C100DBA Server Administration — Timed 10 Questions

Question 1

Refer to the exhibit. Based on the provided YAML configuration snippet, what will be the behavior of the mongod instance regarding memory allocation on a host with 16GB of total system RAM?

Accepted Answer

The internal cache will be capped strictly at 2GB.. Administrators must understand how to manually override default settings to optimize performance or coexist with other applications. By default, WiredTiger uses 50% of (RAM - 1GB). On a 16GB host, the default would be 7.5GB. However, the configuration explicitly restricts the internal cache to 2GB, which is useful when running multiple instances or memory-constrained containers.

Answer

The instance will use 7.5GB for the WiredTiger cache.

Answer

The total resident memory (RSS) will not exceed 2GB.

Answer

The mongod process will fail to start due to a memory mismatch.

Question 2

A production environment requires point-in-time recovery for a 2TB database. Which TWO strategies are most appropriate for minimizing recovery time while ensuring data consistency?

Accepted Answer

Using filesystem-level snapshots on a volume with journaling enabled.. Point-in-time recovery (PITR) is a fundamental requirement for production database administration. Filesystem snapshots provide a fast way to restore large volumes of data, while the oplog (operations log) allows the DBA to replay changes up to a specific second. Combining these ensures that even a 2TB dataset can be restored quickly without full BSON re-indexing.

Answer

Performing a daily mongodump with the --gzip flag.

Answer

Scheduling a weekly mongorestore to a staging environment.

Answer

Running the repairDatabase command after every write operation.

Question 3

When performing a rolling upgrade of a MongoDB replica set from version 5.0 to 6.0, which TWO steps are mandatory to ensure cluster stability and compatibility?

Accepted Answer

Ensure that 'featureCompatibilityVersion' is set to '5.0' before upgrading.. Upgrading a replica set requires a specific sequence to maintain availability and prevent data corruption. Administrators must ensure that the 'featureCompatibilityVersion' (FCV) matches the old version before starting. Upgrading secondaries first ensures that if a failure occurs, the primary can still function on the old version while the administrator troubleshoots the upgrade process.

Answer

Upgrade the primary node first to ensure new features are available.

Answer

Disable all indexes on the collection before starting the upgrade.

Answer

Run 'db.upgrade()' from the admin database on each node.

Question 4

A database administrator notices that the WiredTiger cache is frequently reaching its maximum size, causing excessive eviction. Which configuration change is the most appropriate to manage memory pressure in a standalone node?

Accepted Answer

Decrease the 'wiredTiger.engineConfig.cacheSizeGB' to a value that allows overhead for the OS.. Tuning the WiredTiger cache size is critical for maintaining performance in memory-constrained environments. By adjusting the 'wiredTiger.engineConfig.cacheSizeGB' parameter, administrators can balance the memory allocated to the engine versus the operating system. This prevents the operating system from swapping to disk, which would otherwise degrade latency. Properly sizing the cache ensures that the working set fits efficiently in RAM, reducing the need for constant page evictions and improving overall throughput.

Answer

Increase the 'wiredTiger.engineConfig.cacheSizeGB' value to consume all available system RAM.

Answer

Enable the 'net.compression.snappy' setting to reduce memory footprint.

Answer

Set 'storage.journal.enabled' to false to free up cache space.

Question 5

An administrator is planning a backup strategy for a replica set. Which TWO of the following methods allow for consistent backups without stopping the MongoDB service?

Accepted Answer

Use 'mongodump' on a secondary node with --oplog enabled.. Maintaining data availability during backups is a primary operational requirement. File system snapshots and MongoDB-native tools like mongodump or dedicated backup solutions are designed for this purpose. Understanding these methods is essential for creating reliable recovery points without causing downtime, which is critical for production environments where continuous availability is mandatory for business continuity and compliance with service level agreements.

Answer

Perform a manual copy of the data files while the server is running.

Answer

Stop the mongod process and copy the contents of the 'dbPath'.

Answer

Use the 'mongorestore' utility to stream data to a remote server.

Question 6

Which THREE of the following actions are standard steps when decommissioning a MongoDB node from a replica set?

Accepted Answer

Execute 'rs.remove()' from the primary node.. Decommissioning a node requires careful coordination to prevent cluster instability. Failing to update the configuration or clean up the node can lead to dangling references or unnecessary connection attempts by other members. Following a strict sequence ensures that the remaining replica set maintains its quorum and that the removed node does not interfere with the health or the security profile of the active production cluster.

Answer

Immediately delete the /var/lib/mongodb directory.

Answer

Force a primary election before removal.

Question 7

What is the effect of setting 'writeConcern' to '{ w: "majority" }' on an application's write operations?

Accepted Answer

The write will persist even if the primary node crashes after acknowledgement.. Setting write concern to 'majority' ensures that the data is acknowledged by a majority of the replica set members before the client receives a success response. This guarantees that the write will not be rolled back during an election, which is critical for maintaining data integrity and preventing 'dirty reads' in distributed systems. It is the best practice for applications requiring strong consistency and durability in a production replica set.

Answer

The write operation will be faster than with 'w: 1'.

Answer

It forces the data to be written directly to the disk cache.

Answer

It automatically enables read-your-writes consistency for all reads.

Question 8

Which MongoDB process utility is specifically used to verify the integrity of the data files and the WiredTiger engine metadata?

Accepted Answer

wiredTiger (standalone utility). Data integrity is paramount in database administration. The 'mongod --repair' or the 'wiredTiger' command-line utility tools allow administrators to validate that data files are not corrupted. In scenarios where a server has experienced a hard crash or power failure, these tools provide the only mechanism to assess the physical health of the data files before attempting to restart the database and risking further data loss or corruption.

Answer

mongodump

Answer

mongofiles

Answer

mongostat

Question 9

An administrator needs to rotate the logs without stopping the MongoDB service. Which approach is the most effective?

Accepted Answer

Use the 'logRotate' command via the mongo shell.. Log rotation is essential for preventing log files from consuming all available disk space. By sending a 'SIGUSR1' signal to the mongod process, administrators can trigger an immediate log rotation. This allows for seamless file management, enabling the archiving of old logs and the creation of new ones without impacting the running database process, which is critical for 24/7 production environments that cannot tolerate downtime.

Answer

Restart the mongod service.

Answer

Manually delete the current log file while the service is running.

Answer

Update the 'systemLog.path' in the config file.

Question 10

A MongoDB replica set has three members: Primary, Secondary1, and Secondary2. Secondary1 is in a different data center with 50 ms latency. You notice that write concern 'majority' is taking longer than expected. Which factor is most likely causing the increased latency for majority writes?

Accepted Answer

Secondary1 must acknowledge the write before the majority write concern is satisfied.. In a three-member replica set, majority write concern requires acknowledgment from two members. If one secondary is unavailable or slow, the primary may need to wait for the other secondary, which has high latency. This directly increases the time to satisfy majority write concern. The other options are incorrect because majority does not require all members or a specific secondary.

Answer

The write concern 'majority' is calculated based on the number of data-bearing members, which excludes arbiters.

Answer

The primary must wait for all secondaries to acknowledge before returning.

Answer

Secondary2 is the only member that can satisfy the majority write concern.