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Commit 1a321d99 authored by Evan Read's avatar Evan Read
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Merge branch 'smh-document-lazy-failovers' into 'master' 

Document lazy failovers in Praefect

See merge request gitlab-org/gitlab!62704
parents 9931ea23 71f0dad2
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doc/administration/gitaly/index.md
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......@@ -186,7 +186,8 @@ Gitaly Cluster supports:
- [Strong consistency](praefect.md#strong-consistency) of the secondary replicas.
- [Automatic failover](praefect.md#automatic-failover-and-primary-election-strategies) from the primary to the secondary.
- Reporting of possible data loss if replication queue is non-empty.
- Marking repositories as [read-only](praefect.md#read-only-mode) if data loss is detected to prevent data inconsistencies.
- From GitLab 13.0 to GitLab 14.0, marking repositories as [read-only](praefect.md#read-only-mode)
if data loss is detected to prevent data inconsistencies.
Follow the [Gitaly Cluster epic](https://gitlab.com/groups/gitlab-org/-/epics/1489)
for improvements including
......
doc/administration/gitaly/praefect.md
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......@@ -1239,24 +1239,30 @@ The `per_repository` election strategy solves this problem by electing a primary
repository. Combined with [configurable replication factors](#configure-replication-factor), you can
horizontally scale storage capacity and distribute write load across Gitaly nodes.
Primary elections are run when:
Primary elections are run:
- Praefect starts up.
- The cluster's consensus of a Gitaly node's health changes.
- In GitLab 14.1 and later, lazily. This means that Praefect doesn't immediately elect
a new primary node if the current one is unhealthy. A new primary is elected if it is
necessary to serve a request while the current primary is unavailable.
- In GitLab 13.12 to GitLab 14.0 when:
- Praefect starts up.
- The cluster's consensus of a Gitaly node's health changes.
A Gitaly node is considered:
A valid primary node candidate is a Gitaly node that:
- Healthy if `>=50%` Praefect nodes have successfully health checked the Gitaly node in the
previous ten seconds.
- Unhealthy otherwise.
- Is healthy. A Gitaly node is considered healthy if `>=50%` Praefect nodes have
successfully health checked the Gitaly node in the previous ten seconds.
- Has a fully up to date copy of the repository.
During an election run, Praefect elects a new primary Gitaly node for each repository that has
an unhealthy primary Gitaly node. The election is made:
If there are multiple primary node candidates, Praefect:
- Randomly from healthy secondary Gitaly nodes that are the most up to date.
- Only from Gitaly nodes assigned to the host repository.
- Picks one of them randomly.
- Prioritizes promoting a Gitaly node that is assigned to host the repository. If
there are no assigned Gitaly nodes to elect as the primary, Praefect may temporarily
elect an unassigned one. The unassigned primary is demoted in favor of an assigned
one when one becomes available.
If there are no healthy secondary nodes for a repository:
If there are no valid primary candidates for a repository:
- The unhealthy primary node is demoted and the repository is left without a primary node.
- Operations that require a primary node fail until a primary is successfully elected.
......@@ -1351,23 +1357,37 @@ Migrate to [repository-specific primary nodes](#repository-specific-primary-node
Gitaly Cluster recovers from a failing primary Gitaly node by promoting a healthy secondary as the
new primary.
To minimize data loss, Gitaly Cluster:
In GitLab 14.1 and later, Gitaly Cluster:
- Elects a healthy secondary with a fully up to date copy of the repository as the new primary.
- Repository becomes unavailable if there are no fully up to date copies of it on healthy secondaries.
To minimize data loss in GitLab 13.0 to 14.0, Gitaly Cluster:
- Switches repositories that are outdated on the new primary to [read-only mode](#read-only-mode).
- Elects the secondary with the least unreplicated writes from the primary to be the new primary.
Because there can still be some unreplicated writes, [data loss can occur](#check-for-data-loss).
- Elects the secondary with the least unreplicated writes from the primary to be the new
primary. Because there can still be some unreplicated writes,
[data loss can occur](#check-for-data-loss).
