realaravinth/debian-mirror-gitlab
1
0
Fork 0
Code Issues Pull requests Projects Releases Packages Wiki Activity
debian-mirror-gitlab/doc/administration/gitaly/index.md
Pirate Praveen edb4c77020 New upstream version 14.2.5+ds1
2021-10-27 15:23:28 +05:30

581 lines
26 KiB
Markdown
Raw Blame History

---
stage: Create
group: Gitaly
info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments
type: reference
---
# Gitaly and Gitaly Cluster **(FREE SELF)**
[Gitaly](https://gitlab.com/gitlab-org/gitaly) provides high-level RPC access to Git repositories.
It is used by GitLab to read and write Git data.
Gitaly implements a client-server architecture:
- A Gitaly server is any node that runs Gitaly itself.
- A Gitaly client is any node that runs a process that makes requests of the Gitaly server. These
include, but are not limited to:
- [GitLab Rails application](https://gitlab.com/gitlab-org/gitlab).
- [GitLab Shell](https://gitlab.com/gitlab-org/gitlab-shell).
- [GitLab Workhorse](https://gitlab.com/gitlab-org/gitlab-workhorse).
Gitaly manages only Git repository access for GitLab. Other types of GitLab data aren't accessed
using Gitaly.
GitLab accesses [repositories](../../user/project/repository/index.md) through the configured
[repository storages](../repository_storage_paths.md). Each new repository is stored on one of the
repository storages based on their
[configured weights](../repository_storage_paths.md#configure-where-new-repositories-are-stored). Each
repository storage is either:
- A Gitaly storage with direct access to repositories using [storage paths](../repository_storage_paths.md),
where each repository is stored on a single Gitaly node. All requests are routed to this node.
- A [virtual storage](#virtual-storage) provided by [Gitaly Cluster](#gitaly-cluster), where each
repository can be stored on multiple Gitaly nodes for fault tolerance. In a Gitaly Cluster:
- Read requests are distributed between multiple Gitaly nodes, which can improve performance.
- Write requests are broadcast to repository replicas.
WARNING:
Engineering support for NFS for Git repositories is deprecated. Read the
[deprecation notice](#nfs-deprecation-notice).
## Gitaly
The following shows GitLab set up to use direct access to Gitaly:
![Shard example](img/shard_example_v13_3.png)
In this example:
- Each repository is stored on one of three Gitaly storages: `storage-1`, `storage-2`, or
`storage-3`.
- Each storage is serviced by a Gitaly node.
- The three Gitaly nodes store data on their file systems.
### Gitaly architecture
The following illustrates the Gitaly client-server architecture:
```mermaid
flowchart TD
subgraph Gitaly clients
A[GitLab Rails]
B[GitLab Workhorse]
C[GitLab Shell]
D[...]
end
subgraph Gitaly
E[Git integration]
end
F[Local filesystem]
A -- gRPC --> Gitaly
B -- gRPC--> Gitaly
C -- gRPC --> Gitaly
D -- gRPC --> Gitaly
E --> F
```
### Configure Gitaly
Gitaly comes pre-configured with Omnibus GitLab, which is a configuration
[suitable for up to 1000 users](../reference_architectures/1k_users.md). For:
- Omnibus GitLab installations for up to 2000 users, see [specific Gitaly configuration instructions](../reference_architectures/2k_users.md#configure-gitaly).
- Source installations or custom Gitaly installations, see [Configure Gitaly](configure_gitaly.md).
GitLab installations for more than 2000 users should use Gitaly Cluster.
NOTE:
If not set in GitLab, feature flags are read as false from the console and Gitaly uses their
default value. The default value depends on the GitLab version.
## Gitaly Cluster
Git storage is provided through the Gitaly service in GitLab, and is essential to the operation of
GitLab. When the number of users, repositories, and activity grows, it is important to scale Gitaly
appropriately by:
- Increasing the available CPU and memory resources available to Git before
resource exhaustion degrades Git, Gitaly, and GitLab application performance.
- Increasing available storage before storage limits are reached causing write
operations to fail.
- Removing single points of failure to improve fault tolerance. Git should be
considered mission critical if a service degradation would prevent you from
deploying changes to production.
Gitaly can be run in a clustered configuration to:
- Scale the Gitaly service.
