Symptoms: all threads in BackgroundScheduledPool busy, lot of tasks in replication queue, none of the tasks is executing. 

![Screenshot from 2021-04-16 16-21-44_](https://user-images.githubusercontent.com/1549571/115193796-e767d600-a0ec-11eb-887c-5c9c2412e50c.png)

Related to #16895

Possible workarounds (poor options): increasing the background_schedule_pool_size to huge values, or restart server every time when that happens etc. - 

{
        throw Exception("Cannot alter version column " + backQuoteIfNeed(column_name) +
            " from type " + old_type->getName() +
            " to type " + new_type->getName() + " because new type will change sort order of version column." +
            " The only possible conversion is expansion of the number of bytes of the current type."
            , ErrorCodes::ALTER_OF_COLUMN_IS_FORBIDDEN);
    }
    
    if (new_type->getSizeOfValueInMemory() < old_type->getSizeOfValueInMemory())
    {
        throw Exception("Cannot alter version column " + backQuoteIfNeed(column_name) +
            " from type " + old_type->getName() +
            " to type " + new_type->getName() + " because new type is smaller than current in the number of bytes." +
            " The only possible conversion is expansion of the number of bytes of the current type."
            , ErrorCodes::ALTER_OF_COLUMN_IS_FORBIDDEN);
    }
}
}
void MergeTreeData::checkAlterIsPossible(const AlterCommands & commands, ContextPtr local_context) const
{
    
    StorageInMemoryMetadata new_metadata = getInMemoryMetadata();
    StorageInMemoryMetadata old_metadata = getInMemoryMetadata();
    const auto & settings = local_context->getSettingsRef();
    if (!settings.allow_non_metadata_alters)
    {
        auto mutation_commands = commands.getMutationCommands(new_metadata, settings.materialize_ttl_after_modify, getContext());
        if (!mutation_commands.empty())
            throw Exception(ErrorCodes::ALTER_OF_COLUMN_IS_FORBIDDEN, "The following alter commands: '{}' will modify data on disk, but setting `allow_non_metadata_alters` is disabled", queryToString(mutation_commands.ast()));
    }