Hi all.. 
We are benchmarking couch base and observing a very strange behaviour.

*Setup phase:*

*couchbase cluster machines;*
2 x EC2 r3.xlarge with General purpose 80GB SSD (Not EBS optimised ) , 
 IOPS 240/3000.
couchbase settings:
*Cluster:* 
Data Ram Quota: 22407 MB
Index Ram Quota: 2024 MB
Index Settings (default)
*Bucket:*
Per Node Ram Quota: 22407 MB
Total Bucket Size: 44814 MB (22407 x 2)
Replicas enabled (1)
Disk I/O Optimisation (Low)

* Each node runs all three services 

*couchbase client;*
1 x EC2 m4.xlarge General purpose 20 GB SSD (EBS Optimised), IOPS 60/3000.
The client is running the 'YCSB' benchmark tool.

PS: All the machines are residing within the same VPC and subnet.

*ycsb load couchbase -s -P workloads/workloada -p recordcount=100000000 -p 
core_workload_insertion_retry_limit=3 -p 
couchbase.url=http://HOST:8091/pools -p couchbase.bucket=test -threads 20 | 
tee workloadaLoad.dat*


*Results:*

*While everything works as expected*
The average ops/sec is ~21000
The 'disk write queue' graph is floating between 200K - 600K (periodically 
drained).
The 'temp OOM per sec' graph  is at constant 0.
 
*When things starting to get weird*
After about ~27M documents inserted we start seeing 'disk write queue' is 
constantly rising (Not getting drained)
At about ~8M disk queue size the OOM failures are starting to show them 
selves and the client receives 'Temporary failure' from couchbase.
After 3 retries of each YCSB thread, the client stops after inserting only 
~27% of the overall documents. 
Even when the YCSB client stopped running, the 'disk write queue' is 
asymptotically moving towards 0, and is drained only after  ~15 min.

P.S 
When we benchmark locally on MacBook with 16GB of ram + SSD disk (local 
client + one node server) we do not observe such behaviour and the 'disk 
write queue' is constantly drained in a predictable manner.


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