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Refactor and move wal docs (#16387) 

* Add WAL documentation. Also fix some minor metrics registration details

* Add tests to verify metrics are registered correctly

* refactor and move wal docs

* Updates to the WAL overview page

* updates to enable WAL usage topic

* updates to the monitoring WAL backend topic

* updates for revert WAL topic

* a few tweaks to overview and udpated metadescriptions

* Apply suggestions from code review

Co-authored-by: Paul Banks <pbanks@hashicorp.com>

* make revert docs consistent with enable

* Apply suggestions from code review

Co-authored-by: Paul Banks <pbanks@hashicorp.com>

* address feedback

* address final feedback

* Apply suggestions from code review

Co-authored-by: Jeff Boruszak <104028618+boruszak@users.noreply.github.com>

---------

Co-authored-by: Paul Banks <pbanks@hashicorp.com>
Co-authored-by: trujillo-adam <ajosetru@gmail.com>
Co-authored-by: trujillo-adam <47586768+trujillo-adam@users.noreply.github.com>
Co-authored-by: Jeff Boruszak <104028618+boruszak@users.noreply.github.com>
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88
website/content/docs/agent/config/config-files.mdx
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@ -1586,15 +1586,89 @@ Valid time units are 'ns', 'us' (or 'µs'), 'ms', 's', 'm', 'h'."
## Raft Parameters
- `raft_boltdb` ((#raft_boltdb)) This is a nested object that allows configuring
options for Raft's BoltDB based log store.
- `raft_boltdb` ((#raft_boltdb)) **These fields are deprecated in Consul v1.15.0.
Use [`raft_logstore`](#raft_logstore) instead.** This is a nested
object that allows configuring options for Raft's BoltDB-based log store.
- `NoFreelistSync` ((#NoFreelistSync)) Setting this to `true` will disable
syncing the BoltDB freelist to disk within the raft.db file. Not syncing
the freelist to disk will reduce disk IO required for write operations
at the expense of potentially increasing start up time due to needing
to scan the db to discover where the free space resides within the file.
- `NoFreelistSync` **This field is deprecated in Consul v1.15.0. Use the
[`raft_logstore.boltdb.no_freelist_sync`](#raft_logstore_boltdb_no_freelist_sync) field
instead.** Setting this to `true` disables syncing the BoltDB freelist
to disk within the raft.db file. Not syncing the freelist to disk
reduces disk IO required for write operations at the expense of potentially
increasing start up time due to needing to scan the db to discover where the
free space resides within the file.
- `raft_logstore` ((#raft_logstore)) This is a nested object that allows
configuring options for Raft's LogStore component which is used to persist
logs and crucial Raft state on disk during writes. This was added in Consul
v1.15.0.
- `backend` ((#raft_logstore_backend)) Specifies which storage
engine to use to persist logs. Valid options are `boltdb` or `wal`. Default
is `boltdb`. The `wal` option specifies an experimental backend that
should be used with caution. Refer to
[Experimental WAL LogStore backend](/consul/docs/agent/wal-logstore)
for more information.
- `disable_log_cache` ((#raft_logstore_disable_log_cache)) Disables the in-memory cache for recent logs. We recommend using it for performance testing purposes, as no significant improvement has been measured when the cache is disabled. While the in-memory log cache theoretically prevents disk reads for recent logs, recent logs are also stored in the OS page cache, which does not slow either the `boltdb` or `wal` backend's ability to read them.
- `verification` ((#raft_logstore_verification)) This is a nested object that
allows configuring the online verification of the LogStore. Verification
provides additional assurances that LogStore backends are correctly storing
data. It imposes low overhead on servers and is safe to run in
production. It is most useful when evaluating a new backend
implementation.
Verification must be enabled on the leader to have any effect and can be
used with any backend. When enabled, the leader periodically writes a
special "checkpoint" log message that includes the checksums of all log entries
written to Raft since the last checkpoint. Followers that have verification
enabled run a background task for each checkpoint that reads all logs
directly from the LogStore and then recomputes the checksum. A report is output
as an INFO level log for each checkpoint.
Checksum failure should never happen and indicate unrecoverable corruption
on that server. The only correct response is to stop the server, remove its
data directory, and restart so it can be caught back up with a correct
server again. Please report verification failures including details about
your hardware and workload via GitHub issues. Refer to
[Experimental WAL LogStore backend](/consul/docs/agent/wal-logstore)
for more information.
- `enabled` ((#raft_logstore_verification_enabled)) - Set to `true` to
allow this Consul server to write and verify log verification checkpoints
when elected leader.
- `interval` ((#raft_logstore_verification_interval)) - Specifies the time
interval between checkpoints. There is no default value. You must
configure the `interval` and set [`enabled`](#raft_logstore_verification_enabled)
to `true` to correctly enable intervals. We recommend using an interval
between `30s` and `5m`. The performance overhead is insignificant when the
interval is set to `5m` or less.
- `boltdb` ((#raft_logstore_boltdb)) - Object that configures options for
Raft's `boltdb` backend. It has no effect if the `backend` is not `boltdb`.
- `no_freelist_sync` ((#raft_logstore_boltdb_no_freelist_sync)) - Set to
`true` to disable storing BoltDB's freelist to disk within the
`raft.db` file. Disabling freelist syncs reduces the disk IO required
for write operations, but could potentially increase start up time
because Consul must scan the database to find free space
within the file.
- - `wal` ((#raft_logstore_wal)) - Object that configures the `wal` backend.
Refer to [Experimental WAL LogStore backend](/consul/docs/agent/wal-logstore)
for more information.
- `segment_size_mb` ((#raft_logstore_wal_segment_size_mb)) - Integer value
that represents the target size in MB for each segment file before
rolling to a new segment. The default value is `64` and is suitable for
most deployments. While a smaller value may use less disk space because you
can reclaim space by deleting old segments sooner, the smaller segment that results
may affect performance because safely rotating to a new file more
frequently can impact tail latencies. Larger values are unlikely
to improve performance significantly. We recommend using this
configuration for performance testing purposes.
- `raft_protocol` ((#raft_protocol)) Equivalent to the [`-raft-protocol`
command-line flag](/consul/docs/agent/config/cli-flags#_raft_protocol).

