nats-io/nats-server · error

Partial cache: offset slot index %d is less zero

Error message

Partial cache: offset slot index %d is less zero

What it means

A warning logged by msgBlock.slotInfo() when a negative slot index is requested from the partial cache (cache == nil and slot < 0 case aside, this guards slot < 0). It indicates an internal accounting bug — the computed offset of a sequence into the cache index is negative — and slotInfo returns errPartialCache to the caller.

Source

Thrown at server/filestore.go:6612

	// Make sure we clear the cache since no longer valid.
	mb.clearCacheAndOffset()
	// If we entered with the msgs loaded make sure to reload them.
	if wasLoaded {
		if err := mb.loadMsgsWithLock(); err != nil {
			return err
		}
	}
	return nil
}

// Grab info from a slot.
// Lock should be held.
func (mb *msgBlock) slotInfo(slot int) (uint32, uint32, bool, error) {
	switch {
	case mb.cache == nil: // Shouldn't be possible, but check it anyway.
		return 0, 0, false, errNoCache
	case slot < 0:
		mb.fs.warn("Partial cache: offset slot index %d is less zero", slot)
		return 0, 0, false, errPartialCache
	case slot >= len(mb.cache.idx):
		mb.fs.warn("Partial cache: offset slot index %d is greater than index len %d", slot, len(mb.cache.idx))
		return 0, 0, false, errPartialCache
	}

	bi := mb.cache.idx[slot]
	ri, hashChecked := (bi &^ cbit), (bi&cbit) != 0

	// If this is a deleted slot return here.
	if bi == dbit {
		return 0, 0, false, errDeletedMsg
	}

	// Determine record length
	var rl uint32
	// Need to account for dbit markers in idx.
	// So we will walk until we find valid idx slot to calculate rl.

View on GitHub (pinned to 3a66a489d2)

Solutions

1. Report to maintainers with the stream/sequence numbers: negative slot indices signal an internal bug
2. Verify sequence numbers requested against the block's first sequence (cache fseq)
3. Restart the server to rebuild block caches

When it happens

Trigger: Thrown at server/filestore.go:6612 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of nats-io/nats-server@3a66a489d2 (2026-09-02). Data as JSON: /api/errors/9a7828854f3cafa6. Report an issue: GitHub.