fix: clean up blocksize implementation

It helps to RTFM. `st_blocks` and `st_blksize` are *not* related.
`st_blksize` *really is* the optimal I/O block size. `st_blocks`
is *always* given in multiples of 512 bytes.

CHANGES.md

@@ -9,8 +9,7 @@
 
 - (feature) Added `blocksize` option to the FUSE driver, which allows
   the `st_blksize` value to be configured for the mounted file system.
-  This is necessary on macOS as macFUSE derives the optimum read size
-  from this value. Use along with macFUSE's `iosize` for best effect.
+  Increasing this value can improve throughput for large files.
 
 - (feature) Added experimental `readahead` option to the FUSE driver.
   This can potentially increase throughput when performing sequential

doc/dwarfs.md

@@ -33,11 +33,8 @@ options:
   memory. For more details, see mkdwarfs(1).
 
 - `-o blocksize=`*value*:
-  Size reported for files in `st_blksize`. This is 512 bytes
-  by default on Linux/Windows. On macOS, the value is 256 kiB,
-  as macFUSE uses this to derive the maximum size of a read
-  request. Use this along with macFUSE's `iosize` option to
-  tune throughput.
+  Size reported for files in `st_blksize`. You can use this to
+  optimize throughput in certain situations.
 
 - `-o readahead=`*value*:
   How much data to read ahead when receiving a read request.

src/dwarfs/metadata_v2.cpp

@@ -1554,7 +1554,7 @@ int metadata_<LoggerPolicy>::getattr(inode_view iv, file_stat* stbuf) const {
                                       : file_size(iv, mode);
   stbuf->ino = inode + inode_offset_;
   stbuf->blksize = options_.block_size;
-  stbuf->blocks = (stbuf->size + options_.block_size - 1) / options_.block_size;
+  stbuf->blocks = (stbuf->size + 511) / 512;
   stbuf->uid = iv.getuid();
   stbuf->gid = iv.getgid();
   stbuf->mtime = resolution * (timebase + iv.mtime_offset());

src/dwarfs_main.cpp

@@ -138,11 +138,7 @@ namespace dwarfs {
 
 namespace {
 
-#ifdef __APPLE__
-constexpr size_t const kDefaultBlockSize{static_cast<size_t>(256) << 10};
-#else
-constexpr size_t const kDefaultBlockSize{512};
-#endif
+constexpr size_t const kDefaultBlockSize{static_cast<size_t>(512) << 10};
 
 struct options {
   // std::string isn't standard-layout on MSVC
@@ -1021,7 +1017,7 @@ void usage(std::ostream& os, std::filesystem::path const& progname) {
      << " <image> <mountpoint> [options]\n\n"
      << "DWARFS options:\n"
      << "    -o cachesize=SIZE      set size of block cache (512M)\n"
-     << "    -o blocksize=SIZE      set file block size\n"
+     << "    -o blocksize=SIZE      set file I/O block size (512K)\n"
      << "    -o readahead=SIZE      set readahead size (0)\n"
      << "    -o workers=NUM         number of worker threads (2)\n"
      << "    -o mlock=NAME          mlock mode: (none), try, must\n"