Patent Publication Number: US-11645100-B2

Title: Global cache for container images in a clustered container host system

Description:
Applications today are deployed onto a combination of virtual machines (VMs), containers, application services, and more. For deploying such applications, a container orchestration platform known as Kubernetes® has gained in popularity among application developers. Kubernetes provides a platform for automating deployment, scaling, and operations of application containers across clusters of hosts It offers flexibility in application development and offers several useful tools for scaling. 
     In a Kubernetes system, containers are grouped into a logical unit called a “pod.” Containers in the same pod share the same resources and network, and maintain a degree of isolation from containers in other pods. The pods are distributed across nodes of the Kubernetes system and an image cache is provided on each node to speed up pod deployment. However, when an instance of the same pod is deployed across multiple nodes, and none of the image caches of the nodes have the images of containers that are in the pod, the network can become saturated during the deployment. 
     In addition, the image caches in a Kubernetes system are opaque to the user. Without a view into which images are cached on which nodes, it is not possible to know how quickly pods can be deployed on a node. Thus, the deployment time for a pod becomes non-deterministic because some nodes may have the images cached and some nodes may not. As a result, it can be difficult to make appropriate scheduling decisions. 
     Over time, duplication of cached images across nodes may also result. Because the image binaries are generally not small, the amount of disk space consumed by them can become very large, e.g., N×their size when they are cached on N nodes. Accordingly, pre-seeding of the images in the image cache of each node in a Kubernetes system, which has been employed as a solution to alleviate the network saturation and scheduling problems noted above, is far from ideal because this results in duplication of images in each cache, which would be wasteful. 
     SUMMARY 
     One or more embodiments provide a global cache for container images in a clustered container host system in which containers are executed within VMs. According to one embodiment, the clustered container host system includes a shared storage device for the container hosts, and the global cache for the container images is allocated in the shared storage device. With this configuration, a container can be deployed in a VM running in one host using a container image that has been cached as a result of deploying the same container image in a VM running in another host. 
     A method of managing container images according to one embodiment includes the steps of: in connection with deploying a container in a first VM running in a first host, creating a virtual disk in the shared storage device, storing an image of the container in the virtual disk, mounting the virtual disk to the first VM, and updating a metadata cache to associate the image of the container to the virtual disk; and in connection with deploying the container in a second VM running in a second host, checking the metadata cache to determine that the image of the container is stored in the virtual disk, and mounting the virtual disk to the second VM. 
     Further embodiments include a non-transitory computer-readable storage medium comprising instructions that cause a computer system to carry out the above methods, as well as a computer system configured to carry out the above methods. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a simplified block diagram of a clustered container host system in which embodiments may be implemented. 
         FIG.  2    is a conceptual diagram showing components of the clustered container host system that perform global caching of container images according to embodiments. 
         FIG.  3    is a flow diagram illustrating commands and data that are transmitted between the components illustrated in  FIG.  2    to enable global caching of container images according to embodiments. 
         FIG.  4    is a flow diagram illustrating steps of a garbage collection method according to embodiments. 
     
    
    
     DETAILED DESCRIPTION 
       FIG.  1    is a simplified block diagram of a clustered container host system in which embodiments may be implemented. The system includes a cluster of hosts, hosts  10 A,  10 B,  10 C, which may be constructed on a server grade hardware platform such as an x86 architecture platform. The hardware platform includes one or more central processing units (e.g., CPUs  31  shown in host  10 A), one or more network interface controllers (NICs  32  shown in host  10 A), and system memory (e.g., random access memory (RAM)  33  shown in host  10 A). A virtualization software layer, also referred to herein as a hypervisor (e.g., hypervisor  40  shown in host  10 A), is installed on top of the hardware platform. The hypervisor supports a virtual machine execution space within which multiple VMs may be concurrently instantiated and executed. As shown in  FIG.  1   , the VMs that are concurrently instantiated and executed in host  10 A includes VM  21 , VM  22 , resolver VM  71 , and fetcher VM  72 . In host  10 B, the VMs include VM  23 , VM  24 , a resolver VM that is configured in the same manner as resolver VM  71 , and a fetcher VM that is configured in the same manner as fetcher VM  72 . In host  10 C, the VMs include VM  25 , VM  26 , a resolver VM that is configured in the same manner as resolver VM  71 , and a fetcher VM that is configured in the same manner as fetcher VM  72 . The functions of the resolver VMs and the fetcher VMs will be further described below in conjunction with  FIGS.  2  and  3   . 