31
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@ -294,7 +294,7 @@ This metric should be monitored to ensure that the license doesn't expire to pre
| Metric Name | Description | Unit | Type |
| :-------------------------------- | :--------------------------------------------------------------- | :---- | :---- |
| `consul.raft.boltdb.freelistBytes` | Represents the number of bytes necessary to encode the freelist metadata. When [`raft_boltdb.NoFreelistSync`](/consul/docs/agent/config/config-files#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_logstore.boltdb.no_freelist_sync`](/consul/docs/agent/config/config-files#raft_logstore_boltdb_no_freelist_sync) 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 |
| `consul.raft.boltdb.writeCapacity` | Theoretical write capacity in terms of the number of logs that can be written per second. Each sample outputs what the capacity would be if future batched log write operations were similar to this one. This similarity encompasses 4 things: batch size, byte size, disk performance and boltdb performance. While none of these will be static and its highly likely individual samples of this metric will vary, aggregating this metric over a larger time window should provide a decent picture into how this BoltDB store can perform | logs/second | sample |
@ -337,11 +337,12 @@ indicator of an actual issue, this metric can be used to diagnose why the `consu
is high.
If Bolt DB log storage performance becomes an issue and is caused by free list management then setting
[`raft_boltdb.NoFreelistSync`](/consul/docs/agent/config/config-files#NoFreelistSync) to `true` in the server's configuration
[`raft_logstore.boltdb.no_freelist_sync`](/consul/docs/agent/config/config-files#raft_logstore_boltdb_no_freelist_sync) 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.
Consul includes an experiment backend configuration that you can use instead of BoldDB. Refer to [Experimental WAL LogStore backend](/consul/docs/agent/wal-logstore) for more information.
## Metrics Reference
@ -418,7 +419,7 @@ 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`](/consul/docs/agent/config/config-files#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_logstore.boltdb.no_freelist_sync`](/consul/docs/agent/config/config-files#raft_logstore_boltdb_no_freelist_sync) 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 |
@ -452,6 +453,13 @@ These metrics are used to monitor the health of the Consul servers.
| `consul.raft.last_index` | Represents the raft applied index. | index | gauge |
| `consul.raft.leader.dispatchLog` | Measures the time it takes for the leader to write log entries to disk. | ms | timer |
| `consul.raft.leader.dispatchNumLogs` | Measures the number of logs committed to disk in a batch. | logs | gauge |
| `consul.raft.logstore.verifier.checkpoints_written` | Counts the number of checkpoint entries written to the LogStore. | checkpoints | counter |
| `consul.raft.logstore.verifier.dropped_reports` | Counts how many times the verifier routine was still busy when the next checksum came in and so verification for a range was skipped. If you see this happen, consider increasing the interval between checkpoints with [`raft_logstore.verification.interval`](/consul/docs/agent/config/config-files#raft_logstore_verification) | reports dropped | counter |
| `consul.raft.logstore.verifier.ranges_verified` | Counts the number of log ranges for which a verification report has been completed. Refer to [Monitor Raft metrics and logs for WAL
](/consul/docs/agent/wal-logstore/monitoring) for more information. | log ranges verifications | counter |
| `consul.raft.logstore.verifier.read_checksum_failures` | Counts the number of times a range of logs between two check points contained at least one disk corruption. Refer to [Monitor Raft metrics and logs for WAL
](/consul/docs/agent/wal-logstore/monitoring) for more information. | disk corruptions | counter |
| `consul.raft.logstore.verifier.write_checksum_failures` | Counts the number of times a follower has a different checksum to the leader at the point where it writes to the log. This could be caused by either a disk-corruption on the leader (unlikely) or some other corruption of the log entries in-flight. | in-flight corruptions | counter |
| `consul.raft.leader.lastContact` | Measures the time since the leader was last able to contact the follower nodes when checking its leader lease. It can be used as a measure for how stable the Raft timing is and how close the leader is to timing out its lease.The lease timeout is 500 ms times the [`raft_multiplier` configuration](/consul/docs/agent/config/config-files#raft_multiplier), so this telemetry value should not be getting close to that configured value, otherwise the Raft timing is marginal and might need to be tuned, or more powerful servers might be needed. See the [Server Performance](/consul/docs/install/performance) guide for more details. | ms | timer |
