将已有的集合转化为固定集合 db.runCommand({convertToCapped:"test",size:10000});
//check
db.test.state()
//isCap:true
转的时候size 是collection的大小， 另外创建的时候可以制定，最大的documents size （max 属性）
db.createCollection("collect",{capped:true, size:10000, max:20});
坑：创建后，手动create >20 条纪录时，会出现先进先出的队列效果。
> db.createCollection("testC",{capped:true, size:100, max:2});
{ "ok" : 1 }
> db.testC.insert({'name':'wade', 'time':new Date().toLocaleTimeString()})
WriteResult({ "nInserted" : 1 })
> db.testC.find({})
{ "_id" : ObjectId("55d5fdf2c49e250bbf9e4770"), "name" : "wade", "time" : "00:18:58" }
> db.testC.insert({'name':'wade', 'time':new Date().toLocaleTimeString()})
WriteResult({ "nInserted" : 1 })
> db.testC.find({})
{ "_id" : ObjectId("55d5fdf2c49e250bbf9e4770"), "name" : "wade", "time" : "00:18:58" }
{ "_id" : ObjectId("55d5fe02c49e250bbf9e4771"), "name" : "wade", "time" : "00:19:14" }
> db.testC.insert({'name':'wade', 'time':new Date().toLocaleTimeString()})
WriteResult({ "nInserted" : 1 })
> db.testC.find({})
{ "_id" : ObjectId("55d5fe02c49e250bbf9e4771"), "name" : "wade", "time" : "00:19:14" }
{ "_id" : ObjectId("55d5fe08c49e250bbf9e4772"), "name" : "wade", "time" : "00:19:20" }
>
offline simulating
> db.article.find({}).count()
6
> db.runCommand({convertToCapped:"article",size:1000});
{ "ok" : 1 }
> db.article.insert({ 'title':'article title','name':'wade', 'time':new Date().toLocaleTimeString()})
WriteResult({ "nInserted" : 1 })
> db.article.find({}).count()
7
reach it's limit
> while(i<100){db.article.insert({
'title':'article title','name':'wade', 'time':new Date().toLocaleTimeString()}) ; i++}
> db.article.find({}).count()
37
> while(i<1000){db.article.insert({ 'title':'article title','name':'wade', 'time':new Date().toLocaleTimeString()}) ; i++}
999
> db.article.find({}).count()
37
> db.article.insert({ 'title':'article title','name':'wade123', 'time':new Date().toLocaleTimeString()})
WriteResult({ "nInserted" : 1 })
> db.article.find({}).sort({time:-1})
{ "_id" : ObjectId("55d6092af311196af3d3c3b2"), "title" : "article title", "name" : "wade123", "time" : "01:06:50" }
{ "_id" : ObjectId("55d60911f311196af3d3c3b1"), "title" : "article title", "name" : "wade123", "time" : "01:06:25" }
{ "_id" : ObjectId("55d608d5f311196af3d3c395"), "title" : "article title", "name" : "wade", "time" : "01:05:25" }
结果看起来work
> db.article.find({}).count()
37
around 16:10 - 16:40
- 线下运行ok，但在线上发现，超过了大小，再也新增不了。报错：
> { [MongoError: exception: failing update: objects in a capped ns cannot grow] ...}
- 尝试将原的size增加，看看能否插新数据，还是不行。
- 尝试 undo capped , stackoverflow 后发现只有rename ,and recreate data 的方法。由于创建大量数据会锁库很严重。于是紧紧rename 了这个数据库（没有旧数据关系不大，所以这么做）
at last, 做完db 操作，要跑测试代码.