- Increase fault tolerance.
In this configuration, every Git repository can be stored on multiple Gitaly nodes in the cluster.
Using a Gitaly Cluster increases fault tolerance by:
- Replicating write operations to warm standby Gitaly nodes.
- Detecting Gitaly node failures.
- Automatically routing Git requests to an available Gitaly node.
NOTE:
Technical support for Gitaly clusters is limited to GitLab Premium and Ultimate
customers.
The following shows GitLab set up to access `storage-1`, a virtual storage provided by Gitaly
Cluster:
![Cluster example](img/cluster_example_v13_3.png)
In this example:
- Repositories are stored on a virtual storage called `storage-1`.
- Three Gitaly nodes provide `storage-1` access: `gitaly-1`, `gitaly-2`, and `gitaly-3`.
- The three Gitaly nodes share data in three separate hashed storage locations.
- The [replication factor](#replication-factor) is `3`. There are three copies maintained
of each repository.
The availability objectives for Gitaly clusters are:
- **Recovery Point Objective (RPO):** Less than 1 minute.
Writes are replicated asynchronously. Any writes that have not been replicated
to the newly promoted primary are lost.
[Strong consistency](#strong-consistency) can be used to avoid loss in some
circumstances.
- **Recovery Time Objective (RTO):** Less than 10 seconds.
Outages are detected by a health check run by each Praefect node every
second. Failover requires ten consecutive failed health checks on each
Praefect node.
Faster outage detection, to improve this speed to less than 1 second,
is tracked [in this issue](https://gitlab.com/gitlab-org/gitaly/-/issues/2608).
### Virtual storage
Virtual storage makes it viable to have a single repository storage in GitLab to simplify repository
management.
Virtual storage with Gitaly Cluster can usually replace direct Gitaly storage configurations.
However, this is at the expense of additional storage space needed to store each repository on multiple
Gitaly nodes. The benefit of using Gitaly Cluster virtual storage over direct Gitaly storage is:
- Improved fault tolerance, because each Gitaly node has a copy of every repository.
- Improved resource utilization, reducing the need for over-provisioning for shard-specific peak
loads, because read loads are distributed across Gitaly nodes.
- Manual rebalancing for performance is not required, because read loads are distributed across
Gitaly nodes.
- Simpler management, because all Gitaly nodes are identical.
The number of repository replicas can be configured using a
[replication factor](#replication-factor).
It can
be uneconomical to have the same replication factor for all repositories.
To provide greater flexibility for extremely large GitLab instances,
variable replication factor is tracked in [this issue](https://gitlab.com/groups/gitlab-org/-/epics/3372).
As with normal Gitaly storages, virtual storages can be sharded.
### Moving beyond NFS
WARNING:
Engineering support for NFS for Git repositories is deprecated. Technical support is planned to be
unavailable from GitLab 15.0. No further enhancements are planned for this feature.
[Network File System (NFS)](https://en.wikipedia.org/wiki/Network_File_System)
is not well suited to Git workloads which are CPU and IOPS sensitive.
Specifically:
- Git is sensitive to file system latency. Even simple operations require many
read operations. Operations that are fast on block storage can become an order of
magnitude slower. This significantly impacts GitLab application performance.
- NFS performance optimizations that prevent the performance gap between
block storage and NFS being even wider are vulnerable to race conditions. We have observed
[data inconsistencies](https://gitlab.com/gitlab-org/gitaly/-/issues/2589)
in production environments caused by simultaneous writes to different NFS
clients. Data corruption is not an acceptable risk.
Gitaly Cluster is purpose built to provide reliable, high performance, fault
tolerant Git storage.
Further reading:
- Blog post: [The road to Gitaly v1.0 (aka, why GitLab doesn't require NFS for storing Git data anymore)](https://about.gitlab.com/blog/2018/09/12/the-road-to-gitaly-1-0/)
- Blog post: [How we spent two weeks hunting an NFS bug in the Linux kernel](https://about.gitlab.com/blog/2018/11/14/how-we-spent-two-weeks-hunting-an-nfs-bug/)
### Components
Gitaly Cluster consists of multiple components:
- [Load balancer](praefect.md#load-balancer) for distributing requests and providing fault-tolerant access to
Praefect nodes.