| `consul.raft.leader.oldestLogAge` | The number of milliseconds since the _oldest_ log in the leader's log store was written. This can be important for replication health where write rate is high and the snapshot is large as followers may be unable to recover from a restart if restoring takes longer than the minimum value for the current leader. Compare this with `consul.raft.fsm.lastRestoreDuration` and `consul.raft.rpc.installSnapshot` to monitor. In normal usage this gauge value will grow linearly over time until a snapshot completes on the leader and the log is truncated. Note: this metric won't be emitted until the leader writes a snapshot. After an upgrade to Consul 1.10.0 it won't be emitted until the oldest log was written after the upgrade. | ms | gauge |
| `consul.raft.replication.heartbeat` | Measures the time taken to invoke appendEntries on a peer, so that it doesn't timeout on a periodic basis. | ms | timer |
@ -476,9 +484,20 @@ These metrics are used to monitor the health of the Consul servers.
| `consul.raft.state.follower` | Counts the number of times an agent has entered the follower mode. This happens when a new agent joins the cluster or after the end of a leader election. | follower state entered / interval | counter |
| `consul.raft.transition.heartbeat_timeout` | The number of times an agent has transitioned to the Candidate state, after receive no heartbeat messages from the last known leader. | timeouts / interval | counter |
| `consul.raft.verify_leader` | This metric doesn't have a direct correlation to the leader change. It just counts the number of times an agent checks if it is still the leader or not. For example, during every consistent read, the check is done. Depending on the load in the system, this metric count can be high as it is incremented each time a consistent read is completed. | checks / interval | Counter |
| `consul.rpc.accept_conn` | Increments when a server accepts an RPC connection. | connections | counter |
| `consul.rpc.rate_limit.exceeded` | Number of rate limited requests. Only increments when `rate_limits.mode` is set to `permissive` or `enforcing`. | requests | counter |
| `consul.rpc.rate_limit.log_dropped` | Number of logs for rate limited requests dropped for performance reasons. Only increments when `rate_limits.mode` is set to `permissive` or `enforcing` and the log is unable to print the number of excessive requests. | log lines | counter |
| `consul.raft.wal.head_truncations` | Counts how many log entries have been truncated from the head - i.e. the oldest entries. by graphing the rate of change over time you can see individual truncate calls as spikes. | logs entries truncated | counter |
| `consul.raft.wal.last_segment_age_seconds` | A gauge that is set each time we rotate a segment and describes the number of seconds between when that segment file was first created and when it was sealed. this gives a rough estimate how quickly writes are filling the disk. | seconds | gauge |
| `consul.raft.wal.log_appends` | Counts the number of calls to StoreLog(s) i.e. number of batches of entries appended. | calls | counter |
| `consul.raft.wal.log_entries_read` | Counts the number of log entries read. | log entries read | counter |
| `consul.raft.wal.log_entries_written` | Counts the number of log entries written. | log entries written | counter |
| `consul.raft.wal.log_entry_bytes_read` | Counts the bytes of log entry read from segments before decoding. actual bytes read from disk might be higher as it includes headers and index entries and possible secondary reads for large entries that don't fit in buffers. | bytes | counter |
| `consul.raft.wal.log_entry_bytes_written` | Counts the bytes of log entry after encoding with Codec. Actual bytes written to disk might be slightly higher as it includes headers and index entries. | bytes | counter |
| `consul.raft.wal.segment_rotations` | Counts how many times we move to a new segment file. | rotations | counter |
| `consul.raft.wal.stable_gets` | Counts how many calls to StableStore.Get or GetUint64. | calls | counter |
| `consul.raft.wal.stable_sets` | Counts how many calls to StableStore.Set or SetUint64. | calls | counter |
| `consul.raft.wal.tail_truncations` | Counts how many log entries have been truncated from the head - i.e. the newest entries. by graphing the rate of change over time you can see individual truncate calls as spikes. | logs entries truncated | counter |
| `consul.rpc.accept_conn` | Increments when a server accepts an RPC connection. | connections | counter |
| `consul.rpc.rate_limit.exceeded` | Increments whenever an RPC is over a configured rate limit. In permissive mode, the RPC is still allowed to proceed. | RPCs | counter |
| `consul.rpc.rate_limit.log_dropped` | Increments whenever a log that is emitted because an RPC exceeded a rate limit gets dropped because the output buffer is full. | log messages dropped | counter |
| `consul.catalog.register` | Measures the time it takes to complete a catalog register operation. | ms | timer |
| `consul.catalog.deregister` | Measures the time it takes to complete a catalog deregister operation. | ms | timer |
| `consul.server.isLeader` | Track if a server is a leader(1) or not(0) | 1 or 0 | gauge |

