optimize performance: reduce allocations, buffered I/O, and zero-copy reads

- Read path: eliminate redundant allocation in bsPerformCommand - remove
  the pre-allocation before bs.Read() and the append loop for zero-fill,
  use direct copy and in-place zero-fill instead
- parseHeader: use command pool (getCommand) instead of direct allocation,
  reducing GC pressure on the hot path
- Unmap: use a shared 1MB zero buffer instead of allocating per-descriptor,
  dramatically reducing allocations for large unmap operations
- Network I/O: add 256KB bufio.Writer to iSCSI connections, batching
  small PDU writes into fewer syscalls. Flush after txHandler completes

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

pkg/scsi/backingstore/common.go

@@ -159,11 +159,26 @@ func (bs *FileBackingStore) DataAdvise(offset, length int64, advise uint32) erro
 	return util.Fadvise(bs.file, offset, length, advise)
 }
 
+// unmapZeroBufSize is the size of the reusable zero buffer for unmap operations.
+const unmapZeroBufSize = 1 << 20 // 1MB
+
+// unmapZeroBuf is a pre-allocated zero buffer shared across unmap calls.
+var unmapZeroBuf = make([]byte, unmapZeroBufSize)
+
 func (bs *FileBackingStore) Unmap(descriptors []api.UnmapBlockDescriptor) error {
 	for _, desc := range descriptors {
-		zeros := make([]byte, desc.TL)
-		if _, err := bs.file.WriteAt(zeros, int64(desc.Offset)); err != nil {
-			return err
+		remaining := desc.TL
+		off := int64(desc.Offset)
+		for remaining > 0 {
+			writeLen := remaining
+			if writeLen > unmapZeroBufSize {
+				writeLen = unmapZeroBufSize
+			}
+			if _, err := bs.file.WriteAt(unmapZeroBuf[:writeLen], off); err != nil {
+				return err
+			}
+			off += int64(writeLen)
+			remaining -= writeLen
 		}
 	}
 	return nil