去年的筆記
For instance, if a chunk represents a single shard key value, then MongoDB cannot split the chunk even when the chunk exceeds the size at which splits occur.
如果一個chunk只包含一個分片鍵值，mongodb 就不會split這個chunk,即使這個chunk超過了 chunk需要split時的大小。所以分片鍵的選擇非常重要。
這裏舉個例子，比如我們使用日期（精確到日） 作爲分片鍵，當某一天的數據非常多時，這個分片鍵值（比如2015/12/12）的對應的chunk會非常大，
超過64M,但是這個chunk是不可分割的。這會造成數據在各個分片中不平衡，出現性能問題。
所以我們要以 選擇性高的字段做爲分片鍵，如果這個字段（比如 日誌級別）選擇性低，我們可以再添加一個選擇性高的字段，兩個字段做爲分片鍵。
如果以日期做爲分片鍵，爲了避免大的chunk，我們可以把日期精確到 時分秒 然後做分片鍵。
if your chunk ranges get down to a single key value then no further splits are possible and you get "jumbo" chunks。
以下是 大的chunk的例子：
http://dba.stackexchange.com/questions/72626/mongo-large-chunks-will-not-split
一個常見的錯誤：
Mongos version 3.0.1 Split Chunk Error with Sharding
http://dba.stackexchange.com/questions/96732/mongos-version-3-0-1-split-chunk-error-with-sharding?rq=1
手動切割分片：
http://www.cnblogs.com/xuegang/archive/2012/12/27/2836209.html
一、使用splitFind對可分割的chunk 手動分割。
splitFind(namespace, query),query的值必須包括分片鍵。將一個query指定的chunk,分割爲兩個基本相等大小的chunk。
mongos> db.users003.getShardDistribution()
Shard shard1 at shard1/192.168.137.111:27017,192.168.137.75:27017
data : 212KiB docs : 3359 chunks : 2
estimated data per chunk : 106KiB
estimated docs per chunk : 1679
Shard shard2 at shard2/192.168.137.138:27018,192.168.137.75:27018
data : 211KiB docs : 3337 chunks : 2
estimated data per chunk : 105KiB
estimated docs per chunk : 1668
Shard shard3 at shard3/192.168.137.111:27019,192.168.137.138:27019
data : 209KiB docs : 3304 chunks : 2
estimated data per chunk : 104KiB
estimated docs per chunk : 1652
Totals
data : 633KiB docs : 10000 chunks : 6
Shard shard1 contains 33.58% data, 33.58% docs in cluster, avg obj size on shard : 64B
Shard shard2 contains 33.37% data, 33.37% docs in cluster, avg obj size on shard : 64B
Shard shard3 contains 33.03% data, 33.04% docs in cluster, avg obj size on shard : 64B
mongos>
mongos>
mongos> AllChunkInfo("test1.users003", true);
ChunkID,Shard,ChunkSize,ObjectsInChunk
test1.users003-_id_MinKey,shard1,106368,1662
test1.users003-_id_-6148914691236517204,shard1,108608,1697
test1.users003-_id_-3074457345618258602,shard3,107072,1673
test1.users003-_id_0,shard3,104384,1631
test1.users003-_id_3074457345618258602,shard2,110592,1728
test1.users003-_id_6148914691236517204,shard2,102976,1609
***********Summary Chunk Information***********
Total Chunks: 6
Average Chunk Size (bytes): 106666.66666666667
Empty Chunks: 0
Average Chunk Size (non-empty): 106666.66666666667
mongos> db.users003.count()
10000執行splitFind之後，chunk被分割爲兩個基本相同大小的chunk:
mongos> sh.splitFind("test1.users003",{"name" : "u_100"})
{
"ok" : 0,
"errmsg" : "no shard key found in chunk query { name: \"u_100\" }"
}
mongos> sh.splitFind("test1.users003",{"_id" : ObjectId("568bdf16e05cf980cec8c455")})
{ "ok" : 1 }
mongos>
mongos>
mongos>
mongos>
mongos>
mongos>
mongos> AllChunkInfo("test1.users003", true);
ChunkID,Shard,ChunkSize,ObjectsInChunk
test1.users003-_id_MinKey,shard1,106368,1662
test1.users003-_id_-6148914691236517204,shard1,54272,848
test1.users003-_id_-4665891797978533183,shard1,54336,849
test1.users003-_id_-3074457345618258602,shard3,107072,1673
test1.users003-_id_0,shard3,104384,1631
test1.users003-_id_3074457345618258602,shard2,110592,1728
test1.users003-_id_6148914691236517204,shard2,102976,1609
***********Summary Chunk Information***********
Total Chunks: 7
Average Chunk Size (bytes): 91428.57142857143
Empty Chunks: 0
Average Chunk Size (non-empty): 91428.57142857143
mongos> db.users003.getShardDistribution()
Shard shard1 at shard1/192.168.137.111:27017,192.168.137.75:27017
data : 212KiB docs : 3359 chunks : 3
estimated data per chunk : 70KiB
estimated docs per chunk : 1119
Shard shard2 at shard2/192.168.137.138:27018,192.168.137.75:27018
data : 211KiB docs : 3337 chunks : 2
estimated data per chunk : 105KiB
estimated docs per chunk : 1668
Shard shard3 at shard3/192.168.137.111:27019,192.168.137.138:27019
data : 209KiB docs : 3304 chunks : 2
estimated data per chunk : 104KiB
estimated docs per chunk : 1652
Totals
data : 633KiB docs : 10000 chunks : 7
Shard shard1 contains 33.58% data, 33.58% docs in cluster, avg obj size on shard : 64B
Shard shard2 contains 33.37% data, 33.37% docs in cluster, avg obj size on shard : 64B
Shard shard3 contains 33.03% data, 33.04% docs in cluster, avg obj size on shard : 64B二、使用splitAt對可分割的chunk 手動分割。
splitAt(namespace, query) 官方解釋：
sh.splitAt() splits the original chunk into two chunks. One chunk has a shard key range
that starts with the original lower bound (inclusive) and ends at the specified shard key value (exclusive).