- [Praefect](praefect.md#praefect) nodes for managing the cluster and routing requests to Gitaly nodes.
- [PostgreSQL database](praefect.md#postgresql) for persisting cluster metadata and [PgBouncer](praefect.md#use-pgbouncer),
recommended for pooling Praefect's database connections.
- Gitaly nodes to provide repository storage and Git access.
### Architecture
Praefect is a router and transaction manager for Gitaly, and a required
component for running a Gitaly Cluster.
![Architecture diagram](img/praefect_architecture_v12_10.png)
For more information, see [Gitaly High Availability (HA) Design](https://gitlab.com/gitlab-org/gitaly/-/blob/master/doc/design_ha.md).
### Features
Gitaly Cluster provides the following features:
- [Distributed reads](#distributed-reads) among Gitaly nodes.
- [Strong consistency](#strong-consistency) of the secondary replicas.
- [Replication factor](#replication-factor) of repositories for increased redundancy.
- [Automatic failover](praefect.md#automatic-failover-and-primary-election-strategies) from the
primary Gitaly node to secondary Gitaly nodes.
- Reporting of possible [data loss](praefect.md#check-for-data-loss) if replication queue is
non-empty.
Follow the [Gitaly Cluster epic](https://gitlab.com/groups/gitlab-org/-/epics/1489) for improvements
including [horizontally distributing reads](https://gitlab.com/groups/gitlab-org/-/epics/2013).
#### Distributed reads
> - Introduced in GitLab 13.1 in [beta](https://about.gitlab.com/handbook/product/gitlab-the-product/#alpha-beta-ga) with feature flag `gitaly_distributed_reads` set to disabled.
> - [Made generally available and enabled by default](https://gitlab.com/gitlab-org/gitaly/-/issues/2951) in GitLab 13.3.
> - [Disabled by default](https://gitlab.com/gitlab-org/gitaly/-/issues/3178) in GitLab 13.5.
> - [Enabled by default](https://gitlab.com/gitlab-org/gitaly/-/issues/3334) in GitLab 13.8.
> - [Feature flag removed](https://gitlab.com/gitlab-org/gitaly/-/issues/3383) in GitLab 13.11.
Gitaly Cluster supports distribution of read operations across Gitaly nodes that are configured for
the [virtual storage](#virtual-storage).
All RPCs marked with the `ACCESSOR` option are redirected to an up to date and healthy Gitaly node.
For example, [`GetBlob`](https://gitlab.com/gitlab-org/gitaly/-/blob/v12.10.6/proto/blob.proto#L16).
_Up to date_ in this context means that:
- There is no replication operations scheduled for this Gitaly node.
- The last replication operation is in _completed_ state.
The primary node is chosen to serve the request if:
- There are no up to date nodes.
- Any other error occurs during node selection.
You can [monitor distribution of reads](#monitor-gitaly-cluster) using Prometheus.
#### Strong consistency
> - Introduced in GitLab 13.1 in [alpha](https://about.gitlab.com/handbook/product/gitlab-the-product/#alpha-beta-ga), disabled by default.
> - Entered [beta](https://about.gitlab.com/handbook/product/gitlab-the-product/#alpha-beta-ga) in GitLab 13.2, disabled by default.
> - In GitLab 13.3, disabled unless primary-wins voting strategy is disabled.
> - From GitLab 13.4, enabled by default.
> - From GitLab 13.5, you must use Git v2.28.0 or higher on Gitaly nodes to enable strong consistency.
> - From GitLab 13.6, primary-wins voting strategy and `gitaly_reference_transactions_primary_wins` feature flag were removed from the source code.
By default, Gitaly Cluster guarantees eventual consistency by replicating all writes to secondary
Gitaly nodes after the write to the primary Gitaly node has happened.
Praefect can instead provide strong consistency by creating a transaction and writing changes to all
Gitaly nodes at once.
If enabled, transactions are only available for a subset of RPCs. For more information, see the
[strong consistency epic](https://gitlab.com/groups/gitlab-org/-/epics/1189).
For configuration information, see [Configure strong consistency](praefect.md#configure-strong-consistency).
#### Replication factor
Replication factor is the number of copies Gitaly Cluster maintains of a given repository. A higher
replication factor:
- Offers better redundancy and distribution of read workload.
- Results in higher storage cost.
