在mongodb中使用db.runCommand({fsync:1,lock:1})可以将内存中未写入磁盘的信息写入磁盘，并锁住对数据库改写的操作,读操作可以继续。
一般用于数据库的SNAPSHOT。
由于MONGODB使用的是read/write锁,即读写是冲突的.因此使用lock有几个需要注意的地方，用得不好可能导致需要重启数据库。
例如:
session A:
> db.runCommand({fsync:1,lock:1})
{
"info" : "now locked against writes, use db.$cmd.sys.unlock.findOne() to unlock",
"ok" : 1
}
> db.currentOp()
{ "inprog" : [ ] }
# 此时不做任何写操作,登录到session B,C.
session B:
MongoDB shell version: 1.6.4
connecting to: 127.0.0.1:5281/admin
> db.auth("digoal","DIGOAL")
这里卡住
session C:
MongoDB shell version: 1.6.4
connecting to: 127.0.0.1:5281/admin
> db.auth("digoal","DIGOAL")
这里卡住
session A:
> db.currentOp()
{
"inprog" : [
{
"opid" : 35,
"active" : false,
"lockType" : "read",
"waitingForLock" : true,
"op" : "query",
"ns" : "?dmin.tbl_test",
"query" : {
},
"client" : "127.0.0.1:33874",
"desc" : "conn"
},
{
"opid" : 34,
"active" : true,
"lockType" : "write",
"waitingForLock" : true,
"secs_running" : 18,
"op" : "query",
"ns" : "?",
"query" : {
"authenticate" : 1,
"user" : "digoal",
"nonce" : "28d27268246c42b8",
"key" : "2b29a029bea30bd16c1bc2ba3b53396f"
},
"client" : "127.0.0.1:43981",
"desc" : "conn"
}
],
"fsyncLock" : 1,
"info" : "use db.$cmd.sys.unlock.findOne() to terminate the fsync write/snapshot lock"
}
#释放锁
> db.$cmd.sys.unlock.findOne();
{ "ok" : 1, "info" : "unlock requested" }
> db.currentOp()
{ "inprog" : [ ] }
session B:
认证通过
session C:
认证通过
session A:
# 再次加锁
> db.runCommand({fsync:1,lock:1})
{
"info" : "now locked against writes, use db.$cmd.sys.unlock.findOne() to unlock",
"ok" : 1
}
session B:
> db.tbl_test.find()
{ "_id" : ObjectId("4d003b9c23ce070b7227bc5f"), "a" : 1 }
session C:
> db.tbl_test.find()
{ "_id" : ObjectId("4d003b9c23ce070b7227bc5f"), "a" : 1 }
session D:
MongoDB shell version: 1.6.4
connecting to: 127.0.0.1:5281/admin
> db.auth("digoal","DIGOAL")
这里卡住
再次回到session B,C:
> db.tbl_test.find()
卡住了
session A:
> db.currentOp()
{
"inprog" : [
{
"opid" : 45,
"active" : true,
"lockType" : "write",
"waitingForLock" : true,
"secs_running" : 63,
"op" : "query",
"ns" : "?",
"query" : {
"authenticate" : 1,
"user" : "digoal",
"nonce" : "8240c091997df278",
"key" : "0c848429a5a973e3b25b10c906fc9211"
},
"client" : "127.0.0.1:45638",
"desc" : "conn"
},
{
"opid" : 46,
"active" : false,
"lockType" : "read",
"waitingForLock" : true,
"op" : "query",
"ns" : "?dmin.tbl_test",
"query" : {
},
"client" : "127.0.0.1:43981",
"desc" : "conn"
},
{
"opid" : 49,
"active" : true,
"lockType" : "write",
"waitingForLock" : true,
"secs_running" : 20,
"op" : "query",
"ns" : "?",
"query" : {
"authenticate" : 1,
"user" : "wapprepaid",
"nonce" : "5ae8f2534427f6d2",
"key" : "01e013f0a035f86455d807b4d94f1e44"
},
"client" : "172.16.10.74:2637",
"desc" : "conn"
}
],
"fsyncLock" : 1,
"info" : "use db.$cmd.sys.unlock.findOne() to terminate the fsync write/snapshot lock"
}
认证时需要加载WRITE锁，导致读被堵塞。
另外需要注意的是，如果SESSION A，也就是允许fsync和lock的SESSION被堵塞了的话，只能重启数据库了。
重启前KILL -9 ,删除lock文件.