fix: BatchIO review fixes — linked SQE, ring overflow, resource leak, sync parity

1. HIGH: LinkedWriteFsync now uses SubmitLinkRequests (IOSQE_IO_LINK)
   instead of SubmitRequests, ensuring write+fdatasync execute as a
   linked chain in the kernel. Falls back to sequential on error.

2. HIGH: PreadBatch/PwriteBatch chunk ops by ring capacity to prevent
   "too many requests" rejection when dirty map exceeds ring size (256).

3. MED: CloseBatchIO() added to Flusher, called in BlockVol.Close()
   after final flush to release io_uring ring / kernel resources.

4. MED: Sync parity — both standard and io_uring paths now use
   fdatasync (via platform-specific fdatasync_linux.go / fdatasync_other.go).
   Standard path previously used fsync; now matches io_uring semantics.
   On non-Linux, fdatasync falls back to fsync (only option available).

10 batchio tests, all blockvol tests pass.

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

weed/storage/blockvol/batchio/batchio.go

@@ -33,12 +33,15 @@ type BatchIO interface {
 	// Op.Offset. Returns the first error encountered.
 	PwriteBatch(fd *os.File, ops []Op) error
 
-	// Fsync issues fdatasync on the file.
+	// Fsync issues fdatasync on the file, flushing data to disk without
+	// updating file metadata (mtime, size). On non-Linux platforms where
+	// fdatasync is unavailable, falls back to fsync. Both backends use
+	// identical sync semantics.
 	Fsync(fd *os.File) error
 
-	// LinkedWriteFsync writes buf at offset then fsyncs, as an atomic pair.
-	// On io_uring this is a linked SQE chain (one syscall).
-	// On standard, this is sequential write + fdatasync.
+	// LinkedWriteFsync writes buf at offset then issues fdatasync as a pair.
+	// On io_uring this is a linked SQE chain (one io_uring_enter syscall).
+	// On standard, this is sequential pwrite + fdatasync.
 	LinkedWriteFsync(fd *os.File, buf []byte, offset int64) error
 
 	// Close releases resources (io_uring ring, etc). No-op for standard.

weed/storage/blockvol/batchio/fdatasync_linux.go

@@ -0,0 +1,14 @@
+//go:build linux
+
+package batchio
+
+import (
+	"os"
+	"syscall"
+)
+
+// fdatasync flushes file data to disk without updating metadata (mtime, size).
+// On Linux, this uses the fdatasync(2) syscall directly.
+func fdatasync(fd *os.File) error {
+	return syscall.Fdatasync(int(fd.Fd()))
+}

weed/storage/blockvol/batchio/fdatasync_other.go

@@ -0,0 +1,11 @@
+//go:build !linux
+
+package batchio
+
+import "os"
+
+// fdatasync flushes file data to disk. On non-Linux platforms, fdatasync is
+// not available, so this falls back to fsync via os.File.Sync().
+func fdatasync(fd *os.File) error {
+	return fd.Sync()
+}

weed/storage/blockvol/batchio/iouring_linux.go

@@ -5,7 +5,6 @@ package batchio
 import (
 	"fmt"
 	"os"
-	"syscall"
 
 	"github.com/iceber/iouring-go"
 )
@@ -13,7 +12,8 @@ import (
 // ioUringBatchIO implements BatchIO using Linux io_uring.
 // Requires kernel 5.6+ (linked fsync: 5.10+).
 type ioUringBatchIO struct {
-	ring *iouring.IOURing
+	ring     *iouring.IOURing
+	ringSize int // max SQEs per submission
 }
 
 // NewIOUring creates a BatchIO backed by io_uring with the given ring size.
@@ -25,17 +25,32 @@ func NewIOUring(ringSize uint) (BatchIO, error) {
 		// Kernel doesn't support io_uring — fall back silently.
 		return NewStandard(), nil
 	}
-	return &ioUringBatchIO{ring: ring}, nil
+	return &ioUringBatchIO{ring: ring, ringSize: int(ringSize)}, nil
 }
 
 func (u *ioUringBatchIO) PreadBatch(fd *os.File, ops []Op) error {
 	if len(ops) == 0 {
 		return nil
 	}
+	// Submit in chunks that fit the ring to avoid "too many requests" rejection.
+	for start := 0; start < len(ops); start += u.ringSize {
+		end := start + u.ringSize
+		if end > len(ops) {
+			end = len(ops)
+		}
+		if err := u.preadChunk(fd, ops[start:end]); err != nil {
+			return err
+		}
+	}
+	return nil
+}
 
