Overlaybd

Overlaybd (overlay block device) is a novel layering block-level image format, which is design for container, secure container and applicable to virtual machine. And it is an open-source implementation of paper DADI: Block-Level Image Service for Agile and Elastic Application Deployment. USENIX ATC'20".

Scaling up Without Slowing Down: Accelerating Pod Start Time. KubeCon+CloudNativeCon Europe 2024

Overlaybd is based on PhotonLibOS, which is a high-efficiency LibOS framework.

Overlaybd has 2 core component:

• Overlaybd is a block-device based image format, provideing a merged view of a sequence of block-based layers as a virtual block device.

• Zfile is a compression file format which support seekalbe online decompression.

This repository is an implementation of overlaybd based on TCMU.

Overlaybd can be used as the storage backend of Accelerated Container Image, which is a solution of remote container image by fetching image data on-demand without downloading and unpacking the whole image before the container starts.

Benefits from the universality of block-device, overlaybd is also a widely applicable image format for most runtime, including qemu/kvm and any other runtime supporting block or scsi api.

Overlaybd is a non-core sub-project of containerd.

Setup

System Requirements

Overlaybd provides virtual block devices through TCMU, so the TCMU kernel module is required. TCMU is implemented in the Linux kernel and supported by most Linux distributions.

Check and load the target_core_user module.

modprobe target_core_user

Install From RPM/DEB

You may download our RPM/DEB packages form Release and install.

The binaries are install to /opt/overlaybd/bin/.

Run /opt/overlaybd/bin/overlaybd-tcmu and the log is stored in /var/log/overlaybd.log.

It is better to run overlaybd-tcmu as a service so that it can be restarted after unexpected crashes.

Build From Source

Requirements

To build overlaybd from source code, the following dependencies are required:

• CMake >= 3.14

• gcc/g++ >= 7

• Libaio, libcurl, libnl3, glib2 and openssl runtime and development libraries.

  • CentOS 7/Fedora: sudo yum install libaio-devel libcurl-devel openssl-devel libnl3-devel libzstd-static e2fsprogs-devel
  • CentOS 8: sudo yum install libaio-devel libcurl-devel openssl-devel libnl3-devel libzstd-devel e2fsprogs-devel
  • Debian/Ubuntu: sudo apt install libcurl4-openssl-dev libssl-dev libaio-dev libnl-3-dev libnl-genl-3-dev libgflags-dev libzstd-dev libext2fs-dev
  • Mariner: sudo yum install libaio-devel libcurl-devel openssl-devel libnl3-devel e2fsprogs-devel glibc-devel libzstd-devel binutils ca-certificates-microsoft build-essential

Build

You need git to checkout the source code:

git clone https://github.com/containerd/overlaybd.git
cd overlaybd
git submodule update --init

The whole project is managed by CMake. Binaries and resource files will be installed to /opt/overlaybd/.

mkdir build
cd build
cmake .. # -DCMAKE_BUILD_TYPE=Debug -DCMAKE_EXPORT_COMPILE_COMMANDS=true -DBUILD_TESTING=true
make -j
sudo make install

Considering some libcurl and libopenssl has API changes, if want to build a make-sured compatible version libcurl and openssl, and link to executable as static library.

Noticed that building libcurl and openssl depends on autoconf automake and libtool.

cmake -D BUILD_CURL_FROM_SOURCE=1 ..

If you want to use the original libext2fs instead of our customized libext2fs.

cmake -D ORIGIN_EXT2FS=1 ..

For more information about ORIGIN_EXT2FS go to USERSPACE_CONVERTOR.

If you want to use DSA hardware to accelerate CRC calculation.

cmake -D ENABLE_DSA=1 ..

If you want to use avx512 to accelerate CRC calculation.

cmake -D ENABLE_ISAL=1 ..

If you want to use QAT to accelerate compression/decompression.

cmake -D ENABLE_QAT=1 ..

For more information go to overlaybd/src/overlaybd/zfile/README.md.

Finally, setup a systemd service for overlaybd-tcmu backstore.

sudo systemctl enable /opt/overlaybd/overlaybd-tcmu.service
sudo systemctl start overlaybd-tcmu

Configuration

overlaybd config

Default configure file overlaybd.json is installed to /etc/overlaybd/.

{
    "logConfig": {
        "logLevel": 1,
        "logPath": "/var/log/overlaybd.log"
    },
    "cacheConfig": {
        "cacheType": "file",
        "cacheDir": "/opt/overlaybd/registry_cache",
        "cacheSizeGB": 4
    },
    "gzipCacheConfig": {
        "enable": true,
        "cacheDir": "/opt/overlaybd/gzip_cache",
        "cacheSizeGB": 4
    },
    "credentialConfig": {
        "mode": "file",
        "path": "/opt/overlaybd/cred.json"
    },
    "ioEngine": 0,
    "download": {
        "enable": true,
        "delay": 600,
        "delayExtra": 30,
        "maxMBps": 100
    },
    "p2pConfig": {
        "enable": false,
        "address": "localhost:19145/dadip2p"
    },
    "exporterConfig": {
        "enable": false,
        "uriPrefix": "/metrics",
        "port": 9863,
        "updateInterval": 60000000
    },
    "enableAudit": true,
    "auditPath": "/var/log/overlaybd-audit.log"
}

NOTE: download is the config for background downloading. After an overlaybd device is lauched, a background task will be running to fetch the whole blobs into local directories. After downloading, I/O requests are directed to local files. Unlike other options, download config is reloaded when a device launching.

credential config

Important: The corresponding credential has to be set before launching devices, if the registry is not public.

