**Use case**

Currently, `system.replicas` table has `total_replicas` and `active_replicas` columns which are useful to check/alert when some replicas are down. One problem, these columns are numbers. They tell when there is a problem, but you need to further check ZooKeeper or query all the nodes to detect what exact replicas (hosts) are down/inactive/have problems.

**Describe the solution you'd like**

Add a replicas column maybe as a map which would contain the replica name, and it's status. Or, add 2 array columns with `replica_names` and `active_replica_names`

**Alternative**

Something along the line

```sql
select path, value from system.zookeeper where path IN (
    select concat(path, '/', name) from system.zookeeper where path in (select concat(zookeeper_path, '/replicas') from system.replicas)
)
and name = 'is_active'
```
* If active = true, consider only active replicas.
      * If found, returns replica name and set 'entry->actual_new_part_name' to name of found largest covering part.
      * If not found, returns empty string.
      */
    String findReplicaHavingCoveringPart(LogEntry & entry, bool active);
    String findReplicaHavingCoveringPart(const String & part_name, bool active, String & found_part_name);
    /** Download the specified part from the specified replica.
      * If `to_detached`, the part is placed in the `detached` directory.
      * If quorum != 0, then the node for tracking the quorum is updated.
      * Returns false if part is already fetching right now.
      */
    bool fetchPart(
        const String & part_name,
        const StorageMetadataPtr & metadata_snapshot,
        const String & replica_path,
        bool to_detached,
        size_t quorum,
        zkutil::ZooKeeper::Ptr zookeeper_ = nullptr);
    /** Download the specified part from the specified replica.
      * Used for replace local part on the same s3-shared part in hybrid storage.
      * Returns false if part is already fetching right now.
      */
    bool fetchExistsPart(
        const String & part_name,
        const StorageMetadataPtr & metadata_snapshot,
        const String & replica_path,
        DiskPtr replaced_disk,
        String replaced_part_path);
    
    std::unordered_set<String> currently_fetching_parts;