### Read-only mode
> - Introduced in GitLab 13.0 as [generally available](https://about.gitlab.com/handbook/product/gitlab-the-product/#generally-available-ga).
> - Between GitLab 13.0 and GitLab 13.2, read-only mode applied to the whole virtual storage and occurred whenever failover occurred.
> - [In GitLab 13.3 and later](https://gitlab.com/gitlab-org/gitaly/-/issues/2862), read-only mode applies on a per-repository basis and only occurs if a new primary is out of date.
new primary. If the failed primary contained unreplicated writes, [data loss can occur](#check-for-data-loss).
> - Removed in GitLab 14.1. Instead, repositories [become unavailable](#unavailable-repositories).
When Gitaly Cluster switches to a new primary, repositories enter read-only mode if they are out of
date. This can happen after failing over to an outdated secondary. Read-only mode eases data
recovery efforts by preventing writes that may conflict with the unreplicated writes on other nodes.
In GitLab 13.0 to 14.0, when Gitaly Cluster switches to a new primary, repositories enter
read-only mode if they are out of date. This can happen after failing over to an outdated
secondary. Read-only mode eases data recovery efforts by preventing writes that may conflict
with the unreplicated writes on other nodes.
To enable writes again, an administrator can:
When Gitaly Cluster switches to a new primary In GitLab 13.0 to 14.0, repositories enter
read-only mode if they are out of date. This can happen after failing over to an outdated
secondary. Read-only mode eases data recovery efforts by preventing writes that may conflict
with the unreplicated writes on other nodes.
To enable writes again in GitLab 13.0 to 14.0, an administrator can:
1. [Check](#check-for-data-loss) for data loss.
1. Attempt to [recover](#data-recovery) missing data.
......@@ -1375,21 +1395,38 @@ To enable writes again, an administrator can:
[accept data loss](#enable-writes-or-accept-data-loss) if necessary, depending on the version of
GitLab.
## Unavailable repositories
> - From GitLab 13.0 through 14.0, repositories became read-only if they were outdated on the primary but fully up to date on a healthy secondary. `dataloss` sub-command displays read-only repositories by default through these versions.
> - Since GitLab 14.1, Praefect contains more responsive failover logic which immediately fails over to one of the fully up to date secondaries rather than placing the repository in read-only mode. Since GitLab 14.1, the `dataloss` sub-command displays repositories which are unavailable due to having no fully up to date copies on healthy Gitaly nodes.
A repository is unavailable if all of its up to date replicas are unavailable. Unavailable repositories are
not accessible through Praefect to prevent serving stale data that may break automated tooling.
### Check for data loss
The Praefect `dataloss` sub-command identifies replicas that are likely to be outdated. This can help
identify potential data loss after a failover. The following parameters are
available:
The Praefect `dataloss` subcommand identifies:
- Copies of repositories in GitLab 13.0 to GitLab 14.0 that at are likely to be outdated.
This can help identify potential data loss after a failover.
- Repositories in GitLab 14.1 and later that are unavailable. This helps identify potential
data loss and repositories which are no longer accessible because all of their up-to-date
replicas copies are unavailable.
The following parameters are available:
- `-virtual-storage` that specifies which virtual storage to check. The default behavior is to
display outdated replicas of read-only repositories as they might require administrator action.
- In GitLab 13.3 and later, `-partially-replicated` that specifies whether to display a list of
[outdated replicas of writable repositories](#outdated-replicas-of-writable-repositories).
- `-virtual-storage` that specifies which virtual storage to check. Because they might require
an administrator to intervene, the default behavior is to display:
- In GitLab 13.0 to 14.0, copies of read-only repositories.
- In GitLab 14.1 and later, unavailable repositories.
- In GitLab 14.1 and later, [`-partially-unavailable`](#unavailable-replicas-of-available-repositories)
that specifies whether to include in the output repositories that are available but have
some assigned copies that are not available.
NOTE:
`dataloss` is still in beta and the output format is subject to change.