The other chunk has a shard key range that starts with the specified shard key value (inclusive) as the lower bound
and ends at the original upper bound (exclusive).
三、手動遷移chunk
db.runCommand( { moveChunk : "myapp.users" ,
find : {username : "smith"} ,
to : "mongodb-shard3.example.net" } )
註釋：
moveChunk：一個集合的名字要加上數據庫的名稱：比如test.yql
find：一個查詢語句，指定集合中的符合查詢的數據或者chunk，系統自動查出from 的shard
to： 指向chunk的目的shard
只要目的shard和源sharad同意指定的chunk由目的shard接管，命令就返回。遷移chunk是一個比較複雜的過程，它包括兩個內部通信協議：
1 複製數據，包括在複製過程中的變化的數據
2 確保所有參與遷移的組成部分：目的shard ，源shard ，config server都確定遷移已經完成！
The command will block until the migration is complete.
四、相關腳本
--顯示collection的chunk分佈信息
db.collection.getShardDistribution()
顯示chunk信息腳本：
AllChunkInfo = function(ns, est){
var chunks = db.getSiblingDB("config").chunks.find({"ns" : ns}).sort({min:1}); //this will return all chunks for the ns ordered by min
//some counters for overall stats at the end
var totalChunks = 0;
var totalSize = 0;
var totalEmpty = 0;
print("ChunkID,Shard,ChunkSize,ObjectsInChunk"); // header row
// iterate over all the chunks, print out info for each
chunks.forEach(
function printChunkInfo(chunk) {
var db1 = db.getSiblingDB(chunk.ns.split(".")[0]); // get the database we will be running the command against later
var key = db.getSiblingDB("config").collections.findOne({_id:chunk.ns}).key; // will need this for the dataSize call
// dataSize returns the info we need on the data, but using the estimate option to use counts is less intensive
var dataSizeResult = db1.runCommand({datasize:chunk.ns, keyPattern:key, min:chunk.min, max:chunk.max, estimate:est});
// printjson(dataSizeResult); // uncomment to see how long it takes to run and status
print(chunk._id+","+chunk.shard+","+dataSizeResult.size+","+dataSizeResult.numObjects);
totalSize += dataSizeResult.size;
totalChunks++;
if (dataSizeResult.size == 0) { totalEmpty++ }; //count empty chunks for summary
}
)
print("***********Summary Chunk Information***********");
print("Total Chunks: "+totalChunks);
print("Average Chunk Size (bytes): "+(totalSize/totalChunks));
print("Empty Chunks: "+totalEmpty);
print("Average Chunk Size (non-empty): "+(totalSize/(totalChunks-totalEmpty)));
}
使用示例：
mongos> AllChunkInfo("test1.users001", true);
ChunkID,Shard,ChunkSize,ObjectsInChunk
test1.users001-_id_MinKey,shard3,11347710,171935
test1.users001-_id_-6148914691236517204,shard1,11293458,171113
test1.users001-_id_-3074457345618258602,shard1,11320716,171526
test1.users001-_id_0,shard3,11349096,171956
test1.users001-_id_3074457345618258602,shard2,11340054,171819
test1.users001-_id_6148914691236517204,shard2,11328966,171651
***********Summary Chunk Information***********
Total Chunks: 6
Average Chunk Size (bytes): 11330000
Empty Chunks: 0
Average Chunk Size (non-empty): 11330000