By default, Gitaly Cluster replicates repositories to every storage in a
[virtual storage](#virtual-storage).
For configuration information, see [Configure replication factor](praefect.md#configure-replication-factor).
### Configure Gitaly Cluster
For more information on configuring Gitaly Cluster, see [Configure Gitaly Cluster](praefect.md).
### Migrate to Gitaly Cluster
Whether migrating to Gitaly Cluster because of [NFS support deprecation](index.md#nfs-deprecation-notice)
or to move from single Gitaly nodes, the basic process involves:
1. Create the required storage. Refer to
[repository storage recommendations](faq.md#what-are-some-repository-storage-recommendations).
1. Create and configure [Gitaly Cluster](praefect.md).
1. [Move the repositories](../operations/moving_repositories.md#move-repositories). To migrate to
Gitaly Cluster, existing repositories stored outside Gitaly Cluster must be moved. There is no
automatic migration but the moves can be scheduled with the GitLab API.
## Monitor Gitaly and Gitaly Cluster
You can use the available logs and [Prometheus metrics](../monitoring/prometheus/index.md) to
monitor Gitaly and Gitaly Cluster (Praefect).
Metric definitions are available:
- Directly from Prometheus `/metrics` endpoint configured for Gitaly.
- Using [Grafana Explore](https://grafana.com/docs/grafana/latest/explore/) on a
Grafana instance configured against Prometheus.
### Monitor Gitaly
You can observe the behavior of [queued requests](configure_gitaly.md#limit-rpc-concurrency) using
the Gitaly logs and Prometheus:
- In the [Gitaly logs](../logs.md#gitaly-logs), look for the string (or structured log field)
`acquire_ms`. Messages that have this field are reporting about the concurrency limiter.
- In Prometheus, look for the following metrics:
- `gitaly_rate_limiting_in_progress`.
- `gitaly_rate_limiting_queued`.
- `gitaly_rate_limiting_seconds`.
Although the name of the Prometheus metric contains `rate_limiting`, it's a concurrency limiter,
not a rate limiter. If a Gitaly client makes 1,000 requests in a row very quickly, concurrency
doesn't exceed 1, and the concurrency limiter has no effect.
The following [pack-objects cache](configure_gitaly.md#pack-objects-cache) metrics are available:
- `gitaly_pack_objects_cache_enabled`, a gauge set to `1` when the cache is enabled. Available
labels: `dir` and `max_age`.
- `gitaly_pack_objects_cache_lookups_total`, a counter for cache lookups. Available label: `result`.
- `gitaly_pack_objects_generated_bytes_total`, a counter for the number of bytes written into the
cache.
- `gitaly_pack_objects_served_bytes_total`, a counter for the number of bytes read from the cache.
- `gitaly_streamcache_filestore_disk_usage_bytes`, a gauge for the total size of cache files.
Available label: `dir`.
- `gitaly_streamcache_index_entries`, a gauge for the number of entries in the cache. Available
label: `dir`.
Some of these metrics start with `gitaly_streamcache` because they are generated by the
`streamcache` internal library package in Gitaly.
Example:
```plaintext
gitaly_pack_objects_cache_enabled{dir="/var/opt/gitlab/git-data/repositories/+gitaly/PackObjectsCache",max_age="300"} 1
gitaly_pack_objects_cache_lookups_total{result="hit"} 2
gitaly_pack_objects_cache_lookups_total{result="miss"} 1
gitaly_pack_objects_generated_bytes_total 2.618649e+07
gitaly_pack_objects_served_bytes_total 7.855947e+07
gitaly_streamcache_filestore_disk_usage_bytes{dir="/var/opt/gitlab/git-data/repositories/+gitaly/PackObjectsCache"} 2.6200152e+07
gitaly_streamcache_filestore_removed_total{dir="/var/opt/gitlab/git-data/repositories/+gitaly/PackObjectsCache"} 1
gitaly_streamcache_index_entries{dir="/var/opt/gitlab/git-data/repositories/+gitaly/PackObjectsCache"} 1
```
#### Useful queries
The following are useful queries for monitoring Gitaly:
- Use the following Prometheus query to observe the
[type of connections](configure_gitaly.md#enable-tls-support) Gitaly is serving a production
environment:
```prometheus
sum(rate(gitaly_connections_total[5m])) by (type)
```
- Use the following Prometheus query to monitor the
[authentication behavior](configure_gitaly.md#observe-type-of-gitaly-connections) of your GitLab
installation:
```prometheus
sum(rate(gitaly_authentications_total[5m])) by (enforced, status)
```
In a system where authentication is configured correctly and where you have live traffic, you
see something like this:
```prometheus
{enforced="true",status="ok"} 4424.985419441742
```
There may also be other numbers with rate 0, but you only have to take note of the non-zero numbers.