     In the embodiment illustrated in  FIG.  1   , a storage system for handling input-output operations (IOs) from hosts  10 A,  10 B,  10 C is provided by shared storage  100  (e.g., a network-attached storage device including magnetic disks and/or flash memory) which is on the same network  140  as hosts  10 A,  10 B,  10 C, and is accessed by hosts  10 A,  10 B,  10 C through their respective NICs. In another embodiment, hosts  10 A,  10 B,  10 C each include a host bus adapter through which IOs are sent to a shared storage (including magnetic disks and/or flash memory) in a storage area network (SAN). In both embodiments, hosts  10 A,  10 B,  10 C may also each include a local storage device (e.g., hard disk drive and/or solid-state drive) for handling IOs. In some embodiments, the local storage devices of the hosts are aggregated and provisioned as a virtual SAN device. 
     VM management server  40  is a physical or virtual server that cooperates with hypervisors installed in hosts  10 A,  10 B,  10 C to provision VMs from the hardware resources of hosts  10 A,  10 B,  10 C, and virtual disks for the VMs in the shared storage. The unit of managing the hosts by VM management server  40  is a cluster. A cluster may include any number of hosts and in the embodiment illustrated herein the number of hosts in the cluster is three. 
     A group of containers is executed in VMs in the embodiments to provide isolation from another group of containers running in a different VM. In each VM, a container engine (not shown) runs on top of the VM&#39;s guest operating system (not shown) to provide the execution environment for the containers.  FIG.  1    shows containers C 1 , C 2  running in VM  21 , containers C 1 , C 3  running in VM  22 , containers C 1 , C 3  running in VM  24 , containers C 1 , C 2  running in VM  25 , and containers C 1 , C 3  running in VM  26 . Images of containers C 1 , C 2 , C 3  are stored as virtual disks in a region of shared storage  100  that has been allocated as global cache  120 . According to embodiments, one container image is stored in one virtual disk as read-only and one virtual disk stores only one container image, so there is a one-to-one correspondence between container images and virtual disks that store the container images. Descriptive information about the container images are stored in metadata cache  110 . which is searched to determine whether or not an image of a container that is to be spun up in a VM is stored in global cache  120 . Metadata cache  110  is also stored in shared storage  100  and is accessible by the VMs of hosts  10 A,  10 B,  10 C through their respective NICs. When an image of the container needs to be loaded into memory of a VM so that the container can be executed in the VM, the container image is retrieved from the corresponding virtual disk in global cache  120  if it is present in global cache  120 . If it is not, the container image is fetched from image registry  130 , stored as a read-only virtual disk in global cache  120  (for subsequent retrievals of this container image from global cache  120  without going through image registry  130 ), and loaded into memory of the VM. In some embodiments, the contents of the container images are encrypted prior to being stored in the virtual disks and decrypted when they are loaded into the memory of the VM executing the container. The keys for decrypting the container images may be stored in metadata cache  110 . 
     In the embodiments illustrated herein, metadata cache  110  is a database  111  comprising a plurality of relational database tables. Two such tables are shown in  FIG.  1   , Table 1 and Table 2. Table 1 has a plurality of entries each corresponding to a container image, which is uniquely identified by its URI (uniform resource identifier). The URI of a container image is composed of the address of an image registry  130  with which the container image is registered, an identifier of container repository  135  in which the container image is stored, a name of the container, and a tag. In one example, different tags are attached to different versions of the container image. In Table 1, the container image&#39;s URI is associated with its chain ID, which is depicted as “chainID.” The chain ID is a unique ID that represents the contents of the container image and is generated by hashing the contents of the container image. When the container image consists of a plurality of layers, a layer ID generated for each of the layers by hashing the contents thereof, and the chain ID of the container image is generated as a composite of the layer IDs. Table 2 also has a plurality of entries each corresponding to a container image, but Table 2 is indexed by the chain ID of the container image. In Table 2, the chain ID of the container image is associated with a pointer to a virtual disk that stores the container image. It should be recognized that metadata cache  110  is not limited to a database and may take other forms, such as a file based key-value store or a file system based cache that is on some network attached storage device or an in-memory cache, so long as it is centrally accessible. 
