Using NFS with GitLab

DETAILS: Tier: Free, Premium, Ultimate Offering: Self-managed

NFS can be used as an alternative for object storage but this isn't typically recommended for performance reasons.

For data objects such as LFS, Uploads, and Artifacts, an Object Storage service is recommended over NFS where possible, due to better performance. When eliminating the usage of NFS, there are additional steps you need to take in addition to moving to Object Storage.

NFS cannot be used for repository storage.

For steps you can use to test file system performance, see File System Performance Benchmarking.

Fast lookup of authorized SSH keys

The fast SSH key lookup feature can improve performance of GitLab instances even if they're using block storage.

Fast SSH key lookup is a replacement for authorized_keys (in /var/opt/gitlab/.ssh) using the GitLab database.

NFS increases latency, so fast lookup is recommended if /var/opt/gitlab is moved to NFS.

We are investigating the use of fast lookup as the default.

NFS server

Installing the nfs-kernel-server package allows you to share directories with the clients running the GitLab application:

sudo apt-get update
sudo apt-get install nfs-kernel-server

Required features

File locking: GitLab requires advisory file locking, which is only supported natively in NFS version 4. NFSv3 also supports locking as long as Linux Kernel 2.6.5+ is used. We recommend using version 4 and do not specifically test NFSv3.

Recommended options

When you define your NFS exports, we recommend you also add the following options:

• no_root_squash - NFS usually changes the root user to nobody. This is a good security measure when NFS shares are accessed by many different users. However, in this case only GitLab uses the NFS share so it is safe. GitLab recommends the no_root_squash setting because we need to manage file permissions automatically. Without the setting, you may receive errors when the Linux package tries to alter permissions. GitLab and other bundled components do not run as root but as non-privileged users. The recommendation for no_root_squash is to allow the Linux package to set ownership and permissions on files, as needed. In some cases where the no_root_squash option is not available, the root flag can achieve the same result.
• sync - Force synchronous behavior. Default is asynchronous and under certain circumstances it could lead to data loss if a failure occurs before data has synced.

Due to the complexities of running the Linux package with LDAP and the complexities of maintaining ID mapping without LDAP, in most cases you should enable numeric UIDs and GIDs (which is off by default in some cases) for simplified permission management between systems:

• NetApp instructions
• For non-NetApp devices, disable NFSv4 idmapping by performing opposite of enable NFSv4 idmapper

Disable NFS server delegation

We recommend that all NFS users disable the NFS server delegation feature. This is to avoid a Linux kernel bug which causes NFS clients to slow precipitously due to excessive network traffic from numerous TEST_STATEID NFS messages.

To disable NFS server delegation, do the following:

1. On the NFS server, run:

   echo 0 > /proc/sys/fs/leases-enable
   sysctl -w fs.leases-enable=0

2. Restart the NFS server process. For example, on CentOS run service nfs restart.

NOTE: The kernel bug may be fixed in more recent kernels with this commit. Red Hat Enterprise 7 shipped a kernel update on August 6, 2019 that may also have resolved this problem. You may not need to disable NFS server delegation if you know you are using a version of the Linux kernel that has been fixed. That said, GitLab still encourages instance administrators to keep NFS server delegation disabled.

NFS client

The nfs-common provides NFS functionality without installing server components which we don't need running on the application nodes.

apt-get update
apt-get install nfs-common

Mount options

Here is an example snippet to add to /etc/fstab:

10.1.0.1:/var/opt/gitlab/.ssh /var/opt/gitlab/.ssh nfs4 defaults,vers=4.1,hard,rsize=1048576,wsize=1048576,noatime,nofail,_netdev,lookupcache=positive 0 2
10.1.0.1:/var/opt/gitlab/gitlab-rails/uploads /var/opt/gitlab/gitlab-rails/uploads nfs4 defaults,vers=4.1,hard,rsize=1048576,wsize=1048576,noatime,nofail,_netdev,lookupcache=positive 0 2
10.1.0.1:/var/opt/gitlab/gitlab-rails/shared /var/opt/gitlab/gitlab-rails/shared nfs4 defaults,vers=4.1,hard,rsize=1048576,wsize=1048576,noatime,nofail,_netdev,lookupcache=positive 0 2
10.1.0.1:/var/opt/gitlab/gitlab-ci/builds /var/opt/gitlab/gitlab-ci/builds nfs4 defaults,vers=4.1,hard,rsize=1048576,wsize=1048576,noatime,nofail,_netdev,lookupcache=positive 0 2
10.1.0.1:/var/opt/gitlab/git-data /var/opt/gitlab/git-data nfs4 defaults,vers=4.1,hard,rsize=1048576,wsize=1048576,noatime,nofail,_netdev,lookupcache=positive 0 2

You can view information and options set for each of the mounted NFS file systems by running nfsstat -m and cat /etc/fstab.