To check for repositories with outdated primaries, run:
To check for repositories with outdated primaries or for unavailable repositories, run:
```shell
sudo /opt/gitlab/embedded/bin/praefect -config /var/opt/gitlab/praefect/config.toml dataloss [-virtual-storage <virtual-storage>]
......@@ -1401,13 +1438,20 @@ Every configured virtual storage is checked if none is specified:
sudo /opt/gitlab/embedded/bin/praefect -config /var/opt/gitlab/praefect/config.toml dataloss
```
Repositories which have assigned storage nodes that contain an outdated copy of the repository are listed
in the output. This information is printed for each repository:
Repositories are listed in the output that have either:
- An outdated copy of the repository on the primary, in GitLab 13.0 to GitLab 14.0.
- No healthy and fully up-to-date copies available, in GitLab 14.1 and later.
The following information is printed for each repository:
- A repository's relative path to the storage directory identifies each repository and groups the related
information.
- The repository's current status is printed in parentheses next to the disk path. If the repository's primary
is outdated, the repository is in `read-only` mode and can't accept writes. Otherwise, the mode is `writable`.
- The repository's current status is printed in parentheses next to the disk path:
- In GitLab 13.0 to 14.0, either `(read-only)` if the repository's primary node is outdated
and can't accept writes. Otherwise, `(writable)`.
- In GitLab 14.1 and later, `(unavailable)` is printed next to the disk path if the
repository is unavailable.
- The primary field lists the repository's current primary. If the repository has no primary, the field shows
`No Primary`.
- The In-Sync Storages lists replicas which have replicated the latest successful write and all writes
......@@ -1417,44 +1461,51 @@ in the output. This information is printed for each repository:
is listed next to replica. It's important to notice that the outdated replicas may be fully up to date or contain
later changes but Praefect can't guarantee it.
Whether a replica is assigned to host the repository is listed with each replica's status. `assigned host` is printed
next to replicas which are assigned to store the repository. The text is omitted if the replica contains a copy of
the repository but is not assigned to store the repository. Such replicas aren't kept in-sync by Praefect, but may
act as replication sources to bring assigned replicas up to date.
Additional information includes:
- Whether a node is assigned to host the repository is listed with each node's status.
`assigned host` is printed next to nodes that are assigned to store the repository. The
text is omitted if the node contains a copy of the repository but is not assigned to store
the repository. Such copies aren't kept in sync by Praefect, but may act as replication
sources to bring assigned copies up to date.
- In GitLab 14.1 and later, `unhealthy` is printed next to the copies that are located
on unhealthy Gitaly nodes.
Example output:
```shell
Virtual storage: default
Outdated repositories:
@hashed/3f/db/3fdba35f04dc8c462986c992bcf875546257113072a909c162f7e470e581e278.git (read-only):
@hashed/3f/db/3fdba35f04dc8c462986c992bcf875546257113072a909c162f7e470e581e278.git (unavailable):
Primary: gitaly-1
In-Sync Storages:
gitaly-2, assigned host
gitaly-2, assigned host, unhealthy
Outdated Storages:
gitaly-1 is behind by 3 changes or less, assigned host
gitaly-3 is behind by 3 changes or less
```
A confirmation is printed out when every repository is writable. For example:
A confirmation is printed out when every repository is available. For example:
```shell
Virtual storage: default
All repositories are writable!
All repositories are available!
```
#### Outdated replicas of writable repositories
#### Unavailable replicas of available repositories
> [Introduced](https://gitlab.com/gitlab-org/gitaly/-/issues/3019) in GitLab 13.3.
NOTE:
In GitLab 14.0 and earlier, the flag is `-partially-replicated` and the output shows any repositories with assigned nodes with outdated
copies.
To also list information of repositories whose primary is up to date but one or more assigned
replicas are outdated, use the `-partially-replicated` flag.
To also list information of repositories which are available but are unavailable from some of the assigned nodes,
use the `-partially-unavailable` flag.
A repository is writable if the primary has the latest changes. Secondaries might be temporarily
outdated while they are waiting to replicate the latest changes.
A repository is available if there is a healthy, up to date replica available. Some of the assigned secondary
replicas may be temporarily unavailable for access while they are waiting to replicate the latest changes.
