Overlay

admin



How to Make a Twitch Overlay Go to Placeit’s Twitch Overlay library and choose a template that fits your style & personality to start customizing. You can customize all your colors like for user icon, donation icon, accent color, and your main color. Type your texts in the boxes. Map of the world's electricity, telecoms, oil, and gas infrastructure, using data from OpenStreetMap. WDFLAT is an online platform for streamers, with amazing elements for Twitch and Youtube Gaming – Stream Overlay, Twitch Panels, Twitch Offline, Youtube Banner, Logo Esports, and more others. All templates are Free! Free designs and overlays for live streamers on Twitch, Youtube, Facebook Live, and more. Browse the best looking webcam frames, HUD's, and scene cards.


HOW TO

HowTo Home

Menus

Icon BarMenu IconAccordionTabsVertical TabsTab HeadersFull Page TabsHover TabsTop NavigationResponsive TopnavNavbar with IconsSearch MenuSearch BarFixed SidebarSide NavigationResponsive SidebarFullscreen NavigationOff-Canvas MenuHover Sidenav ButtonsSidebar with IconsHorizontal Scroll MenuVertical MenuBottom NavigationResponsive Bottom NavBottom Border Nav LinksRight Aligned Menu LinksCentered Menu LinkEqual Width Menu LinksFixed MenuSlide Down Bar on ScrollHide Navbar on ScrollShrink Navbar on ScrollSticky NavbarNavbar on ImageHover DropdownsClick DropdownsCascading DropdownDropdown in TopnavDropdown in SidenavResp Navbar DropdownSubnavigation MenuDropupMega MenuMobile MenuCurtain MenuCollapsed SidebarCollapsed SidepanelPaginationBreadcrumbsButton GroupVertical Button GroupSticky Social BarPill NavigationResponsive Header

Images

SlideshowSlideshow GalleryModal ImagesLightboxResponsive Image GridImage GridTab GalleryImage Overlay FadeImage Overlay SlideImage Overlay ZoomImage Overlay TitleImage Overlay IconImage EffectsBlack and White ImageImage TextImage Text BlocksTransparent Image TextFull Page ImageForm on ImageHero ImageBlur Background ImageChange Bg on ScrollSide-by-Side ImagesRounded ImagesAvatar ImagesResponsive ImagesCenter ImagesThumbnailsBorder Around ImageMeet the TeamSticky ImageFlip an ImageShake an ImagePortfolio GalleryPortfolio with FilteringImage ZoomImage Magnifier GlassImage Comparison Slider

Buttons

Alert ButtonsOutline ButtonsSplit ButtonsAnimated ButtonsFading ButtonsButton on ImageSocial Media ButtonsRead More Read LessLoading ButtonsDownload ButtonsPill ButtonsNotification ButtonIcon ButtonsNext/prev ButtonsMore Button in NavBlock ButtonsText ButtonsRound ButtonsScroll To Top Button

Forms

Login FormSignup FormCheckout FormContact FormSocial Login FormRegister FormForm with IconsNewsletterStacked FormResponsive FormPopup FormInline FormClear Input FieldHide Number ArrowsCopy Text to ClipboardAnimated SearchSearch ButtonFullscreen SearchInput Field in NavbarLogin Form in NavbarCustom Checkbox/RadioCustom SelectToggle SwitchCheck CheckboxDetect Caps LockTrigger Button on EnterPassword ValidationToggle Password VisibilityMultiple Step FormAutocompleteTurn off autocompleteTurn off spellcheckFile Upload ButtonEmpty Input Validation

Filters

Filter ListFilter TableFilter ElementsFilter DropdownSort ListSort Table

Tables

Zebra Striped TableCenter TablesFull-width TableSide-by-side TablesResponsive TablesComparison Table

More

Fullscreen VideoModal BoxesDelete ModalTimelineScroll IndicatorProgress BarsSkill BarRange SlidersTooltipsDisplay Element HoverPopupsCollapsibleCalendarHTML IncludesTo Do ListLoadersStar RatingUser RatingOverlay EffectContact ChipsCardsFlip CardProfile CardProduct CardAlertsCalloutNotesLabelsCirclesStyle HRCouponList GroupList Without BulletsResponsive TextCutout TextGlowing TextFixed FooterSticky ElementEqual HeightClearfixResponsive FloatsSnackbarFullscreen WindowScroll DrawingSmooth ScrollGradient Bg ScrollSticky HeaderShrink Header on ScrollPricing TableParallaxAspect RatioResponsive IframesToggle Like/DislikeToggle Hide/ShowToggle Dark ModeToggle TextToggle ClassAdd ClassRemove ClassActive ClassTree ViewRemove PropertyOffline DetectionFind Hidden ElementRedirect WebpageZoom HoverFlip BoxCenter VerticallyCenter Button in DIVTransition on HoverArrowsShapesDownload LinkFull Height ElementBrowser WindowCustom ScrollbarHide ScrollbarShow/Force ScrollbarDevice LookContenteditable BorderPlaceholder ColorText Selection ColorBullet ColorVertical LineDividersAnimate IconsCountdown TimerTypewriterComing Soon PageChat MessagesPopup Chat WindowSplit ScreenTestimonialsSection CounterQuotes SlideshowClosable List ItemsTypical Device BreakpointsDraggable HTML ElementJS Media QueriesSyntax HighlighterJS AnimationsJS String LengthJS ExponentiationJS Default ParametersGet Current URLGet Current Screen SizeGet Iframe Elements