+func (u *ioUringBatchIO) preadChunk(fd *os.File, ops []Op) error {
 	requests := make([]iouring.PrepRequest, len(ops))
+	// fd.Fd() is safe: the os.File stays live through this call.
+	fdInt := int(fd.Fd())
 	for i := range ops {
-		requests[i] = iouring.Pread(int(fd.Fd()), ops[i].Buf, uint64(ops[i].Offset))
+		requests[i] = iouring.Pread(fdInt, ops[i].Buf, uint64(ops[i].Offset))
 	}
 
 	results, err := u.ring.SubmitRequests(requests, nil)
@@ -43,7 +58,6 @@ func (u *ioUringBatchIO) PreadBatch(fd *os.File, ops []Op) error {
 		return fmt.Errorf("iouring PreadBatch submit: %w", err)
 	}
 
-	// Wait for all completions and check results.
 	for i, res := range results {
 		<-res.Done()
 		n, err := res.ReturnInt()
@@ -61,10 +75,24 @@ func (u *ioUringBatchIO) PwriteBatch(fd *os.File, ops []Op) error {
 	if len(ops) == 0 {
 		return nil
 	}
+	// Submit in chunks that fit the ring to avoid "too many requests" rejection.
+	for start := 0; start < len(ops); start += u.ringSize {
+		end := start + u.ringSize
+		if end > len(ops) {
+			end = len(ops)
+		}
+		if err := u.pwriteChunk(fd, ops[start:end]); err != nil {
+			return err
+		}
+	}
+	return nil
+}
 
+func (u *ioUringBatchIO) pwriteChunk(fd *os.File, ops []Op) error {
 	requests := make([]iouring.PrepRequest, len(ops))
+	fdInt := int(fd.Fd())
 	for i := range ops {
-		requests[i] = iouring.Pwrite(int(fd.Fd()), ops[i].Buf, uint64(ops[i].Offset))
+		requests[i] = iouring.Pwrite(fdInt, ops[i].Buf, uint64(ops[i].Offset))
 	}
 
 	results, err := u.ring.SubmitRequests(requests, nil)
@@ -85,6 +113,10 @@ func (u *ioUringBatchIO) PwriteBatch(fd *os.File, ops []Op) error {
 	return nil
 }
 
+// Fsync issues fdatasync via io_uring. fdatasync flushes data to disk without
+// updating file metadata (size, mtime). This is sufficient for pre-allocated
+// extent files where metadata does not change during writes, and matches the
+// standard path behavior (see standard_linux.go).
 func (u *ioUringBatchIO) Fsync(fd *os.File) error {
 	req := iouring.Fdatasync(int(fd.Fd()))
 	result, err := u.ring.SubmitRequest(req, nil)
@@ -99,34 +131,33 @@ func (u *ioUringBatchIO) Fsync(fd *os.File) error {
 	return nil
 }
 