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@ -0,0 +1,143 @@
---
layout: docs
page_title: Enable the experimental WAL LogStore backend
description: >-
Learn how to safely configure and test the experimental WAL backend in your Consul deployment.
---
# Enable the experimental WAL LogStore backend
This topic describes how to safely configure and test the WAL backend in your Consul deployment.
The overall process for enabling the WAL LogStore backend for one server consists of the following steps. In production environments, we recommend starting by enabling the backend on a single server . If you eventually choose to expand the test to further servers, you must repeat these steps for each one.
1. Enable log verification.
1. Select target server to enable WAL.
1. Stop target server gracefully.
1. Remove data directory from target server.
1. Update target server's configuration.
1. Start the target server.
1. Monitor target server raft metrics and logs.
!> **Experimental feature:** The WAL LogStore backend is experimental.
## Requirements
- Consul v1.15 or later is required for all servers in the datacenter. Refer to the [standard upgrade procedure](/consul/docs/upgrading/general-process) and the [1.15 upgrade notes](/consul/docs/upgrading/upgrade-specific#consul-1-15-x) for additional information.
- A Consul cluster with at least three nodes are required to safely test the WAL backend without downtime.
We recommend taking the following additional measures:
- Take a snapshot prior to testing.
- Monitor Consul server metrics and logs, and set an alert on specific log events that occur when WAL is enabled. Refer to [Monitor Raft metrics and logs for WAL](/consul/docs/agent/wal-logstore/monitoring) for more information.
- Enable WAL in a pre-production environment and run it for a several days before enabling it in production.
## Risks
While their likelihood remains low to very low, be aware of the following risks before implementing the WAL backend:
- If WAL corrupts data on a Consul server agent, the server data cannot be recovered. Restart the server with an empty data directory and reload its state from the leader to resolve the issue.
- WAL may corrupt data or contain a defect that causes the server to panic and crash. WAL may not restart if the defect recurs when WAL reads from the logs on startup. Restart the server with an empty data directory and reload its state from the leader to resolve the issue.
- If WAL corrupts data, clients may read corrupted data from the Consul server, such as invalid IP addresses or unmatched tokens. This outcome is unlikely even if a recurring defect causes WAL to corrupt data because replication uses objects cached in memory instead of reads from disk. Restart the server with an empty data directory and reload its state from the leader to resolve the issue.
- If you enable a Consul OSS server to use WAL or enable WAL on a voting server with Consul Enterprise, WAL may corrupt the server's state, become the leader, and replicate the corrupted state to all other servers. In this case, restoring from backup is required to recover a completely uncorrupted state. Test WAL on a non-voting server in Enterprise to prevent this outcome. You can add a new non-voting server to the cluster to test with if there are no existing ones.
## Enable log verification
You must enable log verification on all voting servers in Enterprise and all servers in OSS because the leader writes verification checkpoints.
1. On each voting server, add the following to the server's configuration file:
```hcl
raft_logstore {
verification {
enabled = true
interval = "60s"
}
}
```
1. Restart the server to apply the changes. The `consul reload` command is not sufficient to apply `raft_logstore` configuration changes.
1. Run the `consul operator raft list-peers` command to wait for each server to become a healthy voter before moving on to the next. This may take a few minutes for large snapshots.
When complete, the server's logs should contain verifier reports that appear like the following example:
```log hideClipboard
