File size is updated correctly according to 

> fdatasync() is similar to fsync(), but does not flush modified metadata unless that metadata is needed in order to allow a subsequent data retrieval to be correctly handled.  For example, changes to  st_atime  or  st_mtime  (respec‐
       tively,  time  of  last  access  and  time  of last modification; see inode(7)) do not require flushing because they are not necessary for a subsequent data read to be handled correctly.  On the other hand, a change to the file size
       (st_size, as made by say ftruncate(2)), would require a metadata flush.

And we don't care about modification times.
Should work alright.
{
        if constexpr (dictionary_key_type == DictionaryKeyType::Simple)
            insertColumnsForKeysImpl(keys, columns);
        else
            throw Exception(ErrorCodes::NOT_IMPLEMENTED, "Method insertColumnsForKeys is not supported for complex key storage");
    }
    void insertDefaultKeys(const PaddedPODArray<UInt64> & keys) override
    {
        if constexpr (dictionary_key_type == DictionaryKeyType::Simple)
            insertDefaultKeysImpl(keys);
        else
            throw Exception(ErrorCodes::NOT_IMPLEMENTED, "Method insertDefaultKeysImpl is not supported for complex key storage");
    }
    PaddedPODArray<UInt64> getCachedSimpleKeys() const override
    {
        if constexpr (dictionary_key_type == DictionaryKeyType::Simple)
            return getCachedKeysImpl();
        else
            throw Exception(ErrorCodes::NOT_IMPLEMENTED, "Method getCachedSimpleKeys is not supported for complex key storage");
    }
    bool supportsComplexKeys() const override { return dictionary_key_type == DictionaryKeyType::Complex; }
    ComplexKeysStorageFetchResult fetchColumnsForKeys(