Abstract:
Apparatus for non-disruptive error recovery in a clustered system, comprises a node instance configuration activity quiescing component for quiescing configuration activity at a first virtual node instance; a node instance creation component for creating a second virtual node instance with I/O access blocked; a configuration transfer component for transferring configuration data from the first to the second virtual node instance; a commit component for committing the changeover from the first to the second virtual node instance; a node instance communications blocking component for blocking inbound communications to a first virtual node instance; a node instance communications unblocking component for unblocking I/O access at the second virtual node instance; a communication control component for monitoring outbound communications from the first virtual node instance and signalling completion of all pending outbound communications; and a node instance deletion component for deleting the first virtual node instance.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to technology for recovering from errors in high-availability clustered software, and especially in a storage network controller. 
       BACKGROUND OF THE INVENTION 
       [0002]    In high-availability clustered software, a number of nodes collaborate to deliver a service to users. High-availability clustering requires the existence of a quantity of replicated state or metadata information that the software uses in order to deliver its features. For correct operation of the product it is critical that this cluster state be internally consistent. By internally consistent we mean that for example different layers in the software have the same count of the number of objects, and the like. 
         [0003]    In high-availability clustered software, it is desirable to maintain 100% availability or as close to that target as possible. However, it is possible for software defects to exist in the code. Software error recovery procedures are available for high-availability clustered software. These error recovery procedures allow the cluster to recover if a software failure occurs. The error recovery procedures are designed to ensure that the internal state remains consistent. Unfortunately, software errors can result in inconsistencies in cluster state which can in turn provoke further software failures. These software errors are sometimes only discovered when another failure occurs and this can lead to extensive, and expensive, downtime in production environments. In order to fix these software errors when they occur, fixes are applied to patch the identified error in the cluster state. However, it is not possible to guarantee that there are no further, undiscovered bugs in the cluster state. 
         [0004]    In order to guarantee that there are no further inconsistencies as a result of a previous cluster recovery, a reinstall of the storage virtualization software and a re-initialisation of the cluster state to its initial conditions are necessary, but this is a disruptive procedure. One option for a reinstall is to run a Tier 3 recovery procedure (restoration of data from archive storage) which again is disruptive. Another option is to build a new cluster and configure it identically to the original cluster. The data would have to be transferred (by using, for example, host mirroring). In some systems this can be done without stopping I/O. The disadvantage of this solution is that it is expensive: additional hardware is required (twice as many nodes, additional storage) and it requires considerable resource use to migrate to the new cluster. The introduction of new hardware also introduces the risk of hardware faults that potentially compound the problem. 
         [0005]    It would thus be desirable to have a technological means for recovering from errors in high-availability clustered software, in a manner which is non-disruptive and which is not dependent on additional hardware and resources in form of systems and storage administrator time and effort. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention accordingly provides, in a first aspect, an apparatus for non-disruptive error recovery in a clustered system, comprising: a node instance configuration activity quiescing component for quiescing configuration activity at a first virtual node instance; a node instance creation component for creating a second virtual node instance with I/O access blocked; a node instance configuration transfer component for transferring configuration data from said first virtual node instance to said second virtual node instance; a commit component for committing the changeover from said first virtual node instance to said second virtual node instance; a node instance communications blocking component for blocking inbound communications to a first virtual node instance; a node instance communications unblocking component for unblocking I/O access at said second virtual node instance; a communication control component for monitoring outbound communications from said first virtual node instance and signalling completion of all pending outbound communications; and a node instance deletion component for deleting said first virtual node instance. 