Credentials are reloaded when authentication is required. Credentials have to be updated before expiration if temporary credential is used, otherwise overlaybd keeps reloading until a valid credential is set.

Overlaybd supports serveral credential mode. Here are some example credentialConfig field.

• mode file

  the credentialConfig.path should be similar to '.docker/config.json' like this:

  #### /etc/overlaybd/config.json ####
  {
  "logLevel": 1,
  "logPath": "/var/log/overlaybd.log",
  ...
  "credentialConfig": {
    "mode": "file",
    "path": "/opt/overlaybd/cred.json"
  },
  ...
  }
  #### /opt/overlaybd/cred.json ####
  {
  "auths": {
  "hub.docker.com": {
    "username": "username",
    "password": "password"
  },
  "hub.docker.com/hello/world": {
    "auth": "dXNlcm5hbWU6cGFzc3dvcmQK"
  }
  }
  }

• mode http

  the credentialConfig.path should be a server listening address implemented by developers and can reply to credential information.

#### /etc/overlaybd/config.json ####
{
  "logLevel": 1,
  "logPath": "/var/log/overlaybd.log",
  ...
  "credentialConfig": {
      "mode": "http",
      "path": "localhost:19876/auth"
    },
  ...
}

overlaybd will send http request to the server with remote_url like this:

GET "localhost:19876/auth?remote_url=https://hub.docker.com/v2/overlaybd/ubuntu/blobs/sha256:47e63559a8487efb55b2f1ccea9cfc04110a185c49785fdf1329d1ea462ce5f0" the server response should be formatted as follows:

{
"traceId": "${trace_id}"
"success": true or false
"data": {
"auths": {
"hub.docker.com": {
"username": "username",
"password": "password"
}
}
}
}

we write a sample http server in test/simple_auth_server.cpp

Usage

Use with containerd

Please install overlaybd and refer to Accelerated Container Image. Overlaybd is well integrated with containerd and easy to use.

Standalone Usage

For other scenarios, users can use overlaybd manually. Overlaybd works as a backing store of TCMU, so users can run overlaybd image by interacting with configfs.

Config file

A config file is required to describe an overlaybd image, only local image and registry image are supported. Here is a sample json config file:

{
      "repoBlobUrl": "https://obd.cr.aliyuncs.com/v2/overlaybd/sample/blobs",
      "lowers" : [
          {
              "file" : "/opt/overlaybd/layer0"
          },
          {
              "dir": "/var/lib/containerd/root/io.containerd.snapshotter.v1.overlayfs/snapshots/1000",
              "digest": "sha256:e3b0d67cfa3a37dfed187badc7766e3db64d492c4db2dc4260997b41af1b28f3",
              "size": 43446424
          }
      ],
      "resultFile": "/home/overlaybd/1/result"
}

Start up

Here is an example to start up an overlaybd image. First, create the overlaybd tcmu device.

mkdir -p /sys/kernel/config/target/core/user_1/vol1
echo -n dev_config=overlaybd//root/config.v1.json > /sys/kernel/config/target/core/user_1/vol1/control
echo -n 1 > /sys/kernel/config/target/core/user_1/vol1/enable

Then, create a tcm loop device.

mkdir -p /sys/kernel/config/target/loopback/naa.123456789abcdef/tpgt_1/lun/lun_0
echo -n "naa.123456789abcdef" > /sys/kernel/config/target/loopback/naa.123456789abcdef/tpgt_1/nexus
ln -s /sys/kernel/config/target/core/user_1/vol1 /sys/kernel/config/target/loopback/naa.123456789abcdef/tpgt_1/lun/lun_0/vol1

Then a block device /dev/sdX is generated, overlaybd image can be used locally. Furthermore, overlaybd device can be used on remote hosts by iscsi.

Clean up

Just remove the files and directories in configfs in reverse order.

Writable layer

Overlaybd provides a log-structured writable layer and a sprase-file writable layer. Log-structured layer is append only and converts all writes into sequential writes so that the image build/convert process is usually faster. Sparse-file writable layer is more suitable for container rutime.

Use overlaybd-create to create a writable layer.

  /opt/overlaybd/bin/overlaybd-create ${data_file} ${index_file} ${virtual size}

use -s to for creating sparse-file writable layer. The upper option in overlaybd config file must be set to use a writable layer. Only one writable layer is avialable and it always workes as the top layer.Example:

{
    "repoBlobUrl": ...,
    "lowers" : [
        ...
    ],
    "upper": {
        "index": "${index_file}",
        "data": "${data_file}"
    },
    "resultFile": "/home/overlaybd/1/result"
}

If upper is set, the overlaybd device is launched as a writable device. The differences produced by data writing are stored in the index and data files ofupper.

After writing data and destroying the device, overlaybd-commit command is required to excute to commit the layer into a read-only layer and can be used asa lower layer later.

/opt/overlaybd/bin/overlaybd-commit ${data_file} ${index_file} ${commit_file}

At last, compression may be needed.

/opt/overlaybd/bin/overlaybd-zfile ${commit_file} ${zfile}

The zfile can be used as lower layer with online decompression.

Kernel module

DADI_kmod is a kernel module of overlaybd. It can make local overlaybd-format files as a loop device or device-mapper.

Contributing

Welcome to contribute! CONTRIBUTING

Licenses

Overlaybd is released under the Apache License, Version 2.0.