Website

Make a WebsiteMake a Website (W3.CSS)Make a Website (BS3)Make a Website (BS4)Make a WebBookCenter WebsiteContact SectionAbout PageBig HeaderExample Website

Grid

2 Column Layout3 Column Layout4 Column LayoutExpanding GridList Grid ViewMixed Column LayoutColumn CardsZig Zag LayoutBlog Layout

Overlay Maker

Google

Google ChartsGoogle FontsGoogle Font Pairings

Converters

Convert WeightConvert TemperatureConvert LengthConvert Speed

Estimated reading time: 18 minutes

OverlayFS is a modern union filesystem that is similar to AUFS, but fasterand with a simpler implementation. Docker provides two storage drivers forOverlayFS: the original overlay, and the newer and more stable overlay2.

This topic refers to the Linux kernel driver as OverlayFS and to the Dockerstorage driver as overlay or overlay2.

Note: If you use OverlayFS, use the overlay2 driver rather than theoverlay driver, because it is more efficient in terms of inode utilization.To use the new driver, you need version 4.0 or higher of the Linux kernel,or RHEL or CentOS using version 3.10.0-514 and above.

For more information about differences between overlay vs overlay2, checkDocker storage drivers.

Note: For fuse-overlayfs driver, check Rootless mode documentation.

Prerequisites

OverlayFS is the recommended storage driver, and supported if you meet the followingprerequisites:

  • Version 4.0 or higher of the Linux kernel, or RHEL or CentOS usingversion 3.10.0-514 of the kernel or higher. If you use an older kernel, you needto use the overlay driver, which is not recommended.

  • The overlay and overlay2 drivers are supported on xfs backing filesystems,but only with d_type=true enabled.

    Use xfs_info to verify that the ftype option is set to 1. To format an xfs filesystem correctly, use the flag -n ftype=1.

    Warning: Running on XFS without d_type support now causes Docker toskip the attempt to use the overlay or overlay2 driver. Existinginstalls will continue to run, but produce an error. This is to allowusers to migrate their data. In a future version, this will be a fatalerror, which will prevent Docker from starting.

  • Changing the storage driver makes existing containers and images inaccessibleon the local system. Use docker save to save any images you have built orpush them to Docker Hub or a private registry before changing the storage driver,so that you do not need to re-create them later.

Configure Docker with the overlay or overlay2 storage driver

It is highly recommended that you use the overlay2 driver if possible, ratherthan the overlay driver. The overlay driver is not supported forDocker EE.

To configure Docker to use the overlay storage driver your Docker host must berunning version 3.18 of the Linux kernel (preferably newer) with the overlaykernel module loaded. For the overlay2 driver, the version of your kernel mustbe 4.0 or newer.

Before following this procedure, you must first meet all theprerequisites.

The steps below outline how to configure the overlay2 storage driver. If youneed to use the legacy overlay driver, specify it instead.

  1. Stop Docker.

  2. Copy the contents of /var/lib/docker to a temporary location.

  3. If you want to use a separate backing filesystem from the one used by/var/lib/, format the filesystem and mount it into /var/lib/docker.Make sure add this mount to /etc/fstab to make it permanent.

  4. Edit /etc/docker/daemon.json. If it does not yet exist, create it. Assumingthat the file was empty, add the following contents.

    Docker does not start if the daemon.json file contains badly-formed JSON.

  5. Start Docker.

  6. Verify that the daemon is using the overlay2 storage driver.Use the docker info command and look for Storage Driver andBacking filesystem.

Docker is now using the overlay2 storage driver and has automaticallycreated the overlay mount with the required lowerdir, upperdir, merged,and workdir constructs.

Continue reading for details about how OverlayFS works within your Dockercontainers, as well as performance advice and information about limitations ofits compatibility with different backing filesystems.

How the overlay2 driver works

If you are still using the overlay driver rather than overlay2, seeHow the overlay driver works instead.

OverlayFS layers two directories on a single Linux host and presents them asa single directory. These directories are called layers and the unificationprocess is referred to as a union mount. OverlayFS refers to the lower directoryas lowerdir and the upper directory a upperdir. The unified view is exposedthrough its own directory called merged.