Note there are several options that you should consider using:

soft mount option

It's recommended that you use hard in your mount options, unless you have a specific reason to use soft.

When GitLab.com used NFS, we used soft because there were times when we had NFS servers reboot and soft improved availability, but everyone's infrastructure is different. If your NFS is provided by on-premise storage arrays with redundant controllers, for example, you shouldn't need to worry about NFS server availability.

The NFS man page states:

"soft" timeout can cause silent data corruption in certain cases

Read the Linux man page to understand the difference, and if you do use soft, ensure that you've taken steps to mitigate the risks.

If you experience behavior that might have been caused by writes to disk on the NFS server not occurring, such as commits going missing, use the hard option, because (from the man page):

use the soft option only when client responsiveness is more important than data integrity

Other vendors make similar recommendations, including Recommended mount options for read-write directories and NetApp's knowledge base, they highlight that if the NFS client driver caches data, soft means there is no certainty if writes by GitLab are actually on disk.

Mount points set with the option hard may not perform as well, and if the NFS server goes down, hard causes processes to hang when interacting with the mount point. Use SIGKILL (kill -9) to deal with hung processes. The intr option stopped working in the 2.6 kernel.

nocto mount option

Do not use nocto. Instead, use cto, which is the default.

When using nocto, the dentry cache is always used, up to acdirmax seconds (attribute cache time) from the time it's created.

This results in stale dentry cache issues with multiple clients, where each client can see a different (cached) version of a directory.

From the Linux man page, the important parts:

If the nocto option is specified, the client uses a non-standard heuristic to determine when files on the server have changed.

Using the nocto option may improve performance for read-only mounts, but should be used only if the data on the server changes only occasionally.

We have noticed this behavior in an issue about refs not found after a push, where newly added loose refs can be seen as missing on a different client with a local dentry cache, as described in this issue.

A single NFS mount

It's recommended to nest all GitLab data directories within a mount, that allows automatic restore of backups without manually moving existing data.

mountpoint
└── gitlab-data
    ├── builds
    ├── git-data
    ├── shared
    └── uploads

To do so, configure the Linux package with the paths to each directory nested in the mount point as follows:

Mount /gitlab-nfs then use the following Linux package configuration to move each data location to a subdirectory:

git_data_dirs({"default" => { "path" => "/gitlab-nfs/gitlab-data/git-data"} })
gitlab_rails['uploads_directory'] = '/gitlab-nfs/gitlab-data/uploads'
gitlab_rails['shared_path'] = '/gitlab-nfs/gitlab-data/shared'
gitlab_ci['builds_directory'] = '/gitlab-nfs/gitlab-data/builds'

Run sudo gitlab-ctl reconfigure to start using the central location. Be aware that if you had existing data, you need to manually copy or rsync it to these new locations, and then restart GitLab.

Bind mounts

Instead of changing the configuration in the Linux package, bind mounts can be used to store the data on an NFS mount.

Bind mounts provide a way to specify just one NFS mount and then bind the default GitLab data locations to the NFS mount. Start by defining your single NFS mount point as you typically would in /etc/fstab. Let's assume your NFS mount point is /gitlab-nfs. Then, add the following bind mounts in /etc/fstab:

/gitlab-nfs/gitlab-data/git-data /var/opt/gitlab/git-data none bind 0 0
/gitlab-nfs/gitlab-data/.ssh /var/opt/gitlab/.ssh none bind 0 0
/gitlab-nfs/gitlab-data/uploads /var/opt/gitlab/gitlab-rails/uploads none bind 0 0
/gitlab-nfs/gitlab-data/shared /var/opt/gitlab/gitlab-rails/shared none bind 0 0
/gitlab-nfs/gitlab-data/builds /var/opt/gitlab/gitlab-ci/builds none bind 0 0

Using bind mounts requires you to manually make sure the data directories are empty before attempting a restore. Read more about the restore prerequisites.

