@CanIgnoreReturnValue
    public Builder setBlockSize(int blockSize) {
      checkArgument(blockSize > 0, "blockSize (%s) must be positive", blockSize);
      this.blockSize = blockSize;
      return this;
    }
Sets the block size (in bytes) for the file system to use. All regular files will be
allocated blocks of the given size, so this is the minimum granularity for file size.

<p>The default is 8192 bytes (8 KB).
jimfs/src/main/java/com/google/common/jimfs/Configuration.java