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. - 

}
    return {};
}
bool StorageReplicatedMergeTree::executeLogEntry(LogEntry & entry)
{
    if (entry.type == LogEntry::DROP_RANGE)
    {
        executeDropRange(entry);
        return true;
    }
    if (entry.type == LogEntry::REPLACE_RANGE)
    {
        executeReplaceRange(entry);
        return true;
    }
    const bool is_get_or_attach = entry.type == LogEntry::GET_PART || entry.type == LogEntry::ATTACH_PART;
    if (is_get_or_attach || entry.type == LogEntry::MERGE_PARTS || entry.type == LogEntry::MUTATE_PART)
    {
        
        
        
        DataPartPtr existing_part = getPartIfExists(entry.new_part_name, {MergeTreeDataPartState::PreCommitted});
        if (!existing_part)
            existing_part = getActiveContainingPart(entry.new_part_name);
        
        if (existing_part && getZooKeeper()->exists(fs::path(replica_path) / "parts" / existing_part->name))
        {
            if (!is_get_or_attach || entry.source_replica != replica_name)
                LOG_DEBUG(log, "Skipping action for part {} because part {} already exists.",
                    entry.new_part_name, existing_part->name);