Multiple NFS mounts

When using default Linux package configuration, you need to share 4 data locations between all GitLab cluster nodes. No other locations should be shared. The following are the 4 locations need to be shared:

Other GitLab directories should not be shared between nodes. They contain node-specific files and GitLab code that does not need to be shared. To ship logs to a central location consider using remote syslog. The Linux package provides configuration for UDP log shipping.

Having multiple NFS mounts requires you to manually make sure the data directories are empty before attempting a restore. Read more about the restore prerequisites.

Testing NFS

When you've set up the NFS server and client, you can verify NFS is configured correctly by testing the following commands:

sudo mkdir /gitlab-nfs/test-dir
sudo chown git /gitlab-nfs/test-dir
sudo chgrp root /gitlab-nfs/test-dir
sudo chmod 0700 /gitlab-nfs/test-dir
sudo chgrp gitlab-www /gitlab-nfs/test-dir
sudo chmod 0751 /gitlab-nfs/test-dir
sudo chgrp git /gitlab-nfs/test-dir
sudo chmod 2770 /gitlab-nfs/test-dir
sudo chmod 2755 /gitlab-nfs/test-dir
sudo -u git mkdir /gitlab-nfs/test-dir/test2
sudo -u git chmod 2755 /gitlab-nfs/test-dir/test2
sudo ls -lah /gitlab-nfs/test-dir/test2
sudo -u git rm -r /gitlab-nfs/test-dir

Any Operation not permitted errors means you should investigate your NFS server export options.

NFS in a Firewalled Environment

If the traffic between your NFS server and NFS clients is subject to port filtering by a firewall, then you need to reconfigure that firewall to allow NFS communication.

This guide from The Linux Documentation Project (TDLP) covers the basics of using NFS in a firewalled environment. Additionally, we encourage you to search for and review the specific documentation for your operating system or distribution and your firewall software.

Example for Ubuntu:

Check that NFS traffic from the client is allowed by the firewall on the host by running the command: sudo ufw status. If it's being blocked, then you can allow traffic from a specific client with the command below.

sudo ufw allow from <client_ip_address> to any port nfs

Known issues

Avoid using cloud-based file systems

GitLab strongly recommends against using cloud-based file systems such as:

• AWS Elastic File System (EFS).
• Google Cloud Filestore.
• Azure Files.

Our support team cannot assist with performance issues related to cloud-based file system access.

Customers and users have reported that these file systems don't perform well for the file system access GitLab requires. Workloads where many small files are written in a serialized manner, like git, are not well suited to cloud-based file systems.

If you do choose to use these, avoid storing GitLab log files (for example, those in /var/log/gitlab) there because this also affects performance. We recommend that the log files be stored on a local volume.

For more details on the experience of using a cloud-based file systems with GitLab, see this Commit Brooklyn 2019 video.

Avoid using CephFS and GlusterFS

GitLab strongly recommends against using CephFS and GlusterFS. These distributed file systems are not well-suited for the GitLab input/output access patterns because Git uses many small files and access times and file locking times to propagate makes Git activity very slow.

Avoid using PostgreSQL with NFS

GitLab strongly recommends against running your PostgreSQL database across NFS. The GitLab support team is not able to assist on performance issues related to this configuration.

Additionally, this configuration is specifically warned against in the PostgreSQL Documentation:

PostgreSQL does nothing special for NFS file systems, meaning it assumes NFS behaves exactly like locally-connected drives. If the client or server NFS implementation does not provide standard file system semantics, this can cause reliability problems. Specifically, delayed (asynchronous) writes to the NFS server can cause data corruption problems.

For supported database architecture, see our documentation about configuring a database for replication and failover.

Troubleshooting

Finding the requests that are being made to NFS

In case of NFS-related problems, it can be helpful to trace the file system requests that are being made by using perf:

sudo perf trace -e 'nfs4:*' -p $(pgrep -fd ',' puma)

On Ubuntu 16.04, use:

sudo perf trace --no-syscalls --event 'nfs4:*' -p $(pgrep -fd ',' puma)