+// LinkedWriteFsync submits a pwrite + fdatasync as a linked SQE chain.
+// The kernel executes them in order in a single io_uring_enter() call.
+// Requires kernel 5.10+ for linked fsync. Falls back to sequential on error.
 func (u *ioUringBatchIO) LinkedWriteFsync(fd *os.File, buf []byte, offset int64) error {
-	// Try linked SQE chain: pwrite → fdatasync (one io_uring_enter).
-	// This requires IOSQE_IO_LINK support (kernel 5.3+) and linked fsync (5.10+).
-	// If the ring doesn't support it, fall back to sequential.
-	writeReq := iouring.Pwrite(int(fd.Fd()), buf, uint64(offset))
-	fsyncReq := iouring.Fdatasync(int(fd.Fd()))
-
-	// SubmitRequests with linked flag.
-	results, err := u.ring.SubmitRequests(
+	fdInt := int(fd.Fd())
+	writeReq := iouring.Pwrite(fdInt, buf, uint64(offset))
+	fsyncReq := iouring.Fdatasync(fdInt)
+
+	// SubmitLinkRequests sets IOSQE_IO_LINK on all SQEs except the last,
+	// ensuring the fdatasync executes only after the pwrite completes.
+	results, err := u.ring.SubmitLinkRequests(
 		[]iouring.PrepRequest{writeReq, fsyncReq},
 		nil,
 	)
 	if err != nil {
-		// Fallback to sequential if linking not supported.
-		if errno, ok := err.(syscall.Errno); ok && errno == syscall.EINVAL {
-			if _, werr := fd.WriteAt(buf, offset); werr != nil {
-				return werr
-			}
-			return fd.Sync()
+		// Fallback to sequential if linked submission fails.
+		if _, werr := fd.WriteAt(buf, offset); werr != nil {
+			return werr
 		}
-		return fmt.Errorf("iouring LinkedWriteFsync submit: %w", err)
+		return fdatasync(fd)
 	}
 
 	for i, res := range results {
 		<-res.Done()
-		_, err := res.ReturnInt()
-		if err != nil {
-			return fmt.Errorf("iouring LinkedWriteFsync op[%d]: %w", i, err)
+		_, rerr := res.ReturnInt()
+		if rerr != nil {
+			return fmt.Errorf("iouring LinkedWriteFsync op[%d]: %w", i, rerr)
 		}
 	}
 	return nil

weed/storage/blockvol/batchio/standard.go

@@ -3,7 +3,7 @@ package batchio
 import "os"
 
 // standardBatchIO implements BatchIO with sequential os.File calls.
-// This is functionally identical to calling ReadAt/WriteAt/Sync directly.
+// This is functionally identical to calling ReadAt/WriteAt/fdatasync directly.
 type standardBatchIO struct{}
 
 // NewStandard returns a BatchIO that uses sequential pread/pwrite/fdatasync.
@@ -30,15 +30,18 @@ func (s *standardBatchIO) PwriteBatch(fd *os.File, ops []Op) error {
 	return nil
 }
 
+// Fsync issues fdatasync to flush data to disk. Uses fdatasync(2) on Linux
+// for parity with the io_uring path. Falls back to fsync on other platforms.
 func (s *standardBatchIO) Fsync(fd *os.File) error {
-	return fd.Sync()
+	return fdatasync(fd)
 }
 
+// LinkedWriteFsync writes buf at offset then issues fdatasync, sequentially.
 func (s *standardBatchIO) LinkedWriteFsync(fd *os.File, buf []byte, offset int64) error {
 	if _, err := fd.WriteAt(buf, offset); err != nil {
 		return err
 	}
-	return fd.Sync()
+	return fdatasync(fd)
 }
 
 func (s *standardBatchIO) Close() error {

weed/storage/blockvol/blockvol.go

@@ -1127,8 +1127,9 @@ func (v *BlockVol) Close() error {
 	}
 	var flushErr error
 	if v.flusher != nil {
-		v.flusher.Stop()         // stop background goroutine first (no concurrent flush)
+		v.flusher.Stop()                 // stop background goroutine first (no concurrent flush)
 		flushErr = v.flusher.FlushOnce() // then do final flush safely
+		v.flusher.CloseBatchIO()         // release io_uring ring / kernel resources
 	}
 
 	// Close snapshot delta files.

weed/storage/blockvol/flusher.go

@@ -434,6 +434,15 @@ func (f *Flusher) CheckpointLSN() uint64 {
 	return f.checkpointLSN
 }
 
+// CloseBatchIO releases the batch I/O backend resources (e.g. io_uring ring).
+// Must be called after Stop() and the final FlushOnce().
+func (f *Flusher) CloseBatchIO() error {
+	if f.bio != nil {
+		return f.bio.Close()
+	}
+	return nil
+}
+
 // SetFD replaces the file descriptor used for extent writes. Test-only.
 func (f *Flusher) SetFD(fd *os.File) {
 	f.mu.Lock()