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OverlayFS as a Docker Storage Driver
- 1. OverlayFS as a Docker Storage Driver Tomoya Akase
- 2. Index • OverlayFS • Docker Storage Driver • OverlayFS, as a Docker Storage Driver • Conclusion
- 3. OverlayFS
- 4. What is OverlayFS? • OverlayFS is implementation of Union File System • OverlayFS stacks multiple Directory/Files to construct one directory tree, and it can save volumes • OverlayFS is composed of three layers Reference: https://www.datalight.com/blog/2016/01/27/explaining-overlayfs-%E2%80%93-what-it-does-and-how-it-works/
- 5. Three Layers of OverlayFS • Lower is R/O layer • Upper is R/W layer • Overlay merge Lower and Upper, and you can handle data without being aware of layers Lower Upper Overlay
- 6. OverlayFS, as a FileSystem • File Read: Read from top (if no it exist, read from bottom) • File Write(if exist Upper): normal write • File Write(if exist only Lower): Copy file into Upper Lower Upper Overlay Read Read Lower Upper Overlay Write Copy Read Write Write
- 7. OverlayFS, as a FileSystem • File Remove: if it exist on Lower, overlay will create whiteout file • If you just delete file exist on Upper, you can read the file from Lower Lower Upper Overlay Remove Remove Lower Upper Overlay Can not ReadRemove whiteout file Create Can not Read
- 10. Background of Docker Storage Driver • Docker consists of Image and Container • Image is a bundle of R/O layers • Container is a bundle of Image and thin R/W layers on Image Dockerfile Image R/O layer FROM ubuntu:latest COPY ./sample.sh / RUN apt-get update.. RUN curl https://go.. Container R/O layer FROM ubuntu:latest COPY ./sample.sh / RUN apt-get update.. RUN curl https://go.. R/W layer Build Run
- 11. What is Docker Storage Driver? • Docker Storage Driver stacks these layers to make them appear single Reference: https://docs.docker.com/v17.09/engine/userguide/storagedriver/imagesandcontainers/
- 12. Why Docker Storage Driver is used? • R/O Files should share with others (same as Union FS philosophy) Reference: https://docs.docker.com/v17.09/engine/userguide/storagedriver/imagesandcontainers/
- 13. OverlayFS, as a Docker Storage Driver
- 14. OverlayFS, as a Docker Storage Driver • Docker Storage Driver itself is like Union FS • It has many implementation (overlay2, aufs, devicemapper, btrfs, and etc...), overlay is one of them • Docker official site recommend overlay2 driver • overlay2 can use multiple lower (overlay can only use one lower) Reference: https://docs.docker.com/storage/storagedriver/select-storage-driver/
- 15. Conclusion
- 16. Conclusion • OverlayFS is an File System consisting of R/O layer(Lower), R/W layer(Upper), and an Overlay layer merge Lower and Upper, which can save volumes • Docker uses Image as R/O layer(Lower) and Container as stack(Overlay) of Image and R/W layer(Upper), which can save volumes
- 17. References • shttps://www.slideshare.net/akachochin/overlayfsa-brief-report-of-overlayfs- source-code-reading • https://docs.docker.com/storage/storagedriver/overlayfs-driver/ • https://blog.tiqwab.com/2017/02/18/docker-technology.html • https://qiita.com/fireowl11/items/d1cf03aff691f9c0eff9 • https://qiita.com/awakia/items/af9b46c322905cdce1d7 • http://gihyo.jp/admin/serial/01/linux_containers/0018?page=3 • https://docs.docker.com/storage/storagedriver/ • http://docs.docker.jp/engine/userguide/storagedriver/imagesandcontainers.html