     Container images are registered with image registry  130 , which manages a plurality of container repositories (one of which is shown in  FIG.  1    as container repository  135 ) in which images of all containers registered with image registry  130  are stored. Authentication information is associated with each container repository and access to container images in a particular container repository is governed by the authentication information associated therewith. Once a requester is authenticated to a container repository, image registry  130  permits the requester to fetch the container images from that container registry. 
       FIG.  2    is a conceptual diagram showing components of the clustered container host system that perform global caching of container images according to embodiments.  FIG.  2    conceptually illustrates creation of a new virtual disk for image of container Cn in global cache  120 . In one embodiment, the new virtual disk for the image of container Cn is created in global cache  120  in response to a request to spin up a new container within a VM, which is received by VM management server  20  from an application administrator, for example. 
     Upon receiving the request to spin up a new container, VM management server  20  sends a request for an image of the new container to the resolver VMs. The image request includes the URI of the new container image and credentials of the application administrator. The resolver VMs then carry out the function of resolving the image request, which includes: (1) authenticating the credentials with image registry  130 , (2) acquiring a chain ID of the new container image from image registry  130  and determining if the new container image corresponding to this chain ID is cached in global cache  120  or not, i.e., whether or not the chain ID is present or not present in metadata cache  110 , and (3) acquiring a size of the new container image from image registry  130 . If it is not cached, one of the resolver VMs updates metadata cache  110  to add an entry, which associates the URI of the new container image with the chain ID, in Table 1 and to add an entry for the chain ID in Table 2, and also sends a request to fetch the new container image to the fetcher VMs. The request to fetch includes the URI of the new container image, the credentials of the application administrator, the chain ID of the new container image, and the size of the new container image, the latter two of which were acquired from image registry  130 . 
     In the embodiments described herein, the authentication part of the resolving function is carried out when a new container is spun up within a VM. In other embodiments, the authentication part of the resolving function also may be carried out for a container each time that a container is run. 
     The fetcher VMs carry out the function of fetching in response to the request to fetch the new container image by calling an API of VM management server  20  to create a new virtual disk (also referred to herein as VMDK), the parameters of the API including a size corresponding to the size of the container image acquired from image registry  130 . In response to the API calls for creating a new virtual disk, one of the fetcher VMs receives a pointer to the new virtual disk, updates the entry in Table 2 of metadata cache  110  corresponding to the chain ID to add a pointer to the new virtual disk, and sends a fetch request to image registry  130 , the fetch request including the URI of the new container image and the credentials of the application administrator. In response, image registry  130  retrieves the contents of the new container image from image repository  135  and transmits the contents of the container image to the fetcher VM. Then, the fetcher VM stores the contents of the container image received from image registry  130  in the new virtual disk. 
     After creating the new virtual disk, VM management server  20  instructs the hypervisor supporting the VM in which container Cn is to be spun up, to reconfigure the VM to mount the new virtual disk. Once the VM is reconfigured in this manner, container Cn can be executed within VM  21  according to the contents of its container image stored in the new virtual disk. 
     In some embodiments, the container engine that supports execution of containers in VMs employs an overlay file system. An image of a container executed in such an environment consists of a plurality of layers and these layers need to be mounted on top of each other in the proper order by the overlay file system for execution of the container. Accordingly, when these layers are fetched from image registry  130  and stored in a virtual disk, the fetcher VM, based on information acquired from image registry  130  during the fetching, creates metadata that describes how and in what order the layers should be mounted by the overlay file system, and stores this metadata in the virtual disk for later consumption during mounting of the layers. 