         [0007]    Preferably, a single director component controls said quiescing component, said node instance creation component, said node instance configuration transfer component, said commit component, said node instance communications blocking component, said node instance communications unblocking component, said communication control component and said node instance deletion component. 
         [0008]    Preferably, said single director component is cooperable with one or more filter components to control one or more of said quiescing component, said node instance creation component, said node instance configuration transfer component, said commit component, said node instance communications blocking component, said node instance communications unblocking component, said communication control component and said node instance deletion component. 
         [0009]    In a preferred embodiment, a clustered system comprises an apparatus according to the first aspect. 
         [0010]    In a preferred embodiment, a storage virtualization system comprises an apparatus according to the first aspect. 
         [0011]    In a second aspect, the present invention provides a method or logic arrangement for performing non-disruptive error recovery in a clustered system, comprising the steps of: quiescing configuration activity at a first virtual node instance; creating a second virtual node instance with I/O access blocked; transferring configuration data from said first virtual node instance to said second virtual node instance; committing the changeover from said first virtual node instance to said second virtual node instance; blocking inbound communications to a first virtual node instance; unblocking I/O access at said second virtual node instance; monitoring outbound communications from said first virtual node instance and signalling completion of all pending outbound communications; and deleting said first virtual node instance. 
         [0012]    Preferably, a single directing component controls said steps of quiescing, creating, transferring, committing, blocking, unblocking, monitoring, signalling and deleting. 
         [0013]    Preferably, said single directing component is cooperable with one or more filter components to control one or more of said steps of quiescing, creating, transferring, committing, blocking, unblocking, monitoring, signalling and deleting. 
         [0014]    In a third aspect, there is provided a data carrier having functional data thereon, said functional data comprising functional computer data structures to, when loaded into a computer system and operated upon thereby, enable said computer system to perform all the steps of a method according to the second aspect. 
         [0015]    In a fourth aspect, there is provided a computer program comprising computer program code to, when loaded into a computer system and executed thereon, cause said computer system to perform all the steps of a method according to the second aspect. 
         [0016]    Preferred embodiments of the invention thus contemplate, in their broadest aspect, a technical framework for recovering from errors in high-availability clustered software. 
         [0017]    The preferred embodiment of the invention has the advantage over known systems that it does not require additional hardware (storage virtualization nodes, storage subsystems) and therefore reduces the cost and also the risk of introducing new problems through new hardware. Another advantage is that it does not require host I/O to be stopped. The preferred embodiment is also more efficient than known systems according to the art, as it does not require extra resources in the form of additional storage and system administrator time and effort. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    A preferred embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawing figures, in which: 
           [0019]      FIGS. 1 to 4  show in schematic form an apparatus, or an arrangement of apparatus, in operation, in accordance with a preferred embodiment of the present invention. 
           [0020]      FIG. 5  shows in flowchart form one method or one logic arrangement in which a method of operation according to a preferred embodiment of the present invention may be implemented. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0021]    A preferred embodiment of the present invention is implemented in a storage network having storage virtualization controllers at its nodes. 
         [0022]    In  FIG. 1  is shown an example of a storage area network controller which provides storage virtualization. The diagram shows a single instance of a participating hardware component or arrangement. It will be clear to one of ordinary skill in the art, however, that the various components will be the same for all nodes in a cluster. As is well known to those of ordinary skill in the art, clustering is the use of multiple devices and interconnections to form what is presented externally as a single system. 
         [0023]    In  FIG. 1 , there is shown an arrangement of apparatus  100  comprising a node instance  102 , attached to which is a virtual hard disk  104 . Node instance  102  is operatively connected to filter  106 , which in turn is connected to one or more virtual external devices  108 . Node instance  102  and virtual external devices  108  are operatively connected to director  110 , which is in communication with virtual machine monitor  112 . Virtual machine monitor  112  is connected in conventional fashion to hard disk drive  114  and to one or more external devices  116 . 
