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MongoDB Performance Tuning Questions

MongoDB Inc.10 min read • Published Jan 11, 2022 • Updated Sep 23, 2022
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Most of the challenges related to keeping a MongoDB cluster running at top speed can be addressed by asking a small number of fundamental questions and then using a few crucial metrics to answer them.
By keeping an eye on the metrics related to query performance, database performance, throughput, resource utilization, resource saturation, and other critical "assertion" errors it's possible to find problems that may be lurking in your cluster. Early detection allows you to stay ahead of the game, resolving issues before they affect performance.
These fundamental questions apply no matter how MongoDB is used, whether through MongoDB Atlas, the managed service available on all major cloud providers, or through MongoDB Community or Enterprise editions, which are run in a self-managed manner on-premise or in the cloud.
Each type of MongoDB deployment can be used to support databases at scale with immense transaction volumes and that means performance tuning should be a constant activity.
But the good news is that the same metrics are used in the tuning process no matter how MongoDB is used.
However, as we'll see, the tuning process is much easier in the cloud using MongoDB Atlas where everything is more automatic and prefabricated.
Here are the key questions you should always be asking about MongoDB performance tuning and the metrics that can answer them.

Are all queries running at top speed?

Query problems are perhaps the lowest hanging fruit when it comes to debugging MongoDB performance issues. Finding problems and fixing them is generally straightforward. This section covers the metrics that can reveal query performance problems and what to do if you find slow queries.
Slow Query Log. The time elapsed and the method used to execute each query is captured in MongoDB log files, which can be searched for slow queries. In addition, queries over a certain threshold can be logged explicitly by the MongoDB Database Profiler.
  • When a query is slow, first look to see if it was a collection scan rather than an index scan.
    • Collection scans means all documents in a collection must be read.
    • Index scans limit the number of documents that must be inspected.
  • But remember: indexes have a cost when it comes to writes and updates. Too many indexes that are underutilized can slow down the modification or insertion of new documents. Depending on the nature of your workloads, this may or may not be a problem.
Scanned vs Returned is a metric that can be found in Cloud Manager and in MongoDB Atlas that indicates how many documents had to be scanned in order to return the documents meeting the query.
  • In the absence of indexes, a rarely met ideal for this ratio is 1/1, meaning all documents scanned were returned — no wasted scans. Most of the time however, when scanning is done, documents are scanned that are not returned meaning the ratio is greater than 1.
  • When indexes are used, this ratio can be less than 1 or even 0, meaning you have a covered query. When no documents needed to be scanned, producing a ratio of 0, that means all the data needed was in the index.
  • Scanning huge amounts of documents is inefficient and could indicate problems regarding missing indexes or indicate a need for query optimization.
Scan and Order is an index related metric that can be found in Cloud Manager and MongoDB Atlas.
  • A high Scan and Order number, say 20 or more, indicates that the server is having to sort query results to get them in the right order. This takes time and increases the memory load on the server.
  • Fix this by making sure indexes are sorted in the order in which the queries need the documents, or by adding missing indexes.
WiredTiger Ticket Number is a key indicator of the performance of the WiredTiger storage engine, which, since release 3.2, has been the storage engine for MongoDB.
  • WiredTiger has a concept of read or write tickets that are created when the database is accessed. The WiredTiger ticket number should always be at 128.
  • If the value goes below 128 and stays below that number, that means the server is waiting on something and it's an indication of a problem.
  • The remedy is then to find the operations that are going too slowly and start a debugging process.
  • Deployments of MongoDB using releases older than 3.2 will certainly get a performance boost from migrating to a later version that uses WiredTiger.
Document Structure Antipatterns aren't revealed by a metric but can be something to look for when debugging slow queries. Here are two of the most notorious bad practices that hurt performance.
Unbounded arrays: In a MongoDB document, if an array can grow without a size limit, it could cause a performance problem because every time you update the array, MongoDB has to rewrite the array into the document. If the array is huge, this can cause a performance problem. Learn more at Avoid Unbounded Arrays and Performance Best Practices: Query Patterns and Profiling.
Subdocuments without bounds: The same thing can happen with respect to subdocuments. MongoDB supports inserting documents within documents, with up to 128 levels of nesting. Each MongoDB document, including subdocuments, also has a size limit of 16MB. If the number of subdocuments becomes excessive, performance problems may result.
One common fix to this problem is to move some or all of the subdocuments to a separate collection and then refer to them from the original document. You can learn more about this topic in this blog post.