2023-01-31T14:44:31.174Z [INFO] agent.server.raft.logstore.verifier: verification checksum OK: elapsed=488.463268ms leaderChecksum=f15db83976f2328c rangeEnd=357802 rangeStart=298132 readChecksum=f15db83976f2328c
```
## Select target server to enable WAL
If you are using Consul OSS or Consul Enterprise without non-voting servers, select a follower server to enable WAL. As noted in [Risks](#risks), Consul Enterprise users with non-voting servers should first select a non-voting server, or consider adding another server as a non-voter to test on.
Retrieve the current state of the servers by running the following command:
```shell-session
$ consul operator raft list-peers
```
## Stop target server
Stop the target server gracefully. For example, if you are using `systemd`,
run the following command:
```shell-session
$ systemctl stop consul
```
If your environment uses configuration management automation that might interfere with this process, such as Chef or Puppet, you must disable them until you have completely enabled WAL as a storage backend.
## Remove data directory from target server
Temporarily moving the data directory to a different location is less destructive than deleting it. We recommend moving it in cases where you unsuccessfully enable WAL. Do not use the old data directory (`/data-dir/raft.bak`) for recovery after restarting the server. We recommend eventually deleting the old directory.
The following example assumes the `data_dir` in the server's configuration is `/data-dir` and renames it to `/data-dir.bak`.
```shell-session
$ mv /data-dir/raft /data-dir/raft.bak
```
When switching backends, you must always remove _the entire raft directory_, not just the `raft.db` file or `wal` directory. The log must always be consistent with the snapshots to avoid undefined behavior or data loss.
## Update target server configuration
Add the following to the target server's configuration file:
```hcl
raft_logstore {
backend = "wal"
verification {
enabled = true
interval = "60s"
}
}
```
## Start target server
Start the target server. For example, if you are using `systemd`, run the following command:
```shell-session
$ systemctl start consul
```
Watch for the server to become a healthy voter again.
```shell-session
$ consul operator raft list-peers
```
## Monitor target server Raft metrics and logs
Refer to [Monitor Raft metrics and logs for WAL](/consul/docs/agent/wal-logstore/monitoring) for details.
We recommend leaving the cluster in the test configuration for several days or weeks, as long as you observe no errors. An extended test provides more confidence that WAL operates correctly under varied workloads and during routine server restarts. If you observe any errors, end the test immediately and report them.
If you disabled configuration management automation, consider reenabling it during the testing phase to pick up other updates for the host. You must ensure that it does not revert the Consul configuration file and remove the altered backend configuration. One way to do this is add the `raft_logstore` block to a separate file that is not managed by your automation. This file can either be added to the directory if [`-config-dir`](/consul/docs/agent/config/cli-flags#_config_dir) is used or as an additional [`-config-file`](/consul/docs/agent/config/cli-flags#_config_file) argument.
## Next steps
- If you observe any verification errors, performance anomalies, or other suspicious behavior from the target server during the test, you should immediately follow [the procedure to revert back to BoltDB](/consul/docs/agent/wal-logstore/revert-to-boltdb). Report failures through GitHub.
- If you do not see errors and would like to expand the test further, you can repeat the above procedure on another target server. We suggest waiting after each test expansion and slowly rolling WAL out to other parts of your environment. Once the majority of your servers use WAL, any bugs not yet discovered may result in cluster unavailability.
- If you wish to permanently enable WAL on all servers, repeat the steps described in this topic for each server. Even if `backend = "wal"` is set in logs, servers continue to use BoltDB if they find an existing raft.db file in the data directory.

