Operational Defect Database

BugZero found this defect 2406 days ago.

MongoDB | 408556

[SERVER-30339] Enabling profiling and read concern majority with a workload of reads and no writes causes accumulation of dirty data

Last update date:

12/6/2022

Affected products:

MongoDB Server

Affected releases:

3.2.15

3.4.6

3.5.5

Fixed releases:

No fixed releases provided.

Description:

Info

Start a replica set with these options: mongod --enableMajorityReadConcern --profile 2 --wiredTigerCacheSizeGB 2 ... Then start a workload of reads but no writes: db.c.insert({}) for (var i = 0; ; i++) { if (i%1000 == 0) print(i) db.c.findOne() } Range of pinned ids and dirty data grows, and instance hangs when dirty data in cache reaches 20%:

Top User Comments

bruce.lucas@10gen.com commented on Wed, 26 Jul 2017 18:05:04 +0000: The write every 10 seconds was introduced by SERVER-23892 in 3.3, which explains why behavior is better in 3.4 and 3.5. bruce.lucas@10gen.com commented on Wed, 26 Jul 2017 17:42:00 +0000: Looks like the behavior is better in 3.4 and 3.5: We write an entry to the oplog every 10 seconds even if there are no writes, and that is sufficient to bound the range of pinned ids and dirty data to 10 seconds worth of profile entries. That may be a sufficient fix since this only occurs when there are no other writes generating dirty data. bruce.lucas@10gen.com commented on Wed, 26 Jul 2017 16:23:17 +0000: It appears that the dirty data is due to entries written to the profile, but because there are no db writes that advance the oplog a named snapshot remains in place for an extended period, causing the accumulation of pinned transactions and dirty data.

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