The overlay2 driver natively supports up to 128 lower OverlayFS layers. Thiscapability provides better performance for layer-related Docker commands suchas docker build and docker commit, and consumes fewer inodes on the backingfilesystem.

Image and container layers on-disk

After downloading a five-layer image using docker pull ubuntu, you can seesix directories under /var/lib/docker/overlay2.

Warning: Do not directly manipulate any files or directories within/var/lib/docker/. These files and directories are managed by Docker.

The new l (lowercase L) directory contains shortened layer identifiers assymbolic links. These identifiers are used to avoid hitting the page sizelimitation on arguments to the mount command.

The lowest layer contains a file called link, which contains the name of theshortened identifier, and a directory called diff which contains thelayer’s contents.

The second-lowest layer, and each higher layer, contain a file called lower,which denotes its parent, and a directory called diff which contains itscontents. It also contains a merged directory, which contains the unifiedcontents of its parent layer and itself, and a work directory which is usedinternally by OverlayFS.

To view the mounts which exist when you use the overlay storage driver withDocker, use the mount command. The output below is truncated for readability.

The rw on the second line shows that the overlay mount is read-write.

How the overlay driver works

This content applies to the overlay driver only. Docker recommends using theoverlay2 driver, which works differently. SeeHow the overlay2 driver worksfor overlay2.

OverlayFS layers two directories on a single Linux host and presents them asa single directory. These directories are called layers and the unificationprocess is referred to as a union mount. OverlayFS refers to the lower directoryas lowerdir and the upper directory a upperdir. The unified view is exposedthrough its own directory called merged.

The diagram below shows how a Docker image and a Docker container are layered.The image layer is the lowerdir and the container layer is the upperdir.The unified view is exposed through a directory called merged which iseffectively the containers mount point. The diagram shows how Docker constructsmap to OverlayFS constructs.

Where the image layer and the container layer contain the same files, thecontainer layer “wins” and obscures the existence of the same files in the imagelayer.

The overlay driver only works with two layers. This means that multi-layeredimages cannot be implemented as multiple OverlayFS layers. Instead, each imagelayer is implemented as its own directory under /var/lib/docker/overlay. Hardlinks are then used as a space-efficient way to reference data shared with lowerlayers. The use of hardlinks causes an excessive use of inodes, which is a knownlimitation of the legacy overlay storage driver, and may require additionalconfiguration of the backing filesystem. Refer to the overlayFS and Dockerperformance for details.

To create a container, the overlay driver combines the directory representingthe image’s top layer plus a new directory for the container. The image’s toplayer is the lowerdir in the overlay and is read-only. The new directory forthe container is the upperdir and is writable.

Image and container layers on-disk

The following docker pull command shows a Docker host downloading a Dockerimage comprising five layers.

The image layers

Each image layer has its own directory within /var/lib/docker/overlay/, whichcontains its contents, as shown below. The image layer IDs do not correspond tothe directory IDs.

Warning: Do not directly manipulate any files or directories within/var/lib/docker/. These files and directories are managed by Docker.

The image layer directories contain the files unique to that layer as well ashard links to the data that is shared with lower layers. This allows forefficient use of disk space.

The container layer

Containers also exist on-disk in the Docker host’s filesystem under/var/lib/docker/overlay/. If you list a running container’s subdirectoryusing the ls -l command, three directories and one file exist:

Overlay facebook

The lower-id file contains the ID of the top layer of the image the containeris based on, which is the OverlayFS lowerdir.

The upper directory contains the contents of the container’s read-write layer,which corresponds to the OverlayFS upperdir.

The merged directory is the union mount of the lowerdir and upperdir, whichcomprises the view of the filesystem from within the running container.

The work directory is internal to OverlayFS.

To view the mounts which exist when you use the overlay storage driver withDocker, use the mount command. The output below is truncated for readability.

The rw on the second line shows that the overlay mount is read-write.

How container reads and writes work with overlay or overlay2

Reading files

Consider three scenarios where a container opens a file for read access withoverlay.

  • The file does not exist in the container layer: If a container opens afile for read access and the file does not already exist in the container(upperdir) it is read from the image (lowerdir). This incurs very littleperformance overhead.

  • The file only exists in the container layer: If a container opens a filefor read access and the file exists in the container (upperdir) and not inthe image (lowerdir), it is read directly from the container.

  • The file exists in both the container layer and the image layer: If acontainer opens a file for read access and the file exists in the image layerand the container layer, the file’s version in the container layer is read.Files in the container layer (upperdir) obscure files with the same name inthe image layer (lowerdir).

Modifying files or directories

Consider some scenarios where files in a container are modified.