Is the database performing at top speed?

MongoDB, like most advanced database systems, has thousands of metrics that track all aspects of database performance which includes reading, writing, and querying the database, as well as making sure background maintenance tasks like backups don't gum up the works.
The metrics described in this section all indicate larger problems that can have a variety of causes. Like a warning light on a dashboard, these metrics are invaluable high-level indicators that help you start looking for the causes before the database has a catastrophic failure.
Note: Various ways to get access to all of these metrics are covered below in the Getting Access to Metrics and Setting Up Monitoring section.
Replication lag occurs when a secondary member of a replica set falls behind the primary. A detailed examination of the OpLog related metrics can help get to the bottom of the problems but the causes are often:
  • A networking issue between the primary and secondary, making nodes unreachable
  • A secondary node applying data slower than the primary node
  • Insufficient write capacity in which case you should add more shards
  • Slow operations on the primary node, blocking replication
Locking performance problems are indicated when the number of available read or write tickets remaining reaches zero, which means new read or write requests will be queued until a new read or write ticket is available.
  • MongoDB's internal locking system is used to support simultaneous queries while avoiding write conflicts and inconsistent reads.
  • Locking performance problems can indicate a variety of problems including suboptimal indexes and poor schema design patterns, both of which can lead to locks being held longer than necessary.
Number of open cursors rising without a corresponding growth of traffic is often symptomatic of poorly indexed queries or the result of long running queries due to large result sets.
  • This metric can be another indicator that the kind of query optimization techniques mentioned in the first section are in order.

Is the cluster overloaded?

A large part of performance tuning is recognizing when your total traffic, the throughput of transactions through the system, is rising beyond the planned capacity of your cluster. By keeping track of growth in throughput, it's possible to expand the capacity in an orderly manner. Here are the metrics to keep track of.
Read and Write Operations is the fundamental metric that indicates how much work is done by the cluster. The ratio of reads to writes is highly dependent on the nature of the workloads running on the cluster.
  • Monitoring read and write operations over time allows normal ranges and thresholds to be established.
  • As trends in read and write operations show growth in throughput, capacity should be gradually increased.
Document Metrics and Query Executor are good indications of whether the cluster is actually too busy. These metrics can be found in Cloud Manager and in MongoDB Atlas. As with read and write operations, there is no right or wrong number for these metrics, but having a good idea of what's normal helps you discern whether poor performance is coming from large workload size or attributable to other reasons.
  • Document metrics are updated anytime you return a document or insert a document. The more documents being returned, inserted, updated or deleted, the busier your cluster is.
    • Poor performance in a cluster that has plenty of capacity usually points to query problems.
  • The query executor tells how many queries are being processed through two data points:
    • Scanned - The average rate per second over the selected sample period of index items scanned during queries and query-plan evaluation.
    • Scanned objects - The average rate per second over the selected sample period of documents scanned during queries and query-plan evaluation.
Hardware and Network metrics can be important indications that throughput is rising and will exceed the capacity of computing infrastructure. These metrics are gathered from the operating system and networking infrastructure. To make these metrics useful for diagnostic purposes, you must have a sense of what is normal.
  • In MongoDB Atlas, or when using Cloud Manager, these metrics are easily displayed. If you are running on-premise, it depends on your operating system.
  • There's a lot to track but at a minimum have a baseline range for metrics like:
    • Disk latency
    • Disk IOPS
    • Number of Connections

Is the cluster running out of key resources?

A MongoDB cluster makes use of a variety of resources that are provided by the underlying computing and networking infrastructure. These can be monitored from within MongoDB as well as from outside of MongoDB at the level of computing infrastructure as described in the previous section. Here are the crucial resources that can be easily tracked from within Mongo, especially through Cloud Manager and MongoDB Atlas.
Current number of client connections is usually an effective metric to indicate total load on a system. Keeping track of normal ranges at various times of the day or week can help quickly identify spikes in traffic.
  • A related metric, percentage of connections used, can indicate when MongoDB is getting close to running out of available connections.
Storage metrics track how MongoDB is using persistent storage. In the WiredTiger storage engine, each collection is a file and so is each index. When a document in a collection is updated, the entire document is re-written.
  • If memory space metrics (dataSize, indexSize, or storageSize) or the number of objects show a significant unexpected change while the database traffic stays within ordinary ranges, it can indicate a problem.
  • A sudden drop in dataSize may indicate a large amount of data deletion, which should be quickly investigated if it was not expected.
Memory metrics show how MongoDB is using the virtual memory of the computing infrastructure that is hosting the cluster.
  • An increasing number of page faults or a growing amount of dirty data — data changed but not yet written to disk — can indicate problems related to the amount of memory available to the cluster.
  • Cache metrics can help determine if the working set is outgrowing the available cache.