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---
layout: docs
page_title: WAL LogStore Backend Overview
description: >-
The experimental WAL (write-ahead log) LogStore backend shipped in Consul 1.15 is intended to replace the BoltDB backend, improving performance and log storage issues.
---
# Experimental WAL LogStore backend overview
This topic provides an overview of the experimental WAL (write-ahead log) LogStore backend.
!> **Experimental feature:** The WAL LogStore backend is experimental.
## WAL versus BoltDB
WAL implements a traditional log with rotating, append-only log files. WAL resolves many issues with the existing `LogStore` provided by the BoltDB backend. The BoltDB `LogStore` is a copy-on-write BTree, which is not optimized for append-only, write-heavy workloads.
### BoltDB storage scalability issues
The existing BoltDB log store inefficiently stores append-only logs to disk because it was designed as a full key-value database. It is a single file that only ever grows. Deleting the oldest logs, which Consul does regularly when it makes new snapshots of the state, leaves free space in the file. The free space must be tracked in a `freelist` so that BoltDB can reuse it on future writes. By contrast, a simple segmented log can delete the oldest log files from disk.
A burst of writes at double or triple the normal volume can suddenly cause the log file to grow to several times its steady-state size. After Consul takes the next snapshot and truncates the oldest logs, the resulting file is mostly empty space.
To track the free space, Consul must write extra metadata to disk with every write. The metadata is proportional to the amount of free pages, so after a large burst write latencies tend to increase. In some cases, the latencies cause serious performance degradation to the cluster.
To mitigate risks associated with sudden bursts of log data, Consul tries to limit lots of logs from accumulating in the LogStore. Significantly larger BoltDB files are slower to append to because the tree is deeper and freelist larger. For this reason, Consul's default options associated with snapshots, truncating logs, and keeping the log history have been aggressively set toward keeping BoltDB small rather than using disk IO optimally.
But the larger the file, the more likely it is to have a large freelist or suddenly form one after a burst of writes. For this reason, the many of Consul's default options asssociated with snapshots, truncating logs, and keeping the log history aggressively keep BoltDT small rather than uisng disk IO more efficiently.
Other reliability issues, such as [raft replication capacity issues](/consul/docs/agent/telemetry#raft-replication-capacity-issues), are much simpler to solve without the performance concerns caused by storing more logs in BoltDB.
### WAL approaches storage issues differently
When directly measured, WAL is more performant than BoltDB because it solves a simpler storage problem. Despite this, some users may not notice a significant performance improvement from the upgrade with the same configuration and workload. In this case, the benefit of WAL is that retaining more logs does not affect write performance. As a result, strategies for reducing disk IO with slower snapshots or for keeping logs to permit slower followers to catch up with cluster state are all possible, increasing the reliability of the deployment.
## WAL quality assurance
The WAL backend has been tested thoroughly during development:
- Every component in the WAL, such as [metadata management](https://github.com/hashicorp/raft-wal/blob/main/types/meta.go), [log file encoding](https://github.com/hashicorp/raft-wal/blob/main/types/segment.go) to actual [file-system interaction](https://github.com/hashicorp/raft-wal/blob/main/types/vfs.go) are abstracted so unit tests can simulate difficult-to-reproduce disk failures.
- We used the [application-level intelligent crash explorer (ALICE)](https://github.com/hashicorp/raft-wal/blob/main/alice/README.md) to exhaustively simulate thousands of possible crash failure scenarios. WAL correctly recovered from all scenarios.
- We ran hundreds of tests in a performance testing cluster with checksum verification enabled and did not detect data loss or corruption. We will continue testing before making WAL the default backend.
We are aware of how complex and critical disk-persistence is for your data.
We hope that many users at different scales will try WAL in their environments after upgrading to 1.15 or later and report success or failure so that we can confidently replace BoltDB as the default for new clusters in a future release.

