• Before deploying Gitaly Cluster
  • Known issues
  • Snapshot backup and recovery limitations
  • What to do if you are on Gitaly Cluster experiencing an issue or limitation
• Disk requirements
• Directly accessing repositories
• Gitaly
  • Gitaly architecture
  • Configure Gitaly
  • Gitaly CLI
  • Backing up repositories
  • Bundle URIs
• Gitaly Cluster
  • Comparison to Geo
  • Virtual storage
  • Storage layout
    • Client-generated replica paths
    • Praefect-generated replica paths
    • Identify repositories on disk
    • Atomicity of operations
      • Repository creations
      • Repository deletions
      • Repository moves
  • Components
  • Architecture
  • Features
    • Distributed reads
    • Strong consistency
    • Replication factor
  • Configure Gitaly Cluster
  • Upgrade Gitaly Cluster
  • Downgrade Gitaly Cluster to a previous version
  • Migrate to Gitaly Cluster
  • Migrate off Gitaly Cluster
  • Transition to Gitaly Cluster

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 is present in every GitLab installation and coordinates Git repository storage and retrieval. Gitaly can be:

• A background service operating on a single instance Linux package installation (all of GitLab on one machine).
• Separated onto its own instance and configured in a full cluster configuration, depending on scaling and availability requirements.

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. Gitaly clients are also known as Gitaly consumers and include:
  • GitLab Rails application
  • GitLab Shell
  • GitLab Workhorse
  • GitLab Elasticsearch Indexer
  • GitLab Zoekt Indexer

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.

Before deploying Gitaly Cluster

Gitaly Cluster provides the benefits of fault tolerance, but comes with additional complexity of setup and management. Before deploying Gitaly Cluster, review:

• Existing known issues.
• Snapshot limitations.
• Configuration guidance and Repository storage options to make sure that Gitaly Cluster is the best setup for you.

If you have not yet migrated to Gitaly Cluster, you have two options:

• A sharded Gitaly instance.
• Gitaly Cluster.

Contact your Customer Success Manager or customer support if you have any questions.

Known issues

The following table outlines current known issues impacting the use of Gitaly Cluster. For the current status of these issues, refer to the referenced issues and epics.

Snapshot backup and recovery limitations

Gitaly Cluster does not support snapshot backups. Snapshot backups can cause issues where the Praefect database becomes out of sync with the disk storage. Because of how Praefect rebuilds the replication metadata of Gitaly disk information during a restore, you should use the official backup and restore Rake tasks.

The incremental backup method can be used to speed up Gitaly Cluster backups.

If you are unable to use either method, contact customer support for restoration help.

What to do if you are on Gitaly Cluster experiencing an issue or limitation

Contact customer support for immediate help in restoration or recovery.

Disk requirements

Gitaly and Gitaly Cluster require fast local storage to perform effectively because they are heavy I/O-based processes. Therefore, we strongly recommend that all Gitaly nodes use solid-state drives (SSDs).

These SSDs should have a throughput of at least:

• 8,000 input/output operations per second (IOPS) for read operations.
• 2,000 IOPS for write operations.

These IOPS values are initial recommendations, and may be adjusted to greater or lesser values depending on the scale of your environment’s workload. If you’re running the environment on a cloud provider, refer to their documentation about how to configure IOPS correctly.

For repository data, only local storage is supported for Gitaly and Gitaly Cluster for performance and consistency reasons. Alternatives such as NFS or cloud-based file systems are not supported.

Directly accessing repositories

GitLab doesn’t advise directly accessing Gitaly repositories stored on disk with a Git client or any other tool, 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.

Gitaly

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

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:

Configure Gitaly

Gitaly comes pre-configured with a Linux package installation, which is a configuration suitable for up to 20 RPS / 1,000 users. For:

• Linux package installations for up to 40 RPS / 2,000 users, see specific Gitaly configuration instructions.
• Self-compiled installations or custom Gitaly installations, see Configure Gitaly.

GitLab installations for more than 2000 active users performing daily Git write operation may be best suited by using Gitaly Cluster.

Gitaly CLI

The gitaly command is a command-line interface that provides additional subcommands for Gitaly administrators. For example, the Gitaly CLI is used to:

• Configure custom Git hooks for a repository.
• Validate Gitaly configuration files.
• Verify the internal Gitaly API is accessible.

For more information on the other subcommands, run /opt/gitlab/embedded/bin/gitaly --help.

Backing up repositories

When backing up or syncing repositories using tools other than GitLab, you must prevent writes while copying repository data.

Bundle URIs

You can use Git bundle URIs with Gitaly. For more information, see the Bundle URIs documentation.

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.

The following shows GitLab set up to access storage-1, a virtual storage provided by Gitaly Cluster:

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. Three copies are maintained of each repository.

The availability objectives for Gitaly clusters assuming a single node failure 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 prevents 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.

Improvements to RPO and RTO are proposed in epic 8903.

