Abstract:
A computing apparatus that is capable of taking over processing from a primary computer. The computing apparatus includes a confirmation-based file system cache flush mechanism that prevents loss of uncommitted data during a primary computer failure and a filter driver that is capable of functioning as a network file system driver in a first mode of operation and as an invisible pass through to the underlying local file system driver in a second mode of operation. A computer system that includes a primary and a secondary computer that have these features is also disclosed.

Description:
FIELD OF THE INVENTION 
     The present invention relates to computer systems and, more specifically, to the transfer (or “fail-over”) of processes from a primary computer to a secondary computer during failure of the primary computer. 
     BACKGROUND OF THE INVENTION 
     A primary computer may be any computer that is the preferred application server in a group of two or more computers. A secondary computer may be any computer that takes over or provides “fail-over” protection for a failing primary computer. The secondary computer typically receives the physical file system, network identity and executing applications of the primary during a fail-over. Prior to fail-over, the secondary computer may be both an application processor and a standby computer or simply a standby computer. 
     In some paired, clustered or networked primary and secondary computer arrangements, the physical storage media may be a shared physical volume such as arbitrated and lockable shared disks. In other arrangements, the physical storage media for the primary and secondary computers may be separate physical volumes and include a network based file system volume replication scheme where the contents of the file system stored on the primary physical storage media is also copied to the secondary physical storage media. Representative primary and secondary computer arrangements include those described in U.S. Pat. No. 5,696,895, issued to Hemphill et al for Fault Tolerant Multiple Network Servers, U.S. Pat. No. 5,675,723, issued to Ekrot et al for Multi-Server Fault Tolerance Using In-Band Signaling and U.S. Pat. No. 5,157,663, issued to Major et al for a Fault Tolerant Computer System, as well as those commercially available from NCR Corporation (product name “LifeKeeper”) and Microsoft Corporation (product name “MS Cluster Server”). A network based file system volume replication scheme is also commercially available from NCR Corporation, assignee of the present application, under the product name “Extended Mirroring”. 
     While prior art fail-over schemes have beneficial aspects they also have disadvantageous aspects. One disadvantageous aspect is that a secondary computer cannot reliable access data within the cache or like memory of a failing primary computer. During conventional processing there is almost always data in the cache waiting to be written to the physical storage media. This data which often includes uncommitted or unwritten file system transactions is lost during failure of the primary computer (often resulting in file corruption and difficult or impossible system recovery). 
     Another disadvantageous aspect of prior art fail-over schemes relates to the transfer of computer identity. If the secondary machine is part of a cluster or network or the like and data processed by an application executing on the secondary computer is stored via a network file system owned by the primary computer, then the transfer of the network file system results in the secondary computer aliasing to the primary computer and a re-aliasing back to the secondary computer. This may cause irrational behavior in the network file system on the secondary computer during and after fail-over. 
     Hence a need exists to provide a manner of achieving fail-over from a primary to a secondary computer that protects data in the cache or like memory of the primary and permits the secondary computer to readily adopt the identity of the primary computer. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a computing device that is capable of taking over processing from another computing device in such a manner that data in the file system cache or like memory of the failing computing device is not lost. 
     It is another object of the present invention to provide a computing device with a filter driver that performs network file system transfers in a non-fail-over mode and switches to local file system transfers in a fail-over mode. 
     It is also an object of the present invention to provide a fail-over computing device that receives a physical media transfer confirmation before flushing corresponding data from its file system cache. 
     These and related objects of the present invention are achieved by use of a file system filter driver apparatus and method as described herein. 
     In one embodiment, the present invention provides within a computing device a file system filter driver that exists above the operating system&#39;s file system layer. This filter driver operates as a file system cache coherent network file system in normal mode, and after fail-over acts as a passive pass-through driver to the underlying physical file system. A file system cache flush algorithm is also preferably provided in conjunction with the filter driver so that network file system data transfers are not lost during a primary computer failure. 