         [0024]    In  FIG. 1 , node instance  102  (the first virtual machine) runs the storage virtualization node software. 
         [0025]    Turning to  FIG. 2 , there is shown an apparatus comprising all the elements of  FIG. 1 , and additional elements as follows: node instance  102 ′ is connected to virtual hard disk  104 ′ and to filter  106 ′. The connection between filter  106 ′ and virtual external devices  108 ′ is disabled. Node instance  102 ′ runs a clean instance of a storage virtualization without event history and contains a clean cluster state. 
         [0026]    In operation, Virtual Machine Monitor  112  (VMM) creates and deletes virtual machines (VMs), such as the exemplary node instances  102 ,  102 ′. It virtualizes the I/O devices (e.g. ethernet, serial ports, FC adapter) and makes them accessible to all virtual machines; that is, the VMM receives I/O and distributes it correctly to the target virtual machine. It also allows the virtual machines to send I/O. In one embodiment, this could be achieved using a known virtualization technology such as Xen, but alternative virtualization technologies will be evident to one of ordinary skill in the art. 
         [0027]    Director  110  is the coordination component to control the creation of virtual machines, the switch-over from the first to the second virtual machine and eventually the deletion of the first virtual machine. It has access to both virtual machines, to their virtual I/O devices and to their virtual hard disks. 
         [0028]    Filters  106 ,  106 ′ define the set of events that a VM can receive and send. During normal  1  operation the set of events is the complete set of possible events for the control application. When the second VM is created at node instance  102 ′, its corresponding filter  106 ′ will allow that subset of the events that is required to configure the control application but not events that define its external view. 
         [0029]    There is one active instance of a storage virtualization node, that is, to begin with in the present example, node instance  102 . When the VMM receives a request to refresh the storage virtualization cluster state, the VMM initiates the creation of second instance (in the present example, node instance  102 ′). The director  110  co-ordinates the configuration of the second node instance  102 ′ using the metadata and configuration information from the first node instance  102 . The filter of the second instance  106 ′ ensures that no customer I/O passes through. Access to all virtual external devices is disabled. Where necessary the director  110  provides a false interface to these external devices (e.g. a false UPS). 
         [0030]    Once the second node instance  102 ′ is fully operational the director  110  issues a commit which causes a change in the behaviour of both filters, as shown in  FIG. 3 . The first node instance  102  does not receive external signals any more; however, it can complete outstanding I/Os. The second node instance  102 ′ is now fully operational using all virtual external devices and receiving and completing I/O. The director  110  monitors the outstanding I/Os to be completed by the first instance. 
         [0031]    Once all I/O has completed from the first node instance  102  the director  110  initiates the deletion of the first instance, as shown in  FIG. 4 . All cluster activity then takes place under the control of node instance  102 ′ and using its corresponding attached components  104 ′,  106 ′,  108 ′ in place of node instance  102  and components  104 ,  106 ,  108 . 
         [0032]    The preferred embodiment of the present invention in the form of an apparatus thus advantageously addresses the problem of providing a technical framework for recovering from errors in high-availability clustered software. 
         [0033]      FIG. 5  shows, in flowchart form, a generalized process flow according to a preferred embodiment of the present invention. 
         [0034]    At START step  200 , a single node instance is operational in a normal mode of operating. At step  202 , a cluster refresh request is received. At step  204 , a second node instance is created, but is blocked from external signals. At step  206  the second node instance is configured as described above, using configuration data (but not state data) taken from the first node instance. When the configuration activity of step  206  is complete, a commit is issued at step  208 . At step  210 , the first node instance is blocked from external signals, and at step  212 , I/O at the second node instance is unblocked. At step  214 , all I/Os that were pending at the first node instance have completed, and at step  216 , the first node instance is deleted. Operations continue at step  218 . 
         [0035]    Thus it can be seen that the replacement software in node instance  2  inherits the configuration data from node instance  1 , but does not inherit any possibly faulty state data from node instance  1 . 
         [0036]    The following is a detailed description of one implementation of a node instance changeover procedure: 
         [0000]    1. Flush the cache for all disks of node instance  102  (Flush cache on all storage virtualization nodes and change to cache-disabled mode for all vdisks/disable storage virtualization cache.)
 