Are critical errors on the rise?

MongoDB asserts are documents created, almost always because of an error, that are captured as part of the MongoDB logging process.
  • Monitoring the number of asserts created at various levels of severity can provide a first level indication of unexpected problems. Asserts can be message asserts, the most serious kind, or warning assets, regular asserts, and user asserts.
  • Examining the asserts can provide clues that may lead to the discovery of problems.

Getting Access to Metrics and Setting Up Monitoring

Making use of metrics is far easier if you know the data well: where it comes from, how to get at it, and what it means.
As the MongoDB platform has evolved, it has become far easier to monitor clusters and resolve common problems. In addition, the performance tuning monitoring and analysis has become increasingly automated. For example, MongoDB Atlas through Performance Advisor will now suggest adding indexes if it detects a query performance problem.
But it's best to know the whole story of the data, not just the pretty graphs produced at the end.

Data Sources for MongoDB Metrics

The sources for metrics used to monitor MongoDB are the logs created when MongoDB is running and the commands that can be run inside of the MongoDB system. These commands produce the detailed statistics that describe the state of the system.
Monitoring MongoDB performance metrics (WiredTiger) contains an excellent categorization of the metrics available for different purposes and the commands that can be used to get them. These commands provide a huge amount of detailed information in raw form that looks something like the following screenshot:
WiredTiger Screenshot
This information is of high quality but difficult to use.

Monitoring Environments for MongoDB Metrics

As MongoDB has matured as a platform, specialized interfaces have been created to bring together the most useful metrics.
  • Ops Manager is a management platform for on-premise and private cloud deployments of MongoDB that includes extensive monitoring and alerting capabilities.
  • Cloud Manager is a management platform for self-managed cloud deployments of MongoDB that also includes extensive monitoring and alerting capabilities. (Remember this screenshot reflects the user interface at the time of writing.)
MongoDB Cloud Manager
  • Real Time Performance Panel, part of MongoDB Atlas or MongoDB Ops Manager (requires MongoDB Enterprise Advanced subscription), provides graph or table views of dozens of metrics and is a great way to keep track of many aspects of performance, including most of the metrics discussed earlier.
  • Commercial products like New Relic, Sumo Logic, and DataDog all provide interfaces designed for monitoring and alerting on MongoDB clusters. A variety of open source platforms such as mtools can be used as well.

Performance Management Tools for MongoDB Atlas

MongoDB Atlas has taken advantage of the standardized APIs and massive amounts of data available on cloud platforms to break new ground in automating performance tuning. Also, in addition to the Real Time Performance Panel mentioned above, the Performance Advisor for MongoDB Atlas analyzes queries that you are actually making on your data, determines what's slow and what's not, and makes recommendations for when to add indexes that take into account the indexes already in use.

The Professional Services Option

In a sense, the questions covered in this article represent a playbook for running a performance tuning process. If you're already running such a process, perhaps some new ideas have occurred to you based on the analysis.
Resources like this article can help you achieve or refine your goals if you know the questions to ask and some methods to get there. But if you don't know the questions to ask or the best steps to take, it's wise to avoid trial and error and ask someone with experience. With broad expertise in tuning large MongoDB deployments, professional services can help identify the most effective steps to take to improve performance right away.
Once any immediate issues are resolved, professional services can guide you in creating an ongoing streamlined performance tuning process to keep an eye on and action the metrics important to your deployment.

Wrap Up

We hope this article has made it clear that with a modest amount of effort, it's possible to keep your MongoDB cluster in top shape. No matter what types of workloads are running or where the deployment is located, use the ideas and tools mentioned above to know what's happening in your cluster and address performance problems before they become noticeable or cause major outages.
See the difference with MongoDB Atlas.

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