     The attainment of the foregoing and related advantages and features of the invention should be more readily apparent to those skilled in the art, after review of the following more detailed description of the invention taken together with the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram of clustered or networked computers having a primary and secondary computer in a fail-over pair arrangement in accordance with the present invention. 
     FIG. 2 is a diagram of operating system hierarchy and the location of a filter driver therein in accordance with the present invention. 
     FIG. 3 is a diagram of operating system hierarchy and cache memory protection for a fail-over implementing computer cluster in accordance with the present invention. 
     FIG. 4 is a flow diagram of software functions within the computer cluster of FIG. 3 in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, a diagram of clustered or networked computers having a primary and secondary computer  10 , 20  in a fail-over pair arrangement in accordance with the present invention is shown. Primary computer  10  is the preferred application server of the pair and secondary computer  20  preferably provides fail-over protection for the primary computer. The primary computer includes a processor  11  and file system cache memory  12 , amongst other conventional computer components, and the secondary computer similarly includes a processor  21  and file system cache memory  22 , amongst other conventional components. As shown, the primary and secondary computers are coupled through a router or switch  6  to a plurality of other computers  8 , one or more of which may be tiered secondary computers or the like. 
     It should be recognized that the primary and secondary computer arrangements described herein are applicable to all computer arrangements that have a primary or preferred application server computer and a secondary or fail-over computer, regardless of whether the computers are networked with other computers, in a redundant or non-redundant computer pair or otherwise clustered or combined. Furthermore, the physical storage media may be shared such as a shared disk drive  12  or implemented with a file mirroring replication scheme utilizing a primary physical storage media  15  and a secondary physical storage media  25 , or other suitable storage media. In file mirror replication schemes, data stored on storage media  15  is mirrored on storage media  25  such that secondary computer  20  can readily assume the identity of primary computer  10  and carry on the application programs that primary computer  10  was executing at the time of failure. 
     As alluded to above, during fail-over problems exist assuring the survivability of uncommitted or unwritten file system data or the like and in establishing the apparent identity of the failed primary computer in the secondary computer. To alleviate these problems an appropriate filter driver and related operating system attributes are preferably provided in the secondary computer (or other computer in a tiered fail-over environment). 
     Referring to FIG. 2, a diagram of operating system hierarchy within a computer ( 10 , 20 ) and the location of a filter driver therein in accordance with the present invention is shown. Block  50  represents application software, block  51  represents a conventional fail-over engine, block  52  represents a clustered file system filter driver, block  56  represents a file system driver, block  57  represents a confirmation detection function and block  58  represents the driver of physical storage media  15 , 25 . Interconnect block  59  represents interconnection via a network. 
     As illustrated in FIG. 2, filter driver  52  preferably exists in a layer between the application software and the file system driver and other drivers in the operation system. In this manner, the filter driver may manipulate data before it is passed to underlying layers. Modern operation systems allow for layered driver arrangements as is known. The diagram of FIG. 2 illustrates the location of the clustered file system filter driver within a typical driver stack on a Windows NT server system. 
     Filter driver  52  preferably incorporates a network file system engine  53  such as CIFS/SMB for Windows NT which allows the secondary computer to communicate with the file system software on the primary through normal operation system channels. Standard network file system technologies are preferably used to allow the underlying file system model of locking and synchronization to be used without any enhancement. 
     Referring to FIG. 3, a diagram of operating system hierarchy and cache memory protection for a fail-over implementing computer cluster in accordance with the present invention is shown. FIG. 3 illustrates a plurality of computers  10 , 20  having operating system hierarchies as illustrated in FIG.  2 . In a preferred embodiment, all computers within a cluster are loaded with software that implements the fail-over function described herein, hence the hierarchies of the various machines appear similar (like components have reference numerals with the same tens and ones unit and a different hundreds unit). After (or during) loading, however, the computers are configured to establish at least one primary computer  10  and one secondary computer  20 . Note that there may be many secondary computers, mutual primary-secondary computer arrangements, tiered secondary computers, etc. The clustered file system filter driver features described herein are preferably not enabled or configured for the primary computer, but are present so that the primary computer may be used as a secondary computer should the need arise. For clarity of description and general pedagogical purposes, the operation of one primary computer and one secondary computer are described with reference to FIG.  3 . It is to be understood, however, that other computer arrangements (for example, those alluded to above amongst others) may be implemented that utilize the fail-over provisions described herein and these other computer arrangements are within the scope of the present invention. 