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---
layout: docs
page_title: Monitor Raft metrics and logs for WAL
description: >-
Learn how to monitor Raft metrics emitted the experimental WAL (write-ahead log) LogStore backend shipped in Consul 1.15.
---
# Monitor Raft metrics and logs for WAL
This topic describes how to monitor Raft metrics and logs if you are testing the WAL backend. We strongly recommend monitoring the Consul cluster, especially the target server, for evidence that the WAL backend is not functioning correctly. Refer to [Enable the experimental WAL LogStore backend](/consul/docs/agent/wal-logstore/index) for additional information about the WAL backend.
!> **Upgrade warning:** The WAL LogStore backend is experimental.
## Monitor for checksum failures
Log store verification failures on any server, regardless of whether you are running the BoltDB or WAL backed, are unrecoverable errors. Consul may report the following errors in logs.
### Read failures: Disk Corruption
```log hideClipboard
2022-11-15T22:41:23.546Z [ERROR] agent.raft.logstore: verification checksum FAILED: storage corruption rangeStart=1234 rangeEnd=3456 leaderChecksum=0xc1... readChecksum=0x45...
```
This indicates that the server read back data that is different from what it wrote to disk. This indicates corruption in the storage backend or filesystem.
For convenience, Consul also increments a metric `consul.raft.logstore.verifier.read_checksum_failures` when this occurs.
### Write failures: In-flight Corruption
The following error indicates that the checksum on the follower did not match the leader when the follower received the logs. The error implies that the corruption happened in the network or software and not the log store:
```log hideClipboard
2022-11-15T22:41:23.546Z [ERROR] agent.raft.logstore: verification checksum FAILED: in-flight corruption rangeStart=1234 rangeEnd=3456 leaderChecksum=0xc1... followerWriteChecksum=0x45...
```
It is unlikely that this error indicates an issue with the storage backend, but you should take the same steps to resolve and report it.
The `consul.raft.logstore.verifier.write_checksum_failures` metric increments when this error occurs.
## Resolve checksum failures
If either type of corruption is detected, complete the instructions for [reverting to BoltDB](/consul/docs/agent/wal-logstore/revert-to-boltdb). If the server already uses BoltDB, the errors likely indicate a latent bug in BoltDB or a bug in the verification code. In both cases, you should follow the revert instructions.
Report all verification failures as a [GitHub
issue](https://github.com/hashicorp/consul/issues/new?assignees=&labels=&template=bug_report.md&title=WAL:%20Checksum%20Failure).
In your report, include the following:
- Details of your server cluster configuration and hardware
- Logs around the failure message
- Context for how long they have been running the configuration
- Any metrics or description of the workload you have. For example, how many raft
commits per second. Also include the performance metrics described on this page.
We recommend setting up an alert on Consul server logs containing `verification checksum FAILED` or on the `consul.raft.logstore.verifier.{read|write}_checksum_failures` metrics. The sooner you respond to a corrupt server, the lower the chance of any of the [potential risks](/consul/docs/agent/wal-logstore/enable#risks) causing problems in your cluster.
## Performance metrics
The key performance metrics to watch are:
- `consul.raft.commitTime` measures the time to commit new writes on a quorum of
servers. It should be the same or lower after deploying WAL. Even if WAL is
faster for your workload and hardware, it may not be reflected in `commitTime`
until enough followers are using WAL that the leader does not have to wait for
two slower followers in a cluster of five to catch up.
- `consul.raft.rpc.appendEntries.storeLogs` measures the time spent persisting
logs to disk on each _follower_. It should be the same or lower for
WAL-enabled followers.
- `consul.raft.replication.appendEntries.rpc` measures the time taken for each
`AppendEntries` RPC from the leader's perspective. If this is significantly
higher than `consul.raft.rpc.appendEntries` on the follower, it indicates a
known queuing issue in the Raft library and is unrelated to the backend.
Followers with WAL enabled should not be slower than the others. You can
determine which follower is associated with which metric by running the
`consul operator raft list-peers` command and matching the
`peer_id` label value to the server IDs listed.
- `consul.raft.compactLogs` measures the time take to truncate the logs after a
snapshot. WAL-enabled servers should not be slower than BoltDB servers.
- `consul.raft.leader.dispatchLog` measures the time spent persisting logs to
disk on the _leader_. It is only relevant if a WAL-enabled server becomes a
leader. It should be the same or lower than before when the leader was using
BoltDB.

