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Update telemetry page with advice for monitoring boltdb performance (#12141) 

Co-authored-by: R.B. Boyer <4903+rboyer@users.noreply.github.com>
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Matt Keeler 2022-01-26 11:51:19 -05:00 committed by GitHub
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@ -269,6 +269,60 @@ resources will still work.
This metric should be monitored to ensure that the license doesn't expire to prevent degradation of functionality.
### Bolt DB Performance
| Metric Name | Description | Unit | Type |
| :-------------------------------- | :--------------------------------------------------------------- | :---- | :---- |
| `consul.raft.boltdb.freelistBytes` | Represents the number of bytes necessary to encode the freelist metadata. When [`raft_boltdb.NoFreelistSync`](/docs/agent/options#NoFreelistSync) is set to `false` these metadata bytes must also be written to disk for each committed log. | bytes | gauge |
| `consul.raft.boltdb.logsPerBatch` | Measures the number of logs being written per batch to the db. | logs | sample |
| `consul.raft.boltdb.storeLogs` | Measures the amount of time spent writing logs to the db. | ms | timer |
** Requirements: **
* Consul 1.11.0+
**Why they're important:**
The `consul.raft.boltdb.storeLogs` metric is a direct indicator of disk write performance of a Consul server. If there are issues with the disk or
performance degradations related to Bolt DB, these metrics will show the issue and potentially the cause as well.
**What to look for:**
The primary thing to look for are increases in the `consul.raft.boltdb.storeLogs` times. Its value will directly govern an
upper limit to the throughput of write operations within Consul.
In Consul each write operation will turn into a single Raft log to be committed. Raft will process these
logs and store them within Bolt DB in batches. Each call to store logs within Bolt DB is measured to record how long
it took as well as how many logs were contained in the batch. Writing logs is this fashion is serialized so that
a subsequent log storage operation can only be started after the previous one completed. Therefore the maximum number
of log storage operations that can be performed each second can be calculated with the following equation:
`(1000 ms) / (consul.raft.boltdb.storeLogs ms/op)`. From there we can extrapolate the maximum number of Consul writes
per second by multiplying that value by the `consul.raft.boltdb.logsPerBatch` metric's value. When log storage
operations are becoming slower you may not see an immediate decrease in write throughput to Consul due to increased
batch sizes of the each operation. However, the max batch size allowed is 64 logs. Therefore if the `logsPerBatch`
metric is near 64 and the `storeLogs` metric is seeing increased time to write each batch to disk, then it is likely
that increased write latencies and other errors may occur.
There can be a number of potential issues that can cause this. Often times it could be performance of the underlying
disks that is the issue. Other times it may be caused by Bolt DB behavior. Bolt DB keeps track of free space within
the `raft.db` file. When needing to allocate data it will use existing free space first before further expanding the
file. By default, Bolt DB will write a data structure containing metadata about free pages within the DB to disk for
every log storage operation. Therefore if the free space within the database grows excessively large, such as after
a large spike in writes beyond the normal steady state and a subsequent slow down in the write rate, then Bolt DB
could end up writing a large amount of extra data to disk for each log storage operation. This has the potential
to drastically increase disk write throughput, potentially beyond what the underlying disks can keep up with. To
detect this situation you can look at the `consul.raft.boltdb.freelistBytes` metric. This metric is a count of
the extra bytes that are being written for each log storage operation beyond the log data itself. While not a clear
indicator of an actual issue, this metric can be used to diagnose why the `consul.raft.boltdb.storeLogs` metric
is high.
If Bolt DB log storage performance becomes an issue and is caused by free list management then setting
[`raft_boltdb.NoFreelistSync`](/docs/agent/options#NoFreelistSync) to `true` in the server's configuration
may help to reduce disk IO and log storage operation times. Disabling free list syncing will however increase
the startup time for a server as it must scan the raft.db file for free space instead of loading the already
populated free list structure.
## Metrics Reference
This is a full list of metrics emitted by Consul.
@ -344,7 +398,7 @@ These metrics are used to monitor the health of the Consul servers.
| `consul.raft.applied_index` | Represents the raft applied index. | index | gauge |
| `consul.raft.apply` | Counts the number of Raft transactions occurring over the interval, which is a general indicator of the write load on the Consul servers. | raft transactions / interval | counter |
| `consul.raft.barrier` | Counts the number of times the agent has started the barrier i.e the number of times it has issued a blocking call, to ensure that the agent has all the pending operations that were queued, to be applied to the agent's FSM. | blocks / interval | counter |
| `consul.raft.boltdb.freelistBytes` | Represents the number of bytes necessary to encode the freelist metadata. When `raft_boltdb.NoFreelistSync` is set to `false` these metadata bytes must also be written to disk for each committed log. | bytes | gauge |
| `consul.raft.boltdb.freelistBytes` | Represents the number of bytes necessary to encode the freelist metadata. When [`raft_boltdb.NoFreelistSync`](/docs/agent/options#NoFreelistSync) is set to `false` these metadata bytes must also be written to disk for each committed log. | bytes | gauge |
| `consul.raft.boltdb.freePageBytes` | Represents the number of bytes of free space within the raft.db file. | bytes | gauge |
| `consul.raft.boltdb.getLog` | Measures the amount of time spent reading logs from the db. | ms | timer |
| `consul.raft.boltdb.logBatchSize` | Measures the total size in bytes of logs being written to the db in a single batch. | bytes | sample |