Hi,
Output from  _cluster/stats?human&pretty  is:
{
  "_nodes" : {
    "total" : 1,
    "successful" : 1,
    "failed" : 0
  },
  "cluster_name" : "elasticsearch",
  "cluster_uuid" : "rqh_pT6GT-KD3CMXnBeBfQ",
  "timestamp" : 1609849049336,
  "status" : "yellow",
  "indices" : {
    "count" : 304,
    "shards" : {
      "total" : 304,
      "primaries" : 304,
      "replication" : 0.0,
      "index" : {
        "shards" : {
          "min" : 1,
          "max" : 1,
          "avg" : 1.0
        },
        "primaries" : {
          "min" : 1,
          "max" : 1,
          "avg" : 1.0
        },
        "replication" : {
          "min" : 0.0,
          "max" : 0.0,
          "avg" : 0.0
        }
      }
    },
    "docs" : {
      "count" : 281041112,
      "deleted" : 4256731
    },
    "store" : {
      "size" : "138.4gb",
      "size_in_bytes" : 148653071123
    },
    "fielddata" : {
      "memory_size" : "41kb",
      "memory_size_in_bytes" : 42032,
      "evictions" : 0
    },
    "query_cache" : {
      "memory_size" : "28.3mb",
      "memory_size_in_bytes" : 29725903,
      "total_count" : 336809,
      "hit_count" : 35675,
      "miss_count" : 301134,
      "cache_size" : 31897,
      "cache_count" : 33613,
      "evictions" : 1716
    },
    "completion" : {
      "size" : "0b",
      "size_in_bytes" : 0
    },
    "segments" : {
      "count" : 3158,
      "memory" : "184.9mb",
      "memory_in_bytes" : 193915270,
      "terms_memory" : "94.4mb",
      "terms_memory_in_bytes" : 99046155,
      "stored_fields_memory" : "68.9mb",
      "stored_fields_memory_in_bytes" : 72312120,
      "term_vectors_memory" : "0b",
      "term_vectors_memory_in_bytes" : 0,
      "norms_memory" : "4.6mb",
      "norms_memory_in_bytes" : 4860736,
      "points_memory" : "11.3mb",
      "points_memory_in_bytes" : 11933003,
      "doc_values_memory" : "5.4mb",
      "doc_values_memory_in_bytes" : 5763256,
      "index_writer_memory" : "412.8mb",
      "index_writer_memory_in_bytes" : 432866840,
      "version_map_memory" : "1.7mb",
      "version_map_memory_in_bytes" : 1816675,
      "fixed_bit_set" : "3.6mb",
      "fixed_bit_set_memory_in_bytes" : 3823824,
      "max_unsafe_auto_id_timestamp" : 1609836489917,
      "file_sizes" : { }
    }
  },
  "nodes" : {
    "count" : {
      "total" : 1,
      "coordinating_only" : 0,
      "data" : 1,
      "ingest" : 1,
      "master" : 1,
      "voting_only" : 0
    },
    "versions" : [
      "7.3.0"
    ],
    "os" : {
      "available_processors" : 8,
      "allocated_processors" : 8,
      "names" : [
        {
          "name" : "Linux",
          "count" : 1
        }
      ],
      "pretty_names" : [
        {
          "pretty_name" : "CentOS Linux 7 (Core)",
          "count" : 1
        }
      ],
      "mem" : {
        "total" : "13.5gb",
        "total_in_bytes" : 14542573568,
        "free" : "185mb",
        "free_in_bytes" : 194035712,
        "used" : "13.3gb",
        "used_in_bytes" : 14348537856,
        "free_percent" : 1,
        "used_percent" : 99
      }
    },
    "process" : {
      "cpu" : {
        "percent" : 4
      },
      "open_file_descriptors" : {
        "min" : 6514,
        "max" : 6514,
        "avg" : 6514
      }
    },
    "jvm" : {
      "max_uptime" : "4.8d",
      "max_uptime_in_millis" : 418784197,
      "versions" : [
        {
          "version" : "12.0.1",
          "vm_name" : "OpenJDK 64-Bit Server VM",
          "vm_version" : "12.0.1+12",
          "vm_vendor" : "Oracle Corporation",
          "bundled_jdk" : true,
          "using_bundled_jdk" : true,
          "count" : 1
        }
      ],
      "mem" : {
