简介：
Elasticsearch 由于使用TTL,在文档量很大的时候，如果同时有大量文档过期，可能会导致集群节点OOM。本文记录这一现象，以及问题分析，处理步骤。
1.现象
今天我们的ES出现了OOM。ES版本是2.1.2，日志如下：
[2017-06-21 11:10:12,250][WARN ][monitor.jvm ] [dm_172.23.41.93:20002] [gc][young][535][56] duration [1.4s], collections [1]/[2.3s], total [1.4s]/[10.6s], memory [9.5gb]->[8.5gb]/[15.8gb], all_pools {[young] [1.3gb]->[26.5mb]/[1.4gb]}{[survivor] [191.3mb]->[191.3mb]/[191.3mb]}{[old] [8gb]->[8.3gb]/[14.1gb]}
[2017-06-21 11:14:06,057][WARN ][monitor.jvm ] [dm_172.23.41.93:20002] [gc][young][766][83] duration [2.1s], collections [1]/[3.1s], total [2.1s]/[15.7s], memory [12.6gb]->[11.7gb]/[15.8gb], all_pools {[young] [1.3gb]->[30.1mb]/[1.4gb]}{[survivor] [191.3mb]->[191.3mb]/[191.3mb]}{[old] [11.1gb]->[11.5gb]/[14.1gb]}
[2017-06-21 11:18:20,668][WARN ][monitor.jvm ] [dm_172.23.41.93:20002] [gc][old][985][17] duration [35.9s], collections [1]/[36.2s], total [35.9s]/[38.1s], memory [15.6gb]->[14.2gb]/[15.8gb], all_pools {[young] [1.4gb]->[96.6mb]/[1.4gb]}{[survivor] [148mb]->[0b]/[191.3mb]}{[old] [14gb]->[14.1gb]/[14.1gb]}
内存增长很快，16G的内存，启动后几分钟就满了，而且回收不了。重启还是一样，很快就OOM。多次反复重启，反复OOM，看来集群恢复不了了。
查看索引，有一个1,283,152,933 文档,主分片总大小996.1GB的索引。索引mapping如下：
{
"cluster_name": "es-302",
"metadata": {
"indices": {
"msg_center_log": {
"settings": {
"index": {
"number_of_shards": "5",
"creation_date": "1490263243013",
"analysis": {
"char_filter": {
"extend_to_space": {
"mappings": [
"\"extend1\":\"=>\\b",
"\",\"venderId\":\"=>\\b",
"\",\"extend2\":\"=>\\b",
"\",\"type\"=>\\b"
],
"type": "mapping"
}
},
"analyzer": {
"my_analyzer": {
"filter": "lowercase",
"char_filter": [
"extend_to_space",
"html_strip"
],
"type": "custom",
"tokenizer": "standard"
}
}
},
"number_of_replicas": "1",
"uuid": "ShmjnFHVS0W25E0EphFznA",
"version": {
"created": "2010399"
}
}
},
"mappings": {
"message": {
"_routing": {
"required": true
},
"_ttl": {
"default": 172800000,
"enabled": true
},
"_timestamp": {
"enabled": true
},
"_all": {
"enabled": true
},
"properties": {
"msgType": {
"index": "not_analyzed",
"store": true,
"type": "string"
},
"source": {
"index": "not_analyzed",
"store": true,
"type": "string"
},
"deviceId": {
"index": "not_analyzed",
"store": true,
"type": "string"
},
"msgStatus": {
"index": "not_analyzed",
"store": true,
"type": "string"
},
"content": {
"analyzer": "my_analyzer",
"store": true,
"type": "string"
},
"pushType": {
"index": "not_analyzed",
"store": true,
"type": "string"
},
"pin": {
"index": "not_analyzed",
"store": true,
"type": "string"
},
"extend2": {
"index": "not_analyzed",
"store": true,
"type": "string"
},
"msgNode": {
"index": "not_analyzed",
"store": true,
"type": "string"
},
"msgTime": {
"index": "not_analyzed",
"store": true,
"type": "string"
},
"extend1": {
"index": "not_analyzed",
"store": true,
"type": "string"
},
"serviceNoId": {
"index": "not_analyzed",
"store": true,
"type": "string"
},
"jmMsgType": {
"index": "not_analyzed",
"store": true,
"type": "string"
},
"msgUniqueId": {
"index": "not_analyzed",
"store": true,
"type": "string"
},
"timestamp": {
"index": "not_analyzed",
"store": true,
"type": "string"
}
}
}
},
"aliases": [],
"state": "open"
}
},
"cluster_uuid": "_na_",
"templates": {}
}
}可以看到该索引使用了TTL。
扫描二维码关注公众号，回复：
5072589 查看本文章
2.分析内存
通过jmap获取堆转储快照，9326 是ES进程的pid，命令如下：
jmap -dump:live,format=b,file=/export/dump/dump.hd 9326在Linux上安装mat，进入mat目录，使用mat工具分析dump文件,命令如下：
./ParseHeapDump.sh /export/dump/dump.hd
./ParseHeapDump.sh /export/dump/dump.hd org.eclipse.mat.api:suspects
./ParseHeapDump.sh /export/dump/dump.hd org.eclipse.mat.api:overview