     In the embodiments, the function of resolving and the function of fetching are carried out in a distributed manner. As such, all of the resolver VMs in the cluster of hosts managed by VM management server  20  carry out the function of resolving and all of the fetcher VMs in the cluster of hosts managed by VM management server  20  carry out the function of fetching. Although multiple resolver VMs are carrying out the same resolving function, the process described herein ensures that only one resolver VM completes the resolving function. In the case of a cache miss, the resolver VM that is the first to access metadata cache  110  to determine the cache miss will have a lock on Table 2 and will update Table 2 to include the chain ID in response to the cache miss. Consequently, all subsequent accesses to metadata cache  110  to determine a cache hit or miss on the chain ID will result in a cache hit and will not cause a further updating of Table 2. In the case of a cache hit, multiple resolver VMs will call an API of VM management server  20  to mount a virtual disk corresponding to the cache hit, but VM management server  20  will process only the first of these API calls and ignore the rest. Likewise, for fetching, multiple fetcher VMs will call an API of VM management server  20  to create a new virtual disk, but VM management server  20  will process only the first one of the API calls and ignore the rest. 
       FIG.  3    is a flow diagram illustrating commands and data that are transmitted between the components illustrated in  FIG.  2    in response to a request from an application administrator to spin up a new container within a VM. 
     VM management server  20  at step S 1 , sends a request for the new container image to the resolver VMs in the cluster of hosts managed by VM management server  20 . The image request includes the URI of the new container image and credentials of the application administrator. At step S 2 , each of the resolver VMs sends the URI and the credentials to image registry  130 . If image registry  130  is able to authenticate the credentials at step S 3 , image registry  130  at step S 4  sends the chain ID (which is generated by hashing the contents of the new container image) and a size of the new container image to each resolver VM. 
     Each resolver VM at step S 5  searches metadata cache  110 , in particular Table 2, to determine if the chain ID of the new container image acquired from image registry  130  is or is not present in metadata cache  110 . If it is not present, a cache miss is determined and steps S 6  to S 11  are carried out. If it is present, a cache hit is determined and steps S 12  and S 13  are carried out. 
     At step S 6 , the resolver VM (e.g., the first resolver VM that determined the absence of the chain ID in the metadata cache  110 ) updates metadata cache  110  to add an entry that associates the URI of the new container image with the chain ID to Table 1 and to add an entry for the chain ID in Table 2. At step S 7 , the resolver VM sends a request to fetch the new container image to the fetcher VMs in the cluster of hosts managed by VM management server  20 . The request to fetch includes the URI of the new container image, the credentials of the application administrator, the chain ID of the new container image, and the size of the new container image, the latter two of which were acquired from image registry  130 . 
     Each of the fetcher VMs carries out the function of fetching in response to the request to fetch the new container image. At step S 8 , the fetcher VMs each call an API of VM management server  20  for creating a new virtual disk of the requested size and thereafter mounting the new virtual disk to the VM in which the new container is to be spun up. VM management server  20  responds to only the first one of these API calls by: (1) sending back a pointer to the new virtual disk to that fetcher VM, and (2) instructing the hypervisor supporting the VM (in which the new container image is to be spun up) to reconfigure the VM to mount the new virtual disk (step S 9 ). After responding to the first one of these API calls, VM management server  20  ignores the remainder of these API calls. Upon receiving the pointer to the new virtual disk, the fetcher VM at step S 10  updates the metadata cache  110  using this information and also communicates with image registry  130  to fetch the new container image. Upon fetching the contents of the new container image, the fetcher VM at step S 11  downloads the contents of the container image into the new virtual disk. After completion of step S 11 , the new container is ready to be loaded into the memory of the VM and executed. 
     At step S 12 , which is carried out if the chain ID of the new container image acquired from image registry  130  is present in metadata cache  110 , each resolver VM determines the pointer to the virtual disk that is associated with the chain ID in Table 2 of metadata cache  110 , and calls an API of VM management server  20  for mounting the virtual disk that is located at the determined pointer to the VM in which the new container is to be spun up. VM management server  20  responds to only the first one of these API calls and ignores the rest. Upon receiving the first of these API calls, VM management server  20  at step S 13 , instructs the hypervisor supporting the VM to reconfigure the VM to mount the virtual disk that is located at the determined pointer. After completion of step S 13 , the new container is ready to be loaded into the memory of the VM and executed. 
       FIG.  4    is a flow diagram illustrating steps of a garbage collection method according to embodiments. The garbage collection method of  FIG.  4    is triggered when global cache  120  is running low on available capacity, and is initiated by a dynamic resource scheduler (DRS) of VM management server  20 . 
