使用MongoDB默认设置，最大块大小为64Mb，最大文档大小为16Mb（https://docs.mongodb.org/manual/reference/limits/）。
我遇到了需要拆分的大块问题，但拆分失败了。
我不明白为什么。假设我们有大文档，如果用文档（16Mb）创建一个块，然后是一个新文档（32Mb），依此类推，直到我们达到64Mb，然后发生拆分过程。为什么会失败？为什么不把这个块分成2个16Mb文件的两半？
或者在文件类似的情况下：
10Mb + 16Mb + 16Mb + 16Mb + 16Mb = 74Mb（需要拆分）
它可以做到：
10Mb + 16Mb + 16Mb = 42Mb
16Mb + 16Mb = 32Mb
为什么会失败？
MongoDB日志：
2016-04-06T12:10:53.424-0700 I SHARDING [Balancer] ns: reports.storePoints going to move { id: "reports.storePoints-storeId-9150629136201875191", ns: "reports.storePoints", min: { storeId: -9150629136201875191 }, max: { storeId: -9148794391694793543 }, version: Timestamp 666000|1, versionEpoch: ObjectId('54ac908475d02f0bdb362171'), lastmod: Timestamp 666000|1, lastmodEpoch: ObjectId('54ac908475d02f0bdb362171'), shard: "rs1" } from: rs1 to: rs2 tag []
2016-04-06T12:10:53.477-0700 I SHARDING [Balancer] moving chunk ns: reports.storePoints moving ( ns: reports.storePoints, shard: rs1:rs1/host1:27017,host2:27017, lastmod: 666|1||000000000000000000000000, min: { storeId: -9150629136201875191 }, max: { storeId: -9148794391694793543 }) rs1:rs1/host1:27017,host1:27017 -> rs2:rs2/host2:27017,host2:27017
2016-04-06T12:10:54.052-0700 I SHARDING [Balancer] moveChunk result: { chunkTooBig: true, estimatedChunkSize: 111379744, ok: 0.0, errmsg: "chunk too big to move", $gleStats: { lastOpTime: Timestamp 1455673163000|2, electionId: ObjectId('55b838ecf80a354067abdbbf') } }
2016-04-06T12:10:54.053-0700 I SHARDING [Balancer] balancer move failed: { chunkTooBig: true, estimatedChunkSize: 111379744, ok: 0.0, errmsg: "chunk too big to move", $gleStats: { lastOpTime: Timestamp 1455673163000|2, electionId: ObjectId('55b838ecf80a354067abdbbf') } } from: rs1 to: rs2 chunk: min: { storeId: -9150629136201875191 } max: { storeId: -9148794391694793543 }
2016-04-06T12:10:54.053-0700 I SHARDING [Balancer] performing a split because migrate failed for size reasons
2016-04-06T12:10:54.063-0700 I SHARDING [Balancer] split results: CannotSplit chunk not full enough to trigger auto-split
2016-04-06T12:10:54.063-0700 I SHARDING [Balancer] marking chunk as jumbo: ns: reports.storePoints, shard: rs1:rs1/host1:27017,host2:27017, lastmod: 666|1||000000000000000000000000, min: { storeId: -9150629136201875191 }, max: { storeId: -9148794391694793543 }