The only non-zero number should have `enforced="true",status="ok"`. If you have other non-zero
numbers, something is wrong in your configuration.
The `status="ok"` number reflects your current request rate. In the example above, Gitaly is
handling about 4000 requests per second.
- Use the following Prometheus query to observe the [Git protocol versions](../git_protocol.md)
being used in a production environment:
```prometheus
sum(rate(gitaly_git_protocol_requests_total[1m])) by (grpc_method,git_protocol,grpc_service)
```
### Monitor Gitaly Cluster
To monitor Gitaly Cluster (Praefect), you can use these Prometheus metrics:
- `gitaly_praefect_read_distribution`, a counter to track [distribution of reads](#distributed-reads).
It has two labels:
- `virtual_storage`.
- `storage`.
They reflect configuration defined for this instance of Praefect.
- `gitaly_praefect_replication_latency_bucket`, a histogram measuring the amount of time it takes
for replication to complete once the replication job starts. Available in GitLab 12.10 and later.
- `gitaly_praefect_replication_delay_bucket`, a histogram measuring how much time passes between
when the replication job is created and when it starts. Available in GitLab 12.10 and later.
- `gitaly_praefect_node_latency_bucket`, a histogram measuring the latency in Gitaly returning
health check information to Praefect. This indicates Praefect connection saturation. Available in
GitLab 12.10 and later.
To monitor [strong consistency](#strong-consistency), you can use the following Prometheus metrics:
- `gitaly_praefect_transactions_total`, the number of transactions created and voted on.
- `gitaly_praefect_subtransactions_per_transaction_total`, the number of times nodes cast a vote for
a single transaction. This can happen multiple times if multiple references are getting updated in
a single transaction.
- `gitaly_praefect_voters_per_transaction_total`: the number of Gitaly nodes taking part in a
transaction.
- `gitaly_praefect_transactions_delay_seconds`, the server-side delay introduced by waiting for the
transaction to be committed.
- `gitaly_hook_transaction_voting_delay_seconds`, the client-side delay introduced by waiting for
the transaction to be committed.
## Do not bypass Gitaly
GitLab doesn't advise directly accessing Gitaly repositories stored on disk with a Git client,
because Gitaly is being continuously improved and changed. These improvements may invalidate
your assumptions, resulting in performance degradation, instability, and even data loss. For example:
- Gitaly has optimizations such as the [`info/refs` advertisement cache](https://gitlab.com/gitlab-org/gitaly/blob/master/doc/design_diskcache.md),
that rely on Gitaly controlling and monitoring access to repositories by using the official gRPC
interface.
- [Gitaly Cluster](#gitaly-cluster) has optimizations, such as fault tolerance and
[distributed reads](#distributed-reads), that depend on the gRPC interface and database
to determine repository state.
WARNING:
Accessing Git repositories directly is done at your own risk and is not supported.
## Direct access to Git in GitLab
Direct access to Git uses code in GitLab known as the "Rugged patches".
Before Gitaly existed, what are now Gitaly clients accessed Git repositories directly, either:
- On a local disk in the case of a single-machine Omnibus GitLab installation.
- Using NFS in the case of a horizontally-scaled GitLab installation.
In addition to running plain `git` commands, GitLab used a Ruby library called
[Rugged](https://github.com/libgit2/rugged). Rugged is a wrapper around
[libgit2](https://libgit2.org/), a stand-alone implementation of Git in the form of a C library.
Over time it became clear that Rugged, particularly in combination with
[Unicorn](https://yhbt.net/unicorn/), is extremely efficient. Because `libgit2` is a library and
not an external process, there was very little overhead between:
- GitLab application code that tried to look up data in Git repositories.
- The Git implementation itself.
