juicedata/juicefs · error

not supported

Error message

not supported

What it means

memcache.stage is a stub: the in-memory cache manager does not support staging blocks for writeback, so it always returns this error. Any code path that tries to stage uploaded/pending blocks into a memory-only cache will get "not supported".

Source

Thrown at pkg/chunk/mem_cache.go:210

				break
			}
			if v.atime.Before(cutoff) {
				deleted++
				freed += int64(cap(v.page.Data))
				c.metrics.cacheEvicts.Add(1)
				c.delete(k, v.page)
			}
		}
		c.Unlock()
		if deleted > 0 {
			logger.Debugf("Expired cache blocks: %d blocks (%s), remaining: %d blocks (%s)", deleted, humanize.IBytes(uint64(freed)), len(c.pages), humanize.IBytes(uint64(c.used)))
		}
		time.Sleep(interval / 1000 * time.Duration((cnt+1-deleted)*1000/(cnt+1)))
	}
}

func (c *memcache) stage(key string, data []byte, tierID uint8) (string, error) {
	return "", errors.New("not supported")
}
func (c *memcache) uploaded(key string, size int)    {}
func (c *memcache) isEmpty() bool                    { return false }
func (c *memcache) getMetrics() *cacheManagerMetrics { return c.metrics }

View on GitHub (pinned to c9a67b23e8)

Solutions

1. Use a disk-based --cache-dir (real path) when writeback/staging is required; memory cache cannot stage.
2. Drop --writeback when using memory cache so blocks are uploaded synchronously instead of staged.
3. If staging was intended for the memory cache, use disk cache or restructure to upload directly.

Example fix

// before
juicefs mount --cache-dir memory --writeback sqlite3://test.db /mnt/jfs
// after
juicefs mount --cache-dir /var/jfsCache --writeback sqlite3://test.db /mnt/jfs

func stagingSupported(cacheDir string) bool {
	return cacheDir != "memory"
}

if _, err := cache.Stage(key, data); err != nil {
	if err.Error() == "not supported" {
		return uploadDirect(key, data)
	}
	return err
}

When it happens

Trigger: Calling stage() on a memcache instance, i.e. running with --cache-dir set to memory (or cache manager resolved to memcache) while a code path requests block staging (writeback flow).

Common situations: Mounting with --cache-dir memory together with --writeback, where staging to disk is impossible; tests that exercise stage against the memory cache.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

• invalid hour number
• space not enough on device
• concurrent staging limit reached
• page is already released
• ErrNotSUP

AI-assisted analysis of juicedata/juicefs@c9a67b23e8 (2026-09-06). Data as JSON: /api/errors/41b94cea3b3cf902. Report an issue: GitHub.