# MongoDB replicaset performance problem **URL:** \ **Category:** Percona Server for MongoDB\ **Created:** [January 18, 2021, 6:40am UTC](https://forums.percona.com/t/mongodb-replicaset-performance-problem/8818 "2021-01-18T06:40:41Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![Cihan\_Tunali](https://sea1.discourse-cdn.com/flex019/user_avatar/forums.percona.com/cihan_tunali/32/4574_2.png) [@Cihan\_Tunali](https://forums.percona.com/u/Cihan_Tunali)\ **Post date:** [January 18, 2021, 6:40am UTC](https://forums.percona.com/t/mongodb-replicaset-performance-problem/8818/1 "2021-01-18T06:40:41Z")
Hello all, I don’t know maybe my problem is normal behavior of MongoDB but I need to find a solution for that. So, I am using 3 node replicaset with 1 primary, 1 secondary and 1 arbiter. My servers are powerfull and can handle 3K ops . When secondary is down for a reason, MongoDB slows down significant like from 3000 ops to 50 ops. As I said that may be normal behavior. I need to solve this slowness when a node is down. - I can not remove down node and re-add it when become online because data is big, i can not wait to re-sync. - when I lost a node, I restore it from backup and open it. It catches from primary in a minute. MongoDB version is 4.2.4. do you have any suggestion about my problem? Thank you! ---
**Author:** ![Akira\_Kurogane](https://sea1.discourse-cdn.com/flex019/user_avatar/forums.percona.com/akira_kurogane/32/1280_2.png) [@Akira\_Kurogane](https://forums.percona.com/u/Akira_Kurogane)\ **Post date:** [January 18, 2021, 7:56am UTC](https://forums.percona.com/t/mongodb-replicaset-performance-problem/8818/2 "2021-01-18T07:56:57Z")
Maybe it’s a write-concern issue. PSA (Primary, Secondary and Arbiter) replicasets since v3.6 or v4.0 have been finicky about it. “w:majority” is default and writes cannot be made to a majority when a node is going and all you have is a ‘PDA’ instead of PSA. A simple differential diagnostic test to perform to see if this hypothesis is hitting the mark or not is to change the write concern to just w:1. ---
**Author:** ![Cihan\_Tunali](https://sea1.discourse-cdn.com/flex019/user_avatar/forums.percona.com/cihan_tunali/32/4574_2.png) [@Cihan\_Tunali](https://forums.percona.com/u/Cihan_Tunali)\ **Post date:** [January 28, 2021, 9:24am UTC](https://forums.percona.com/t/mongodb-replicaset-performance-problem/8818/3 "2021-01-28T09:24:36Z")
Hi @Akira_Kurogane , I found what couses slowness when a node is down. It is because “Flow Control”, by default is “enableFlowControl” is “true”. When I disabled it, my slowness problem is solved. Right now, I am searching why Flow Control is needed. Thank you! ---
**Author:** ![Akira\_Kurogane](https://sea1.discourse-cdn.com/flex019/user_avatar/forums.percona.com/akira_kurogane/32/1280_2.png) [@Akira\_Kurogane](https://forums.percona.com/u/Akira_Kurogane)\ **Post date:** [January 29, 2021, 3:32am UTC](https://forums.percona.com/t/mongodb-replicaset-performance-problem/8818/4 "2021-01-29T03:32:18Z")
Hi Ghan. Ah, I should have thought of that. Sorry, I was focused on some other issues with PSAs, caused by majority read concern, that have been reported several times recently. As I like to say, “Flow Control” is a marketing name. What is really is a primary throttle mechanism. For a lot of detail, including why a primary should be throttled, please see slide #28 onwards in this 2019 presentation: > **[whats-new-in-psmdb.pdf](https://www.percona.com/sites/default/files/ple19-slides/day1-pm/whats-new-in-psmdb.pdf)** If your goal is to allow running with only one data-bearing node out of two up at a time you will need to disable flow control in v4.2+. But you already worked that out so kudos. ---
**Author:** ![Cihan\_Tunali](https://sea1.discourse-cdn.com/flex019/user_avatar/forums.percona.com/cihan_tunali/32/4574_2.png) [@Cihan\_Tunali](https://forums.percona.com/u/Cihan_Tunali)\ **Post date:** [January 29, 2021, 6:33am UTC](https://forums.percona.com/t/mongodb-replicaset-performance-problem/8818/5 "2021-01-29T06:33:23Z")
Thanks @Akira_Kurogane , my goal is ; when a node down at PSA replicas, I dont want any slowness while using MongoDB. I know it is because of Flow Control but I am not knowing why I need Flow control if I am using w:1 as write concern ? I tried to change its configuration (flowControlTargetLagSeconds, flowControlWarnThresholdSeconds ) but nothing changed, only Flow control activation time increased/decreased. ![image](https://us1.discourse-cdn.com/flex019/uploads/percona1/original/2X/e/e29f7948cd54153afd33acaeaf4b9d7048a834dd.png) At above SS, I think yellow ones retired. If I can change red ones, I may continue to use Flow control, otherwise I will disable it incase its consequences. ---
**Author:** ![Akira\_Kurogane](https://sea1.discourse-cdn.com/flex019/user_avatar/forums.percona.com/akira_kurogane/32/1280_2.png) [@Akira\_Kurogane](https://forums.percona.com/u/Akira_Kurogane)\ **Post date:** [February 5, 2021, 1:12pm UTC](https://forums.percona.com/t/mongodb-replicaset-performance-problem/8818/6 "2021-02-05T13:12:14Z")
Hi. Regardless of whether apps are happy with w:1 write consistency a MongoDB replica set has a duty to make sure replication doesn’t get too far behind. If it does then nice elections become impossible; and sudden failover would have a large amount of rolled back ops. Having said that I expect a dead secondary to be something that flow control will ignore. I should retract this statement I made last time: > If your goal is to allow running with only one data-bearing node out of two up at a time you will need to disable flow control in v4.2+. On more thought slowness isn’t a side effect I expect when the secondary is down; I only expect it to occur when the secondary is slow and lagging \> 5(?) secs (by default).