Gitaly and Gitaly Cluster

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:

Gitaly manages only Git repository access for GitLab. Other types of GitLab data aren’t accessed using Gitaly.

GitLab accesses repositories through the configured repository storages. Each new repository is stored on one of the repository storages based on their configured weights. Each repository storage is either:

  • A Gitaly storage with direct access to repositories using storage paths, where each repository is stored on a single Gitaly node. All requests are routed to this node.
  • A virtual storage provided by 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.
cautionEngineering support for NFS for Git repositories is deprecated. Read the deprecation notice.

Gitaly

The following shows GitLab set up to use direct access to Gitaly:

Shard example

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:

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. For:

GitLab installations for more than 2000 users should use Gitaly Cluster.

noteIf 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.
noteTechnical 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

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 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 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.

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.

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.

As with normal Gitaly storages, virtual storages can be sharded.

Moving beyond NFS

cautionEngineering 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) 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 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:

Components

Gitaly Cluster consists of multiple components:

  • Load balancer for distributing requests and providing fault-tolerant access to Praefect nodes.
  • Praefect nodes for managing the cluster and routing requests to Gitaly nodes.
  • PostgreSQL database for persisting cluster metadata and 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

For more information, see Gitaly High Availability (HA) Design.

Features

Gitaly Cluster provides the following features:

Follow the Gitaly Cluster epic for improvements including horizontally distributing reads.

Distributed reads

Version history

Gitaly Cluster supports distribution of read operations across Gitaly nodes that are configured for the virtual storage.

All RPCs marked with the ACCESSOR option are redirected to an up to date and healthy Gitaly node. For example, GetBlob.

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.

To track distribution of read operations, you can use the gitaly_praefect_read_distribution Prometheus counter metric. It has two labels:

  • virtual_storage.
  • storage.

They reflect configuration defined for this instance of Praefect.

Strong consistency

Version history
  • Introduced in GitLab 13.1 in alpha, disabled by default.
  • Entered beta 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.

For configuration information, see 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.

For configuration information, see Configure replication factor.

Configure Gitaly Cluster

For more information on configuring Gitaly Cluster, see Configure Gitaly Cluster.

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, that rely on Gitaly controlling and monitoring access to repositories by using the official gRPC interface.
  • Gitaly Cluster has optimizations, such as fault tolerance and distributed reads, that depend on the gRPC interface and database to determine repository state.
cautionAccessing 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. Rugged is a wrapper around libgit2, 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, 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, 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”:

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.

cautionIf 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.

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:

GitLab recommends:

We welcome your feedback on this process. You can: