MongoDB插入速度突然变得极慢，什么原因？

客户端用的pymongo，程序运行一个周了，之前插入速度都非常快，大概半个小时1千万条，今天突然发现插入速度变得非常慢。
插入目标是goods集合，goods集合现在有1.2亿条数据。

> db.goods.count() 

123535156 

goods集合的索引如下。

> db.goods.getIndexes();
 [
         {
                 "v" : 1,
                 "key" : {
                         "_id" : 1
                 },
                 "ns" : "shop.goods",
                 "name" : "_id_"
         },
         {
                 "v" : 1,
                 "key" : {
                         "item_id" : 1,
                         "updated_at" : -1
                 },
                 "unique" : true,
                 "ns" : "shop.goods",
                 "name" : "item_id_1_updated_at_-1"
         },
         {
                 "v" : 1,
                 "key" : {
                         "updated_at" : 1
                 },
                 "ns" : "shop.goods",
                 "name" : "updated_at_1"
         },
         {
                 "v" : 1,
                 "key" : {
                         "item_id" : 1
                 },
                 "ns" : "shop.goods",
                 "name" : "item_id_1"
         }
 ]

机器目前有足够的内存，CPU使用率也不高。

qi@ubuntu :~$ free -m
              total       used       free     shared    buffers     cached
 Mem:         32162      31965        196          0         37      19739
 -/+ buffers/cache:      12189      19972
 Swap:         8177         59       8118
 

Tasks: 198 total,   1 running, 197 sleeping,   0 stopped,   0 zombie
 Cpu0  :  2.4%us,  1.3%sy,  0.0%ni, 15.2%id, 80.8%wa,  0.0%hi,  0.3%si,  0.0%st
 Cpu1  :  0.0%us,  0.0%sy,  0.0%ni,100.0%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0%st
 Cpu2  :  0.3%us,  0.0%sy,  0.0%ni, 99.3%id,  0.3%wa,  0.0%hi,  0.0%si,  0.0%st
 Cpu3  :  0.0%us,  0.3%sy,  0.0%ni, 99.7%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0%st
 Cpu4  :  0.0%us,  0.0%sy,  0.0%ni,100.0%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0%st
 Cpu5  :  0.0%us,  0.0%sy,  0.0%ni,100.0%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0%st
 Cpu6  :  0.0%us,  0.0%sy,  0.0%ni,100.0%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0%st
 Cpu7  : 20.5%us,  2.0%sy,  0.0%ni, 76.8%id,  0.0%wa,  0.0%hi,  0.7%si,  0.0%st
 Cpu8  :  0.0%us,  0.0%sy,  0.0%ni,100.0%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0%st
 Cpu9  :  0.0%us,  0.0%sy,  0.0%ni,100.0%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0%st
 Cpu10 :  0.3%us,  0.0%sy,  0.0%ni, 99.7%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0%st
 Cpu11 :  0.0%us,  0.0%sy,  0.0%ni,100.0%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0%st
 Cpu12 :  0.0%us,  0.0%sy,  0.0%ni,100.0%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0%st
 Cpu13 :  1.0%us,  1.0%sy,  0.0%ni, 97.7%id,  0.0%wa,  0.0%hi,  0.3%si,  0.0%st
 Cpu14 :  0.0%us,  0.0%sy,  0.0%ni,100.0%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0%st
 Cpu15 :  0.0%us,  0.0%sy,  0.0%ni,100.0%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0%st
 Mem:  32934196k total, 30379792k used,  2554404k free,    37928k buffers
 Swap:  8374268k total,    61388k used,  8312880k free, 17884864k cached

是什么原因造成的呢？