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 was first proposed by Alibaba Cloud and widely used in Alibaba cloud services. It became a sub-project of containerd in 2021.
As an image format, overlaybd has 2 core component:
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Overlaybd is a layering image format, provideing a merged view of a sequence of block-based layers as a virtual block device. SPEC
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Zfile is a compression file format which support seekalbe online decompression. SPEC
Sub-project of containerd, contains the storage service of overlaybd image format, provideing a merged view of a sequence of block-based layers as a virtual block device. Now this service contains an implementation of overlaybd based on TCMU, and will provide an implementation based on ublk in the future.
This service is based on PhotonLibOS, which is a high-efficiency LibOS framework.
Sub-project of containerd, 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. This repositry contains a containerd snapshotter and image conversion tools for overlaybd.
Use p2p protocol to speed up HTTP file download for registry in large-scale clusters.
Overlaybd is a block-device-based image format, which has much lower complexity than filesystem-based implementations. For example, cross-layer hardlink and non-copy commands like chown are very complex for filesystem-based image without copying up, but is natively supported by overlaybd.
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Overlaybd outputs virtual block devices through TCMU, which is a linux kernel module and widely supported in most operation systems. Overlaybd backstore can recover from failures or crashes, which is difficult for FUSE-based image formats.
For sync communication catch us in the #overlaybd slack channels on Cloud Native Computing Foundation's (CNCF) slack - cloud-native.slack.com. Everyone is welcome to join and chat. Get Invite to CNCF slack.