fix: require explicit build tags for io_uring backends, add implementation logging

All three io_uring backends (iceber, giouring, raw) now require explicit
build tags — no tag means standard-only. Each backend registers its name
via IOUringImpl so startup logs show compiled implementation alongside
requested/selected backend mode.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

weed/storage/blockvol/batchio/batchio.go

@@ -23,6 +23,11 @@ import (
 // Callers decide whether to fall back to standard or fail.
 var ErrIOUringUnavailable = errors.New("batchio: io_uring unavailable")
 
+// IOUringImpl reports the compiled io_uring implementation name.
+// Set by each iouring_*_linux.go file. Empty string means no io_uring
+// backend was compiled in (standard only).
+var IOUringImpl string
+
 // Op represents a single I/O operation: read or write buf at offset.
 type Op struct {
 	Buf    []byte

weed/storage/blockvol/batchio/iouring_giouring_linux.go

@@ -10,6 +10,8 @@ import (
 	"github.com/pawelgaczynski/giouring"
 )
 
+func init() { IOUringImpl = "giouring" }
+
 // giouringBatchIO implements BatchIO using giouring (direct liburing port).
 // No goroutines or channels — direct SQE/CQE ring manipulation.
 // Requires kernel 6.0+.

weed/storage/blockvol/batchio/iouring_iceber_linux.go

@@ -9,6 +9,8 @@ import (
 	"github.com/iceber/iouring-go"
 )
 
+func init() { IOUringImpl = "iceber" }
+
 // ioUringBatchIO implements BatchIO using Linux io_uring.
 // Requires kernel 5.6+ (linked fsync: 5.10+).
 type ioUringBatchIO struct {

weed/storage/blockvol/batchio/iouring_other.go

@@ -1,9 +1,12 @@
-//go:build !linux || no_iouring
+//go:build !linux || (!iouring_iceber && !iouring_giouring && !iouring_raw)
 
 package batchio
 
-// NewIOUring returns ErrIOUringUnavailable on non-Linux platforms
-// or when io_uring is disabled via the no_iouring build tag.
+// NewIOUring returns ErrIOUringUnavailable when no io_uring backend is
+// compiled in. On Linux, use one of these build tags to enable a backend:
+//   - iouring_iceber   (iceber/iouring-go library)
+//   - iouring_giouring (pawelgaczynski/giouring library)
+//   - iouring_raw      (raw syscall wrappers, zero deps)
 func NewIOUring(ringSize uint) (BatchIO, error) {
 	return nil, ErrIOUringUnavailable
 }

weed/storage/blockvol/batchio/iouring_raw_linux.go

@@ -1,4 +1,4 @@
-//go:build linux && !no_iouring && !iouring_iceber && !iouring_giouring
+//go:build linux && iouring_raw
 
 package batchio
 
@@ -10,6 +10,8 @@ import (
 	"unsafe"
 )
 
+func init() { IOUringImpl = "raw" }
+
 // Raw io_uring syscall numbers.
 const (
 	sysIOUringSetup    = 425

weed/storage/blockvol/blockvol.go

@@ -1165,25 +1165,31 @@ func (v *BlockVol) Close() error {
 //   - "auto": tries io_uring, falls back to standard with warning
 //   - "io_uring": requires io_uring, returns error if unavailable
 func newBatchIO(mode IOBackendMode, logger *log.Logger) (batchio.BatchIO, string, error) {
+	impl := batchio.IOUringImpl // compiled-in implementation ("iceber", "giouring", "raw", or "")
+	if impl == "" {
+		impl = "none"
+	}
+
 	switch mode {
 	case IOBackendIOUring:
 		bio, err := batchio.NewIOUring(256)
 		if err != nil {
-			return nil, "", fmt.Errorf("io_uring requested but unavailable: %w", err)
+			return nil, "", fmt.Errorf("io_uring requested but unavailable (compiled=%s): %w", impl, err)
 		}
-		logger.Printf("io backend: requested=io_uring selected=io_uring")
+		logger.Printf("io backend: requested=io_uring implementation=%s selected=io_uring", impl)
 		return bio, "io_uring", nil
 
 	case IOBackendAuto:
 		bio, err := batchio.NewIOUring(256)
 		if err != nil {
-			logger.Printf("io backend: requested=auto selected=standard reason=%v", err)
+			logger.Printf("io backend: requested=auto implementation=%s selected=standard reason=%v", impl, err)
 			return batchio.NewStandard(), "standard", nil
 		}
-		logger.Printf("io backend: requested=auto selected=io_uring")
+		logger.Printf("io backend: requested=auto implementation=%s selected=io_uring", impl)
 		return bio, "io_uring", nil
 
 	default: // IOBackendStandard or empty
+		logger.Printf("io backend: requested=standard implementation=%s selected=standard", impl)
 		return batchio.NewStandard(), "standard", nil
 	}
 }