```shell
sudo /opt/gitlab/embedded/bin/praefect -config /var/opt/gitlab/praefect/config.toml dataloss [-virtual-storage <virtual-storage>] [-partially-replicated]
sudo /opt/gitlab/embedded/bin/praefect -config /var/opt/gitlab/praefect/config.toml dataloss [-virtual-storage <virtual-storage>] [-partially-unavailable]
```
Example output:
......@@ -1462,7 +1513,7 @@ Example output:
```shell
Virtual storage: default
Outdated repositories:
@hashed/3f/db/3fdba35f04dc8c462986c992bcf875546257113072a909c162f7e470e581e278.git (writable):
@hashed/3f/db/3fdba35f04dc8c462986c992bcf875546257113072a909c162f7e470e581e278.git:
Primary: gitaly-1
In-Sync Storages:
gitaly-1, assigned host
......@@ -1471,14 +1522,14 @@ Virtual storage: default
gitaly-3 is behind by 3 changes or less
```
With the `-partially-replicated` flag set, a confirmation is printed out if every assigned replica is fully up to
date.
With the `-partially-unavailable` flag set, a confirmation is printed out if every assigned replica is fully up to
date and healthy.
For example:
```shell
Virtual storage: default
All repositories are up to date!
All repositories are fully available on all assigned storages!
```
### Check repository checksums
......@@ -1486,30 +1537,50 @@ Virtual storage: default
To check a project's repository checksums across on all Gitaly nodes, run the
[replicas Rake task](../raketasks/praefect.md#replica-checksums) on the main GitLab node.
### Accept data loss
WARNING:
`accept-dataloss` causes permanent data loss by overwriting other versions of the repository. Data
[recovery efforts](#data-recovery) must be performed before using it.
If it is not possible to bring one of the up to date replicas back online, you may have to accept data
loss. When accepting data loss, Praefect marks the chosen replica of the repository as the latest version
and replicates it to the other assigned Gitaly nodes. This process overwrites any other version of the
repository so care must be taken.
```shell
sudo /opt/gitlab/embedded/bin/praefect -config /var/opt/gitlab/praefect/config.toml accept-dataloss
-virtual-storage <virtual-storage> -repository <relative-path> -authoritative-storage <storage-name>
```
### Enable writes or accept data loss
Praefect provides the following sub-commands to re-enable writes:
WARNING:
`accept-dataloss` causes permanent data loss by overwriting other versions of the repository.
Data [recovery efforts](#data-recovery) must be performed before using it.
- In GitLab 13.2 and earlier, `enable-writes` to re-enable virtual storage for writes after data
recovery attempts.
Praefect provides the following subcommands to re-enable writes or accept data loss:
```shell
sudo /opt/gitlab/embedded/bin/praefect -config /var/opt/gitlab/praefect/config.toml enable-writes -virtual-storage <virtual-storage>
```
- In GitLab 13.2 and earlier, `enable-writes` to re-enable virtual storage for writes after
data recovery attempts:
- [In GitLab 13.3](https://gitlab.com/gitlab-org/gitaly/-/merge_requests/2415) and later,
`accept-dataloss` to accept data loss and re-enable writes for repositories after data recovery
attempts have failed. Accepting data loss causes current version of the repository on the
authoritative storage to be considered latest. Other storages are brought up to date with the
authoritative storage by scheduling replication jobs.
```shell
sudo /opt/gitlab/embedded/bin/praefect -config /var/opt/gitlab/praefect/config.toml enable-writes -virtual-storage <virtual-storage>
```
- In GitLab 13.3 and later, if it is not possible to bring one of the up to date nodes back
online, you may have to accept data loss:
```shell
sudo /opt/gitlab/embedded/bin/praefect -config /var/opt/gitlab/praefect/config.toml accept-dataloss -virtual-storage <virtual-storage> -repository <relative-path> -authoritative-storage <storage-name>
```
WARNING:
`accept-dataloss` causes permanent data loss by overwriting other versions of the repository. Data
[recovery efforts](#data-recovery) must be performed before using it.
When accepting data loss, Praefect:
1. Marks the chosen copy of the repository as the latest version.
1. Replicates the copy to the other assigned Gitaly nodes.
This process overwrites any other copy of the repository so care must be taken.
## Data recovery
......
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