     The method  FIG.  4    begins at step  410 , where DRS selects a VMDK in global cache  120  for possible deletion. At step  412 , DRS determines the cost of replacing the VMDK, wherein the cost of replacing the VMDK is based on the following factors: (1) likelihood of being used again based on how frequently it has been used in the past (more frequently used images are more costly to replace), (2) its size (bigger images are more costly to replace), and (3) when it was last used (more recently used images are more costly to replace). 
     The process loops through steps  410  and  412  if it is determined at step  414  that all of the VMDKs stored in global cache  120  have not been analyzed. If they have, step  416  is executed where the VMDK that costs the least to replace is deleted, e.g., by calling an API of VM management server  20  to delete the VMDK. After step  416 , it is determined at step  418 , whether or not sufficient space has been freed up in global cache  120 . If sufficient space has not been freed up, the process returns to step  416 , where the VMDK having the next lowest cost to replace is deleted. If sufficient space has been freed up, the process ends. 
     Embodiments provide a global cache, which in comparison to per-node caching employed in conventional implementations, reduces the spin-up time for a container, provides better estimates on how long it will take to spin up a container, and eliminates redundant storing of the same container images. 
     The various embodiments described herein may employ various computer-implemented operations involving data stored in computer systems. For example, these operations may require physical manipulation of physical quantities-usually, though not necessarily, these quantities may take the form of electrical or magnetic signals, where they or representations of them are capable of being stored, transferred, combined, compared, or otherwise manipulated. Further, such manipulations are often referred to in terms such as producing, identifying, determining, or comparing. Any operations described herein that form part of one or more embodiments of the invention may be useful machine operations. In addition, one or more embodiments of the invention also relate to a device or an apparatus for performing these operations. The apparatus may be specially constructed for specific required purposes, or it may be a general-purpose computer selectively activated or configured by a computer program stored in the computer. In particular, various general-purpose machines may be used with computer programs written in accordance with the teachings herein, or it may be more convenient to construct a more specialized apparatus to perform the required operations. 
     The various embodiments described herein may be practiced with other computer system configurations including hand-held devices, microprocessor systems, microprocessor-based or programmable consumer electronics, minicomputers, mainframe computers, and the like. 
     One or more embodiments of the present invention may be implemented as one or more computer programs or as one or more computer program modules embodied in one or more computer readable media. The term computer readable medium refers to any data storage device that can store data which can thereafter be input to a computer system. Computer readable media may be based on any existing or subsequently developed technology for embodying computer programs in a manner that enables them to be read by a computer. Examples of a computer readable medium include a hard drive, NAS, read-only memory (ROM), RAM (e.g., flash memory device), Compact Disk (e.g., CD-ROM, CD-R, or CD-RW), Digital Versatile Disk (DVD), magnetic tape, and other optical and non-optical data storage devices. The computer readable medium can also be distributed over a network coupled computer system so that the computer readable code is stored and executed in a distributed fashion. 
     Although one or more embodiments of the present invention have been described in some detail for clarity of understanding, it will be apparent that certain changes and modifications may be made within the scope of the claims. Accordingly, the described embodiments are to be considered as illustrative and not restrictive, and the scope of the claims is not to be limited to details given herein but may be modified within the scope and equivalents of the claims. In the claims, elements and/or steps do not imply any particular order of operation, unless explicitly stated in the claims. 
     Virtualization systems in accordance with the various embodiments may be implemented as hosted embodiments, non-hosted embodiments or as embodiments that tend to blur distinctions between the two, are all envisioned. Furthermore, various virtualization operations may be wholly or partially implemented in hardware. For example, a hardware implementation may employ a look-up table for modification of storage access requests to secure non-disk data. 
     Many variations, modifications, additions, and improvements are possible, regardless the degree of virtualization. The virtualization software can therefore include components of a host, console, or guest operating system that performs virtualization functions. Plural instances may be provided for components, operations or structures described herein as a single instance. Finally, boundaries between various components, operations and data stores are somewhat arbitrary, and particular operations are illustrated in the context of specific illustrative configurations. Other allocations of functionality are envisioned and may fall within the scope of the invention. In general, structures and functionalities presented as separate components in exemplary configurations may be implemented as a combined structure or component. Similarly, structures and functionalities presented as a single component may be implemented as separate components. These and other variations, modifications, additions, and improvements may fall within the scope of the appended claims.