  • Writing to a file for the first time: The first time a container writesto an existing file, that file does not exist in the container (upperdir).The overlay/overlay2 driver performs a copy_up operation to copy the filefrom the image (lowerdir) to the container (upperdir). The container thenwrites the changes to the new copy of the file in the container layer.

    However, OverlayFS works at the file level rather than the block level. Thismeans that all OverlayFS copy_up operations copy the entire file, even if thefile is very large and only a small part of it is being modified. This canhave a noticeable impact on container write performance. However, two thingsare worth noting:

    • The copy_up operation only occurs the first time a given file is writtento. Subsequent writes to the same file operate against the copy of the filealready copied up to the container.

    • OverlayFS only works with two layers. This means that performance shouldbe better than AUFS, which can suffer noticeable latencies when searchingfor files in images with many layers. This advantage applies to bothoverlay and overlay2 drivers. overlayfs2 is slightlyless performant than overlayfs on initial read, because it must lookthrough more layers, but it caches the results so this is only a smallpenalty.

  • Deleting files and directories:

    • When a file is deleted within a container, a whiteout file is created inthe container (upperdir). The version of the file in the image layer(lowerdir) is not deleted (because the lowerdir is read-only). However,the whiteout file prevents it from being available to the container.

    • When a directory is deleted within a container, an opaque directory iscreated within the container (upperdir). This works in the same way as awhiteout file and effectively prevents the directory from being accessed,even though it still exists in the image (lowerdir).

  • Renaming directories: Calling rename(2) for a directory is allowed onlywhen both the source and the destination path are on the top layer.Otherwise, it returns EXDEV error (“cross-device link not permitted”).Your application needs to be designed to handle EXDEV and fall back to a“copy and unlink” strategy.

Overlay Cabinet Hinges

Overlay network

OverlayFS and Docker Performance

Both overlay2 and overlay drivers are more performant than aufs anddevicemapper. In certain circumstances, overlay2 may perform better thanbtrfs as well. However, be aware of the following details.

  • Page Caching. OverlayFS supports page cache sharing. Multiple containersaccessing the same file share a single page cache entry for that file. Thismakes the overlay and overlay2 drivers efficient with memory and a goodoption for high-density use cases such as PaaS.

  • copy_up. As with AUFS, OverlayFS performs copy-up operationswhenever a container writes to a file for the first time. This can add latencyinto the write operation, especially for large files. However, once the filehas been copied up, all subsequent writes to that file occur in the upperlayer, without the need for further copy-up operations.

    The OverlayFS copy_up operation is faster than the same operation with AUFS,because AUFS supports more layers than OverlayFS and it is possible to incurfar larger latencies if searching through many AUFS layers. overlay2 supportsmultiple layers as well, but mitigates any performance hit with caching.

  • Inode limits. Use of the legacy overlay storage driver can cause excessiveinode consumption. This is especially true in the presence of a large numberof images and containers on the Docker host. The only way to increase thenumber of inodes available to a filesystem is to reformat it. To avoid runninginto this issue, it is highly recommended that you use overlay2 if at allpossible.

Performance best practices

The following generic performance best practices also apply to OverlayFS.

  • Use fast storage: Solid-state drives (SSDs) provide faster reads andwrites than spinning disks.

  • Use volumes for write-heavy workloads: Volumes provide the best and mostpredictable performance for write-heavy workloads. This is because they bypassthe storage driver and do not incur any of the potential overheads introducedby thin provisioning and copy-on-write. Volumes have other benefits, such asallowing you to share data among containers and persisting your data even ifno running container is using them.

Limitations on OverlayFS compatibility

To summarize the OverlayFS’s aspect which is incompatible with otherfilesystems:

  • open(2): OverlayFS only implements a subset of the POSIX standards.This can result in certain OverlayFS operations breaking POSIX standards. Onesuch operation is the copy-up operation. Suppose that your application callsfd1=open('foo', O_RDONLY) and then fd2=open('foo', O_RDWR). In this case,your application expects fd1 and fd2 to refer to the same file. However, dueto a copy-up operation that occurs after the second calling to open(2), thedescriptors refer to different files. The fd1 continues to reference the filein the image (lowerdir) and the fd2 references the file in the container(upperdir). A workaround for this is to touch the files which causes thecopy-up operation to happen. All subsequent open(2) operations regardless ofread-only or read-write access mode reference the file in thecontainer (upperdir).

    yum is known to be affected unless the yum-plugin-ovl package is installed.If the yum-plugin-ovl package is not available in your distribution such asRHEL/CentOS prior to 6.8 or 7.2, you may need to run touch /var/lib/rpm/*before running yum install. This package implements the touch workaroundreferenced above for yum.

  • rename(2): OverlayFS does not fully support the rename(2) system call.Your application needs to detect its failure and fall back to a “copy andunlink” strategy.

container, storage, driver, OverlayFS, overlay2, overlay