Comparison to Geo

Gitaly Cluster and Geo both provide redundancy. However the redundancy of:

• Gitaly Cluster provides fault tolerance for data storage and is invisible to the user. Users are not aware when Gitaly Cluster is used.
• Geo provides replication and disaster recovery for an entire instance of GitLab. Users know when they are using Geo for replication. Geo replicates multiple data types, including Git data.

The following table outlines the major differences between Gitaly Cluster and Geo:

For more information, see:

• Geo use cases.
• Geo architecture.

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 standard Gitaly storages, virtual storages can be sharded.

Storage layout

Gitaly Cluster’s virtual storages provide an abstraction that looks like a single storage but actually consists of multiple physical storages. Gitaly Cluster has to replicate each operation to each physical storage. Operations may succeed on some of the physical storages but fail on others.

Partially applied operations can cause problems with other operations and leave the system in a state it can’t recover from. To avoid these types of problems, each operation should either fully apply or not apply at all. This property of operations is called atomicity.

GitLab controls the storage layout on the repository storages. GitLab instructs the repository storage where to create, delete, and move repositories. These operations create atomicity issues when they are being applied to multiple physical storages. For example:

• GitLab deletes a repository while one of its replicas is unavailable.
• GitLab later recreates the repository.

As a result, the stale replica that was unavailable at the time of deletion may cause conflicts and prevent recreation of the repository.

These atomicity issues have caused multiple problems in the past with:

• Geo syncing to a secondary site with Gitaly Cluster.
• Backup restoration.
• Repository moves between repository storages.

Gitaly Cluster provides atomicity for these operations by storing repositories on the disk in a special layout that prevents conflicts that could occur due to partially applied operations.

Client-generated replica paths

Repositories are stored in the storages at the relative path determined by the Gitaly client. These paths can be identified by them not beginning with the @cluster prefix. The relative paths follow the hashed storage schema.

Praefect-generated replica paths

When Gitaly Cluster creates a repository, it assigns the repository a unique and permanent ID called the repository ID. The repository ID is internal to Gitaly Cluster and doesn’t relate to any IDs elsewhere in GitLab. If a repository is removed from Gitaly Cluster and later moved back, the repository is assigned a new repository ID and is a different repository from Gitaly Cluster’s perspective. The sequence of repository IDs always increases, but there may be gaps in the sequence.

The repository ID is used to derive a unique storage path called replica path for each repository on the cluster. The replicas of a repository are all stored at the same replica path on the storages. The replica path is distinct from the relative path:

• The relative path is a name the Gitaly client uses to identify a repository, together with its virtual storage, that is unique to them.
• The replica path is the actual physical path in the physical storages.

Praefect translates the repositories in the RPCs from the virtual (virtual storage, relative path) identifier into physical repository (storage, replica_path) identifier when handling the client requests.

The format of the replica path for:

• Object pools is @cluster/pools/<xx>/<xx>/<repository ID>. Object pools are stored in a different directory than other repositories. They must be identifiable by Gitaly to avoid pruning them as part of housekeeping. Pruning object pools can cause data loss in the linked repositories.
• Other repositories is @cluster/repositories/<xx>/<xx>/<repository ID>

For example, @cluster/repositories/6f/96/54771.

The last component of the replica path, 54771, is the repository ID. This can be used to identify the repository on the disk.

<xx>/<xx> are the first four hex digits of the SHA256 hash of the string representation of the repository ID. These digits are used to balance the repositories evenly into subdirectories to avoid overly large directories that might cause problems on some file systems. In this case, 54771 hashes to 6f960ab01689464e768366d3315b3d3b2c28f38761a58a70110554eb04d582f7 so the first four digits are 6f and 96.

Identify repositories on disk

Use the praefect metadata subcommand to:

• Retrieve a repository’s virtual storage and relative path from the metadata store. After you have the hashed storage path, you can use the Rails console to retrieve the project path.
• Find where a repository is stored in the cluster with either:
  • The virtual storage and relative path.
  • The repository ID.

The repository on disk also contains the project path in the Git configuration file. The configuration file can be used to determine the project path even if the repository’s metadata has been deleted. Follow the instructions in hashed storage’s documentation.

Atomicity of operations

Gitaly Cluster uses the PostgreSQL metadata store with the storage layout to ensure atomicity of repository creation, deletion, and move operations. The disk operations can’t be atomically applied across multiple storages. However, PostgreSQL guarantees the atomicity of the metadata operations. Gitaly Cluster models the operations in a manner that the failing operations always leave the metadata consistent. The disks may contain stale state even after successful operations. This situation is expected and the leftover state does not interfere with future operations but may use up disk space unnecessarily until a clean up is performed.

There is on-going work on a background crawler that cleans up the leftover repositories from the storages.

Repository creations

When creating repositories, Praefect:

1. Reserves a repository ID from PostgreSQL, which is atomic and no two creations receive the same ID.
2. Creates replicas on the Gitaly storages in the replica path derived from the repository ID.
3. Creates metadata records after the repository is successfully created on disk.