2. Stop all configuration activity on node instance  102 .
 
3. Copy Tier 3 metadata and configuration data of node instance  102  to the virtual hard disk of node instance  102 ′.
 
4. Create node instance  102 ′ and use Tier 3 metadata and storage virtualization configuration backup data from the virtual hard disk to mirror configuration of instance  1 .
 
5. All I/O continues to pass through node instance  102  until node instance  102 ′ is fully operational.
 
6. Node instance  102 ′ does not yet receive any I/O. Its cache remains empty, as does the cache of node instance  102 .
 
7. Once node instance  102 ′ is identical (in terms of its configuration and extent map, but not of its event history) to node instance  102 , a commit takes place which switches to node instance  102 ′
 
8. Node instance  102 ′ is removed.
 
9. The cache on node instance  102 ′ is enabled.
 
         [0037]    It will be clear to one of ordinary skill in the art that the above description relates to a single entity in a cluster, and that the same process needs to be coordinated on all the nodes in the cluster. This is achieved through the director  110 . Once all node instances  102 ′ in the cluster are completely set up as described above, the node instances  102 ′ notify all the directors  110  which then inform the corresponding node instances  102 . In this way a controlled switch over can take place. 
         [0038]    The preferred embodiment of the present invention in the form of a method or logic arrangement thus advantageously addresses the problem of providing a technical framework for recovering from errors in high-availability clustered software. 
         [0039]    The preferred embodiment thus operates by activating virtualisation of the resources of all storage virtualization nodes using a virtualisation technology when a non-disruptive reinstall/refresh of the cluster state is requested. This allows two instances of storage virtualization software to run on the same storage virtualization node—the original version with possible inconsistencies in cluster state and a fresh version which has its cluster state reinitialised. The fresh version copies the minimum of data required from the original version in order to reproduce the original configuration with its extent map. Once the fresh versions of storage virtualization software are operational on all nodes, I/O is redirected to the fresh cluster without impacting host systems or storage subsystems. The original version is deleted once the switch is complete and virtualisation of the storage resources can be deactivated. In one example, a known virtualization technology such as Xen can be used, but it will be clear to one of ordinary skill in the art that other equivalent virtualization technologies are equally applicable. 
         [0040]    It will be clear to one of ordinary skill in the art that all or part of the method of the preferred embodiments of the present invention may suitably and usefully be embodied in a logic apparatus, or a plurality of logic apparatus, comprising logic elements arranged to perform the steps of the method and that such logic elements may comprise hardware components, firmware components or a combination thereof. 
         [0041]    It will be equally clear to one of skill in the art that all or part of a logic arrangement according to the preferred embodiments of the present invention may suitably be embodied in a logic apparatus comprising logic elements to perform the steps of the method, and that such logic elements may comprise components such as logic gates in, for example a programmable logic array or application-specific integrated circuit. Such a logic arrangement may further be embodied in enabling elements for temporarily or permanently establishing logic structures in such an array or circuit using, for example, a virtual hardware descriptor language, which may be stored and transmitted using fixed or transmittable carrier media. 
         [0042]    It will be appreciated that the method and arrangement described above may also suitably be carried out fully or partially in software running on one or more processors (not shown in the figures), and that the software may be provided in the form of one or more computer program elements carried on any suitable data-carrier (also not shown in the figures) such as a magnetic or optical disk or the like. Channels for the transmission of data may likewise comprise storage media of all descriptions as well as signal-carrying media, such as wired or wireless signal-carrying media. 
         [0043]    The present invention may further suitably be embodied as a computer program product for use with a computer system. Such an implementation may comprise a series of computer-readable instructions either fixed on a tangible medium, such as a computer readable medium, for example, diskette, CD-ROM, ROM, or hard disk, or transmittable to a computer system, via a modem or other interface device, over either a tangible medium, including but not limited to optical or analogue communications lines, or intangibly using wireless techniques, including but not limited to microwave, infrared or other transmission techniques. The series of computer readable instructions embodies all or part of the functionality previously described herein. 
         [0044]    Those skilled in the art will appreciate that such computer readable instructions can be written in a number of programming languages for use with many computer architectures or operating systems. Further, such instructions may be stored using any memory technology, present or future, including but not limited to, semiconductor, magnetic, or optical, or transmitted using any communications technology, present or future, including but not limited to optical, infrared, or microwave. It is contemplated that such a computer program product may be distributed as a removable medium with accompanying printed or electronic documentation, for example, shrink-wrapped software, pre-loaded with a computer system, for example, on a system ROM or fixed disk, or distributed from a server or electronic bulletin board over a network, for example, the Internet or World Wide Web. 
         [0045]    In an alternative, the preferred embodiment of the present invention may be realized in the form of computer implemented method of deploying a service comprising steps of deploying computer program code operable to, when deployed into a computer infrastructure and executed thereon, cause said computer system to perform all the steps of the method. 
         [0046]    It will be clear to one skilled in the art that many improvements and modifications can be made to the foregoing exemplary embodiment without departing from the scope of the present invention.