     Operation of the computers of FIG. 3 is now described with a shared disk arrangement being described first followed by description of a file mirroring arrangement. With respect to applications being processed on the secondary computer, in non-fail-over mode data to be written to disk is both ( 1 ) temporarily stored in the secondary computer&#39;s file system cache memory  22  and ( 2 ) transferred via a conventional network file system engine to the primary computer for storage. At the primary computer, the data is processed by the primary computer&#39;s file system driver  156  and written by physical storage media driver  158  to the storage media ( 12  for shared and  15  for mirrored). Once the subject data has been successfully written to the storage media, confirmation logic or driver  157  propagates an appropriate write confirmation signal to confirmation logic or driver  257  within secondary computer  20 . Upon receipt of the write confirmation signal, confirmation driver  257  invokes a file system cache flush algorithm (represented by block  254 ) that flushes the subject data from the secondary computers file system cache memory  22 . 
     If the primary computer fails before the subject data is successfully written to the physical storage media owned by the primary computer, then in prior art systems this data is lost. Making use of the cache flush algorithm to retain data until writes to a physical media are confirmed eliminates the loss of this data. 
     With respect to file mirroring implementations, blocks  155 , 255  (adjacent confirmation drivers  157 , 257 ) represent this function. When data is to be written to physical storage media  15 , file mirroring logic  155  implements the replication of this data to a mirrored physical storage media (such as drive  25  associated with or owned by the secondary computer). After a successful write to both physical storage media  15  and physical storage media  25 , a confirmation signal is generated by the respective confirmation logic units  157 , 257  that results in a flush of the subject data from the secondary computer&#39;s file system cache memory  22 . 
     When a primary computer fails, however, the primary computer is not able to send back a confirmation signal. In conjunction with a time-out for the confirmation signal and other conventional test signals, the secondary computer can ascertain when a primary computer failure has occurred. In response to detecting a primary computer failure, the secondary computer assumes the identity of the primary computer. This entails adopting the primary computer&#39;s network file system identity and related conventional fail-over processing. The file system filter driver  252  reconfigures itself through a software implemented switch to support local file system data transfers as opposed to network file system data transfers. In this manner, filter driver  252  supports both (1) the retention of uncommitted file system cache memory data and (2) filter driver mediated identity transfer from a primary computer to a secondary computer. In other words, if there are outstanding file system transactions in the secondary computer&#39;s file system cache, then these transactions are written through the secondary computer&#39;s underlying file system to the physical storage media associated therewith (shared or mirrored). 
     In addition, the path through the filter driver is reconfigured to pass file system transactions to the underlying physical file system, and not through to the network file system protocol engine. 
     Referring to FIG. 4, a flow diagram of software functions within the computer cluster of FIG. 3 in accordance with the present invention is shown. In step  301  a determination is made as to whether a primary computer failure has occurred. If a failure has not occurred, then the operation of the filter driver  252  is unchanged (step  303 ). If a failure has occurred, then uncommitted data is written to the physical storage media owned by the secondary (step  305 ), the filter driver switches from network file system operation to local file system operation (step  306 ) and conventional fail-over and identity assuming processing is performed (step  307 ). It should be recognized that the processing of steps  305 - 307  is not necessarily serial in the manner portrayed diagrammatically. 
     While the invention has been described in connection with specific embodiments thereof, it will be understood that it is capable of further modification, and this application is intended to cover any variations, uses, or adaptations of the invention following, in general, the principles of the invention and including such departures from the present disclosure as come within known or customary practice in the art to which the invention pertains and as may be applied to the essential features hereinbefore set forth, and as fall within the scope of the invention and the limits of the appended claims.