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---
layout: docs
page_title: Revert to BoltDB
description: >-
Learn how to revert Consul to the BoltDB backend after enabled the WAL (write-ahead log) LogStore backend shipped in Consul 1.15.
---
# Revert storage backend to BoltDB from WAL
This topic describes how to revert your Consul storage backend from the experimental WAL LogStore backend to the default BoltDB.
The overall process for reverting to BoltDB consists of the following steps. Repeat the steps for all Consul servers that you need to revert.
1. Stop target server gracefully.
1. Remove data directory from target server.
1. Update target server's configuration.
1. Start target server.
## Stop target server gracefully
Stop the target server gracefully. For example, if you are using `systemd`,
run the following command:
```shell-session
$ systemctl stop consul
```
If your environment uses configuration management automation that might interfere with this process, such as Chef or Puppet, you must disable them until you have completely revereted the storage backend.
## Remove data directory from target server
Temporarily moving the data directory to a different location is less destructive than deleting it. We recommend moving the data directory instead of deleted it in cases where you unsuccessfully enable WAL. Do not use the old data directory (`/data-dir/raft.bak`) for recovery after restarting the server. We recommend eventually deleting the old directory.
The following example assumes the `data_dir` in the server's configuration is `/data-dir` and renames it to `/data-dir.wal.bak`.
```shell-session
$ mv /data-dir/raft /data-dir/raft.wal.bak
```
When switching backend, you must always remove _the entire raft directory_ not just the `raft.db` file or `wal` directory. This is because the log must always be consistent with the snapshots to avoid undefined behavior or data loss.
## Update target server's configuration
Modify the `backend` in the target server's configuration file:
```hcl
raft_logstore {
backend = "boltdb"
verification {
enabled = true
interval = "60s"
}
}
```
## Start target server
Start the target server. For example, if you are using `systemd`, run the following command:
```shell-session
$ systemctl start consul
```
Watch for the server to become a healthy voter again.
```shell-session
$ consul operator raft list-peers
```
### Clean up old data directories
If necessary, clean up any `raft.wal.bak` directories. Replace `/data-dir` with the value you specified in your configuration file.
```shell-session
$ rm /data-dir/raft.bak
```

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