Because the combination of Rugged and Unicorn was so efficient, the GitLab application code ended up
with lots of duplicate Git object lookups. For example, looking up the default branch commit a dozen
times in one request. We could write inefficient code without poor performance.
When we migrated these Git lookups to Gitaly calls, we suddenly had a much higher fixed cost per Git
lookup. Even when Gitaly is able to re-use an already-running `git` process (for example, to look up
a commit), you still have:
- The cost of a network roundtrip to Gitaly.
- Inside Gitaly, a write/read roundtrip on the Unix pipes that connect Gitaly to the `git` process.
Using GitLab.com to measure, we reduced the number of Gitaly calls per request until the loss of
Rugged's efficiency was no longer felt. It also helped that we run Gitaly itself directly on the Git
file servers, rather than by using NFS mounts. This gave us a speed boost that counteracted the
negative effect of not using Rugged anymore.
Unfortunately, other deployments of GitLab could not remove NFS like we did on GitLab.com, and they
got the worst of both worlds:
- The slowness of NFS.
- The increased inherent overhead of Gitaly.
The code removed from GitLab during the Gitaly migration project affected these deployments. As a
performance workaround for these NFS-based deployments, we re-introduced some of the old Rugged
code. This re-introduced code is informally referred to as the "Rugged patches".
### How it works
The Ruby methods that perform direct Git access are behind
[feature flags](../../development/gitaly.md#legacy-rugged-code), disabled by default. It wasn't
convenient to set feature flags to get the best performance, so we added an automatic mechanism that
enables direct Git access.
When GitLab calls a function that has a "Rugged patch", it performs two checks:
- Is the feature flag for this patch set in the database? If so, the feature flag setting controls
the GitLab use of "Rugged patch" code.
- If the feature flag is not set, GitLab tries accessing the file system underneath the
Gitaly server directly. If it can, it uses the "Rugged patch":
- If using Puma and [thread count](../../install/requirements.md#puma-threads) is set
to `1`.
The result of these checks is cached.
To see if GitLab can access the repository file system directly, we use the following heuristic:
- Gitaly ensures that the file system has a metadata file in its root with a UUID in it.
- Gitaly reports this UUID to GitLab by using the `ServerInfo` RPC.
- GitLab Rails tries to read the metadata file directly. If it exists, and if the UUID's match,
assume we have direct access.
Direct Git access is enable by default in Omnibus GitLab because it fills in the correct repository
paths in the GitLab configuration file `config/gitlab.yml`. This satisfies the UUID check.
WARNING:
If directly copying repository data from a GitLab server to Gitaly, ensure that the metadata file,
default path `/var/opt/gitlab/git-data/repositories/.gitaly-metadata`, is not included in the transfer.
Copying this file causes GitLab to use the Rugged patches for repositories hosted on the Gitaly server,
leading to `Error creating pipeline` and `Commit not found` errors, or stale data.
### Transition to Gitaly Cluster
For the sake of removing complexity, we must remove direct Git access in GitLab. However, we can't
remove it as long some GitLab installations require Git repositories on NFS.
There are two facets to our efforts to remove direct Git access in GitLab:
- Reduce the number of inefficient Gitaly queries made by GitLab.
- Persuade administrators of fault-tolerant or horizontally-scaled GitLab instances to migrate off
NFS.
The second facet presents the only real solution. For this, we developed
[Gitaly Cluster](#gitaly-cluster).
## NFS deprecation notice
Engineering support for NFS for Git repositories is deprecated. Technical support is planned to be
unavailable from GitLab 15.0. No further enhancements are planned for this feature.
Additional information:
- [Recommended NFS mount options and known issues with Gitaly and NFS](../nfs.md#upgrade-to-gitaly-cluster-or-disable-caching-if-experiencing-data-loss).
- [GitLab statement of support](https://about.gitlab.com/support/statement-of-support.html#gitaly-and-nfs).
GitLab recommends:
- Creating a [Gitaly Cluster](#gitaly-cluster) as soon as possible.
- [Moving your repositories](#migrate-to-gitaly-cluster) from NFS-based storage to Gitaly
Cluster.
We welcome your feedback on this process. You can:
- Raise a support ticket.
- [Comment on the epic](https://gitlab.com/groups/gitlab-org/-/epics/4916).
Reference in a new issue View git blame Copy permalink