Node down

When a node does not respond to requests, OpsCenter marks the node as down. To determine whether a node is down, each agent gets a list of nodes that its node suspects are down based on information from Cassandra returned via JMX. Based on that information, Opscenterd determines whether a node is truly down based on status reported by other nodes, or if a node is simply flapping and erroneously reporting all other nodes down. Nodes marked with a down status are clearly indicated in the Nodes Ring View. For even more awareness and visibility, see adding an alert for down nodes for further instructions.

Write requests

The number of write requests per second. Monitoring the number of writes over a given time period can give you an idea of system write workload and usage patterns.

Write request latency

The response time (in milliseconds) for successful write operations. The time period starts when a node receives a client write request, and ends when the node responds back to the client.

Read requests

The number of read requests per second. Monitoring the number of reads over a given time period can give you an idea of system read workload and usage patterns.

Read request latency

The response time (in milliseconds) for successful read operations. The time period starts when a node receives a client read request, and ends when the node responds back to the client.

CPU usage

The percentage of time that the CPU was busy, which is calculated by subtracting the percentage of time the CPU was idle from 100 percent.

Load

Load is a measure of the amount of work that a computer system performs. An idle computer has a load number of 0 and each process using or waiting for CPU time increments the load number by 1.

Heap max

The maximum amount of shared memory allocated to the JVM heap for Cassandra processes.

Heap used

The amount of shared memory in use by the JVM heap for Cassandra processes.

JVM CMS collection count

The number of concurrent mark-sweep (CMS) garbage collections performed by the JVM per second.

JVM ParNew collection count

The number of parallel new-generation garbage collections performed by the JVM per second.

JVM CMS collection time

The time spent collecting CMS garbage in milliseconds per second (ms/sec).

JVM ParNew collection time

The time spent performing ParNew garbage collections in ms/sec.

Data size

The size of table data (in gigabytes) that has been loaded/inserted into Cassandra, including any storage overhead and system metadata.

Compactions pending

The number of compaction operations that are queued and waiting for system resources in order to run. The optimal number of pending compactions is 0 (or at most a very small number). A value greater than 0 indicates that read operations are in I/O contention with compaction operations, which usually manifests itself as declining read performance.

Total bytes compacted

The number of SSTable data compacted in bytes per second.

Total compactions

The number of compactions (minor or major) performed per second.

Flush sorter tasks pending

The flush sorter process performs the first step in the overall process of flushing memtables to disk as SSTables. The optimal number of pending flushes is 0 (or at most a very small number).

Flushes pending

The flush process flushes memtables to disk as SSTables. This metric shows the number of memtables queued for the flush process. The optimal number of pending flushes is 0 (or at most a very small number).

Gossip tasks pending

Cassandra uses a protocol called gossip to discover location and state information about the other nodes participating in a Cassandra cluster. In Cassandra, the gossip process runs once per second on each node and exchanges state messages with up to three other nodes in the cluster. Gossip tasks pending shows the number of gossip messages and acknowledgments queued and waiting to be sent or received. The optimal number of pending gossip tasks is 0 (or at most a very small number).

Hinted hand-off pending

While a node is offline, other nodes in the cluster will save hints about rows that were updated during the time the node was unavailable. When a node comes back online, its corresponding replicas will begin streaming the missed writes to the node to catch it up. The hinted hand-off pending metric tracks the number of hints that are queued and waiting to be delivered once a failed node is back online again. High numbers of pending hints are commonly seen when a node is brought back online after some down time. Viewing this metric can help you determine when the recovering node has been made consistent again.

Internal response pending

The number of pending tasks from various internal tasks such as nodes joining and leaving the cluster.

Manual repair tasks pending

The number of operations still to be completed when you run anti-entropy repair on a node. It will only show values greater than 0 when a repair is in progress. It is not unusual to see a large number of pending tasks when a repair is running, but you should see the number of tasks progressively decreasing.

Memtable postflushers pending

The memtable post flush process performs the final step in the overall process of flushing memtables to disk as SSTables. The optimal number of pending flushes is 0 (or at most a very small number).

Migrations pending

The number of pending tasks from system methods that have modified the schema. Schema updates have to be propagated to all nodes, so pending tasks for this metric can manifest in schema disagreement errors.

Miscellaneous tasks pending

The number of pending tasks from other miscellaneous operations that are not ran frequently.

Read requests pending

The number of read requests that have arrived into the cluster but are waiting to be handled. During low or moderate read load, you should see 0 pending read operations (or at most a very low number).

Read repair tasks pending

The number of read repair operations that are queued and waiting for system resources in order to run. The optimal number of pending read repairs is 0 (or at most a very small number). A value greater than 0 indicates that read repair operations are in I/O contention with other operations.

Replicate on write tasks pending

When an insert or update to a row is written, the affected row is replicated to all other nodes that manage a replica for that row. This is called the ReplicateOnWriteStage. This metric tracks the pending tasks related to this stage of the write process. During low or moderate write load, you should see 0 pending replicate on write tasks (or at most a very low number).

Request response pending

Streaming of data between nodes happens during operations such as bootstrap and decommission when one node sends large numbers of rows to another node. The metric tracks the progress of the streamed rows from the receiving node.

Streams pending

Streaming of data between nodes happens during operations such as bootstrap and decommission when one node sends large numbers of rows to another node. The metric tracks the progress of the streamed rows from the sending node.

Write requests pending

The number of write requests that have arrived into the cluster but are waiting to be handled. During low or moderate write load, you should see 0 pending write operations (or at most a very low number).

Local writes

The write load on a table measured in operations per second. This metric includes all writes to a given table, including write requests forwarded from other nodes.

Local write latency

The response time in milliseconds for successful write operations on a table. The time period starts when nodes receive a write request, and ends when nodes respond.

Local reads

The read load on a table measured in operations per second. This metric includes all reads to a given table, including read requests forwarded from other nodes.

Local read latency

The response time in microseconds for successful read operations on a table. The time period starts when a node receives a read request, and ends when the node responds.

Table key cache hits

The number of read requests that resulted in the requested row key being found in the key cache.

Table key cache requests

The total number of read requests on the row key cache.

Table key cache hit rate

The key cache hit rate indicates the effectiveness of the key cache for a given table by giving the percentage of cache requests that resulted in a cache hit.

Table row cache hits

The number of read requests that resulted in the read being satisfied from the row cache.

Table row cache requests

The total number of read requests on the row cache.

Table row cache hit rate

The key cache hit rate indicates the effectiveness of the row cache for a given table by giving the percentage of cache requests that resulted in a cache hit.

Table bloom filter space used

The size of the bloom filter files on disk.

Table bloom filter false positives

The number of false positives, which occur when the bloom filter said the row existed, but it actually did not exist in absolute numbers.

Table bloom filter false positive ratio

The fraction of all bloom filter checks resulting in a false positive.

Live disk used

The current size of live SSTables for a table. It is expected that SSTable size will grow over time with your write load as compaction processes continue doubling the size of SSTables. Monitor the current state of compaction for a given table using this metric together with SSTable count.

Total disk used

The current size of the data directories for the table including space not reclaimed by obsolete objects.

SSTable count

The current number of SSTables for a table. When table memtables are persisted to disk as SSTables, this metric increases to the configured maximum before the compaction cycle is repeated. Monitor the current state of compaction for a given table using this metric together with live disk used.

Pending reads and writes

The number of pending reads and writes on a table. Pending operations indicate Cassandra is not keeping up with the workload. A value of zero indicates healthy throughput.