        "heap_used" : "3.8gb",
        "heap_used_in_bytes" : 4176793776,
        "heap_max" : "5.9gb",
        "heap_max_in_bytes" : 6372720640
      },
      "threads" : 101
    },
    "fs" : {
      "total" : "199.8gb",
      "total_in_bytes" : 214639316992,
      "free" : "51.9gb",
      "free_in_bytes" : 55795838976,
      "available" : "51.9gb",
      "available_in_bytes" : 55795838976
    },
    "plugins" : [ ],
    "network_types" : {
      "transport_types" : {
        "security4" : 1
      },
      "http_types" : {
        "security4" : 1
      }
    },
    "discovery_types" : {
      "zen" : 1
    },
    "packaging_types" : [
      {
        "flavor" : "default",
        "type" : "rpm",
        "count" : 1
      }
    ]
  }
}
At time of elasticsearch crush i see in elasticsearch.log:
[2020-12-27T13:42:35,964][ERROR][o.e.x.s.a.e.ReservedRealm] [certuy-elk.cert.uy] failed to retrieve password hash for reserved user [kibana]
org.elasticsearch.action.UnavailableShardsException: at least one primary shard for the index [.security-7] is unavailable
at org.elasticsearch.xpack.security.support.SecurityIndexManager.getUnavailableReason(SecurityIndexManager.java:181) ~[x-pack-security-7.3.0.jar:7.3.0]
at org.elasticsearch.xpack.security.authc.esnative.NativeUsersStore.getReservedUserInfo(NativeUsersStore.java:525) [x-pack-security-7.3.0.jar:7.3.0]
at org.elasticsearch.xpack.security.authc.esnative.ReservedRealm.getUserInfo(ReservedRealm.java:212) [x-pack-security-7.3.0.jar:7.3.0]
at org.elasticsearch.xpack.security.authc.esnative.ReservedRealm.doAuthenticate(ReservedRealm.java:93) [x-pack-security-7.3.0.jar:7.3.0]
at org.elasticsearch.xpack.security.authc.support.CachingUsernamePasswordRealm.authenticateWithCache(CachingUsernamePasswordRealm.java:166) [x-pack-security-7.3.0.jar:7.3.0]
at org.elasticsearch.xpack.security.authc.support.CachingUsernamePasswordRealm.authenticate(CachingUsernamePasswordRealm.java:103) [x-pack-security-7.3.0.jar:7.3.0]
at org.elasticsearch.xpack.security.authc.AuthenticationService$Authenticator.lambda$consumeToken$15(AuthenticationService.java:364) [x-pack-security-7.3.0.jar:7.3.0]
at org.elasticsearch.xpack.core.common.IteratingActionListener.run(IteratingActionListener.java:102) [x-pack-core-7.3.0.jar:7.3.0]
at org.elasticsearch.xpack.security.authc.AuthenticationService$Authenticator.consumeToken(AuthenticationService.java:407) [x-pack-security-7.3.0.jar:7.3.0]
at org.elasticsearch.xpack.security.authc.AuthenticationService$Authenticator.lambda$extractToken$11(AuthenticationService.java:335) [x-pack-security-7.3.0.jar:7.3.0]
at org.elasticsearch.xpack.security.authc.AuthenticationService$Authenticator.extractToken(AuthenticationService.java:345) [x-pack-security-7.3.0.jar:7.3.0]
at org.elasticsearch.xpack.security.authc.AuthenticationService$Authenticator.lambda$checkForApiKey$3(AuthenticationService.java:288) [x-pack-security-7.3.0.jar:7.3.0]
at org.elasticsearch.action.ActionListener$1.onResponse(ActionListener.java:62) [elasticsearch-7.3.0.jar:7.3.0]
at org.elasticsearch.xpack.security.authc.ApiKeyService.authenticateWithApiKeyIfPresent(ApiKeyService.java:357) [x-pack-security-7.3.0.jar:7.3.0]
at org.elasticsearch.xpack.security.authc.AuthenticationService$Authenticator.checkForApiKey(AuthenticationService.java:269) [x-pack-security-7.3.0.jar:7.3.0]
at org.elasticsearch.xpack.security.authc.AuthenticationService$Authenticator.lambda$authenticateAsync$0(AuthenticationService.java:252) [x-pack-security-7.3.0.jar:7.3.0]
at org.elasticsearch.action.ActionListener$1.onResponse(ActionListener.java:62) [elasticsearch-7.3.0.jar:7.3.0]
However after restarting index is recovered.