./ParseHeapDump.sh /export/dump/dump.hd org.eclipse.mat.api:top_components生成三个报告：
dump_Top_Components.zip
dump_System_Overview.zip
dump_Leak_Suspects.zip
将这三个文件下载下来，解压，浏览可以看到：
可以看到 org.elasticsearch.indices.ttl.IndicesTTLService$PurgerThread这个就是问题所在。
3.问题分析
查看ES源码 org.elasticsearch.indices.ttl.IndicesTTLService构造函数如下：PurgerThread
@Inject
public IndicesTTLService(Settings settings, ClusterService clusterService, IndicesService indicesService, NodeSettingsService nodeSettingsService, TransportBulkAction bulkAction) {
super(settings);
this.clusterService = clusterService;
this.indicesService = indicesService;
TimeValue interval = this.settings.getAsTime("indices.ttl.interval", TimeValue.timeValueSeconds(60));//60s 执行一次
this.bulkAction = bulkAction;
this.bulkSize = this.settings.getAsInt("indices.ttl.bulk_size", 10000);//文档过期后，一次bulk发送删除请求的文档数
this.purgerThread = new PurgerThread(EsExecutors.threadName(settings, "[ttl_expire]"), interval);//启动定时任务，清理ttl过期文档，默认周期60s,
nodeSettingsService.addListener(new ApplySettings());
}IndicesTTLService构造函数中启动了一个清除过期文档的定时周期任务，PurgerThread，任务定义如下：
@Override
public void run() {
try {
while (running.get()) {
try {
List<IndexShard> shardsToPurge = getShardsToPurge();//获取需要进行ttl处理的主分片
purgeShards(shardsToPurge);//获取需要删除的文档，发送bulk delete请求purgeShards()方法如下：
private void purgeShards(List<IndexShard> shardsToPurge) {
for (IndexShard shardToPurge : shardsToPurge) {
Query query = shardToPurge.indexService().mapperService().smartNameFieldType(TTLFieldMapper.NAME).rangeQuery(null, System.currentTimeMillis(), false, true);
Engine.Searcher searcher = shardToPurge.acquireSearcher("indices_ttl");
try {
logger.debug("[{}][{}] purging shard", shardToPurge.routingEntry().index(), shardToPurge.routingEntry().id());
ExpiredDocsCollector expiredDocsCollector = new ExpiredDocsCollector();
searcher.searcher().search(query, expiredDocsCollector);//调用Lucene，获取需要删除的文档，
List<DocToPurge> docsToPurge = expiredDocsCollector.getDocsToPurge();//需要删除的文档
BulkRequest bulkRequest = new BulkRequest();
for (DocToPurge docToPurge : docsToPurge) {
bulkRequest.add(new DeleteRequest().index(shardToPurge.routingEntry().index()).type(docToPurge.type).id(docToPurge.id).version(docToPurge.version).routing(docToPurge.routing));
bulkRequest = processBulkIfNeeded(bulkRequest, false);//如果bulk 请求删除的文档数达到了indices.ttl.bulk_size，发送bulk delete 请求
}
processBulkIfNeeded(bulkRequest, true);通过对比上面的dump分析图表，可以确定占用13.7G内存的就是下面这个
List<DocToPurge> docsToPurge = expiredDocsCollector.getDocsToPurge();//需要删除的文档
同时有63,191,376 个TTL过期的文档需要删除，org.elasticsearch.indices.ttl.IndicesTTLService$DocToPurge对象将内存占满了。
| Class Name | Objects | Shallow Heap | Retained Heap |
| org.elasticsearch.indices.ttl.IndicesTTLService$PurgerThread | 1 | 136 | >= 14,761,919,848 |
| org.elasticsearch.indices.ttl.IndicesTTLService$PurgerThread | 1 | 24 | >= 14,761,340,104 |
| org.elasticsearch.indices.ttl.IndicesTTLService$DocToPurge | 63,191,376 | 2,022,124,032 | >= 14,480,975,760 |
| org.elasticsearch.indices.ttl.IndicesTTLService$DocToPurge | 189,639,704 | 4,551,352,896 | >= 12,465,602,816 |
4.解决方案
通过动态设置索引的参数
index.ttl.disable_purge
[experimental] Disables the purge of expired docs on the current index.
禁用掉ttl，使ES不对ttl过期的文档进行处理。
PUT index1/_settings
{
"ttl.disable_purge": true
}
这样ES才能正常启动起来。然后想办法主动删除过期的文档。
配置参考这里:https://www.elastic.co/guide/en/elasticsearch/reference/2.1/index-modules.html
总结：Elasticsearch 高版本已经取消了TTL功能，所以最好不要用TTL，如果需要删除文档，可以每天创建一个索引，到期后直接删除过期的索引，方便快捷，这也是官方推荐的方案。