /api/errors/9ff5f0d1dc91929a. Report an issue: GitHub.