Even if two concurrent operations create the same repository, they’d be stored in different directories on the storages and not conflict. The first to complete creates the metadata record and the other operation fails with an “already exists” error. The failing creation leaves leftover repositories on the storages. There is on-going work on a background crawler that clean up the leftover repositories from the storages.

The repository IDs are generated from the repositories_repository_id_seq in PostgreSQL. In the above example, the failing operation took one repository ID without successfully creating a repository with it. Failed repository creations are expected lead to gaps in the repository IDs.

Repository deletions

A repository is deleted by removing its metadata record. The repository ceases to logically exist as soon as the metadata record is deleted. PostgreSQL guarantees the atomicity of the removal and a concurrent delete fails with a “not found” error. After successfully deleting the metadata record, Praefect attempts to remove the replicas from the storages. This may fail and leave leftover state in the storages. The leftover state is eventually cleaned up.

Repository moves

Unlike Gitaly, Gitaly Cluster doesn’t move the repositories in the storages but only virtually moves the repository by updating the relative path of the repository in the metadata store.

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.

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

Features

Gitaly Cluster provides the following features:

• Distributed reads among Gitaly nodes.
• Strong consistency of the secondary replicas.
• Replication factor of repositories for increased redundancy.
• Automatic failover from the primary Gitaly node to secondary Gitaly nodes.
• Reporting of possible data loss if replication queue isn’t empty.

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

Distributed reads

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:

• No up-to-date nodes exist.
• Any other error occurs during node selection.

If you have a large, heavily modified repository (like a multi-gigabyte monorepo), the primary node can service most or all requests if changes come in faster than Praefect can replicate to the secondaries. When this occurs, CI/CD jobs and other repository traffic are bottlenecked by the capacity of the primary node.

You can monitor distribution of reads using Prometheus.

Strong consistency

Gitaly Cluster provides strong consistency by writing changes synchronously to all healthy, up-to-date replicas. If a replica is outdated or unhealthy at the time of the transaction, the write is asynchronously replicated to it.

Strong consistency is the primary replication method. A subset of operations still use replication jobs (eventual consistency) instead of strong consistency. Refer to the strong consistency epic for more information.

If strong consistency is unavailable, Gitaly Cluster guarantees eventual consistency. In this case. Gitaly Cluster replicates all writes to secondary Gitaly nodes after the write to the primary Gitaly node has occurred.

For more information on monitoring strong consistency, see the Gitaly Cluster Prometheus metrics documentation.

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.

Upgrade Gitaly Cluster

To upgrade a Gitaly Cluster, follow the documentation for zero-downtime upgrades.

Downgrade Gitaly Cluster to a previous version

If you need to roll back a Gitaly Cluster to an earlier version, some Praefect database migrations may need to be reverted.

To downgrade a Gitaly Cluster (assuming multiple Praefect nodes):

1. Stop the Praefect service on all Praefect nodes:

   gitlab-ctl stop praefect
   

2. Downgrade the GitLab package to the older version on one of the Praefect nodes.

3. On the downgraded node, check the state of Praefect migrations:

   /opt/gitlab/embedded/bin/praefect -config /var/opt/gitlab/praefect/config.toml sql-migrate-status
   

4. Count the number of migrations with unknown migration in the APPLIED column.

5. On a Praefect node that has not been downgraded, perform a dry run of the rollback to validate which migrations to revert. <CT_UNKNOWN> is the number of unknown migrations reported by the downgraded node.

   /opt/gitlab/embedded/bin/praefect -config /var/opt/gitlab/praefect/config.toml sql-migrate <CT_UNKNOWN>
   

6. If the results look correct, run the same command with the -f option to revert the migrations:

   /opt/gitlab/embedded/bin/praefect -config /var/opt/gitlab/praefect/config.toml sql-migrate -f <CT_UNKNOWN>
   

7. Downgrade the GitLab package on the remaining Praefect nodes and start the Praefect service again:

   gitlab-ctl start praefect
   

Migrate to Gitaly Cluster

Before migrating to Gitaly Cluster:

• Review Before deploying Gitaly Cluster.
• Upgrade to the latest possible version of GitLab, to take advantage of improvements and bug fixes.

To migrate to Gitaly Cluster:

1. Create the required storage. Refer to repository storage recommendations.
2. Create and configure Gitaly Cluster.
3. Configure the existing Gitaly instance to use TCP, if not already configured that way.
4. Move the 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.

Even if you don’t use the default repository storage, you must ensure it is configured. Read more about this limitation.

Migrate off Gitaly Cluster

If the limitations and tradeoffs of Gitaly Cluster are found to be not suitable for your environment, you can Migrate off Gitaly Cluster to a sharded Gitaly instance:

1. Create and configure a new Gitaly server.
2. Move the repositories to the newly created storage. You can move them by shard or by group, which gives you the opportunity to spread them over multiple Gitaly servers.

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.

Two facets of our efforts to remove direct Git access in GitLab are:

• 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.