Abstract:
An apparatus and method for connecting a plurality of computing devices, e.g. web servers, database servers, etc., to a plurality of storage devices, such as disks, disk arrays, tapes, etc., by using a stream-oriented (circuit oriented) switch that has high throughput, but that requires non-negligible time for reconfiguration is disclosed. An example of such stream-oriented switch is an optical switch. The preferred embodiment comprises a plurality of communication ports for connection to servers, and plurality of ports for connection to storage devices. The system decodes the requests from the computing devices and uses this information to create circuits, e.g. optical paths in embodiments where the stream-oriented switch is an optical switch, through the stream-oriented switch. The system uses these circuits to route traffic between the computing devices and the storage devices. Buffering data and control in the device memory is used to improve overall throughput and reduce the time spent on reconfigurations.

Description:
This application claims the benefit of Provisional Application No. 60/292,106, filed May 17, 2001. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The invention relates to the storage of information. More particularly, the invention relates to a stream-oriented interconnect for networked computer storage. 
     2. Description of the Prior Art 
     The properties of typical traffic between computing devices and storage devices are fundamentally different from those of the typical flows between computing devices. For example, the latency between the time a request is issued for a disk read and/or write operation and the time the operation is performed by the disk can run into the multiple milliseconds due to factors such as disk seek time and disk rotational delays. As opposed to typical Internet traffic, which tends to have short bursts of traffic to different servers, the traffic in a storage network tends to have significantly longer packets and, in many cases, is stream (circuit) oriented and predictable. In addition, while in general it is very important to minimize the latency in computer networks, the latency of write-to-disk operations is relatively unimportant in many cases. 
     Known stream-oriented switches, e.g. optical interconnects based on micro-mirrors, electro-optic, thermo-optic, acousto-optic, bubbles, etc., are capable of very high throughput but have relatively long switching/reconfiguration times. Conversely, conventional electronic packet switches have lower maximum throughput than optical switches, but have significantly faster switching times. Switching time, as used herein, refers to the time that elapses between a command to create a connection between ports and the time when the data can start flowing through the system. 
     It would be advantageous to provide an apparatus and method for connecting a plurality of computing devices, e.g. web servers, database servers, etc., to a plurality of storage devices, such as disks, disk arrays, tapes, etc., by using a stream-oriented (circuit oriented) switch that has high throughput, but that requires non-negligible time for reconfiguration. 
     SUMMARY OF THE INVENTION 
     The presently preferred embodiment of the invention comprises an apparatus and method for connecting a plurality of computing devices, e.g. web servers, database servers, etc., to a plurality of storage devices, such as disks, disk arrays, tapes, etc., by using a stream-oriented (circuit oriented) switch that has high throughput, but that requires non-negligible time for reconfiguration. An example of such stream-oriented switch is an optical switch. 
     The preferred embodiment comprises a plurality of communication ports for connection to servers, and a plurality of ports for connection to storage devices. The system decodes the requests from the computing devices and uses this information to create circuits, e.g. optical paths in embodiments where the stream-oriented switch is an optical switch, through the stream-oriented switch. The system uses these circuits to route traffic between the computing devices and the storage devices. Buffering of data and control in the device memory is used to improve overall throughput and reduce the time spent on reconfigurations. 
     An alternative embodiment of the system uses two interconnects, in which all devices are connected to one or both interconnects. One of the interconnects is the stream-oriented switch described above. The second interconnect is a conventional packet switch which can switch traffic on a packet-by-packet basis, e.g. electronic packet switch. In contrast to the stream-oriented switch, the packet switch has much smaller overall throughput, but requires much less time for reconfiguration. The stream-oriented switch is used for switching relatively large data streams, e.g. reading a large file, while the packet switch is used for control and the rest of the data traffic. 
     A further embodiment of the invention comprises a system that uses a statistical prediction algorithm, e.g. HMM, Hidden Markov Model, to predict traffic flows. It then uses a statistical decision algorithm, e.g. MDP, Markov Decision Process, to decide on how to reconfigure the stream-oriented switch at every moment in time. The system attempts to optimize certain customer selectable measures, such as throughput, latency, etc., by configuring the stream-oriented switch such that the selected measures are optimized for the predicted traffic, while routing the remainder of the traffic, i.e. the predicted traffic that could not be accommodated by the high-throughput interconnect, as well as the unpredicted traffic, through the packet switch. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block schematic diagram that shows device for switching storage-related data using an optical switch according to the invention; 
         FIG. 2  is a block schematic diagram that shows section of a dual fabric interconnect according to the invention; 
         FIG. 3  is a block schematic diagram that shows a dual fabric section after an optical switch is switched to disk B according to the invention; and 
         FIG. 4  is a block schematic diagram that shows a generalized overview of a dual fabric architecture according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Networked Storage Using an Optical Switch 
     As shown in  FIG. 1 , the herein disclosed invention, a stream-oriented, optical switch  12  (also referred to herein as an optical switch), connects a plurality of computing devices  14  to a plurality of storage devices  16 . The presently preferred embodiment of the invention comprises an optical switching device  11 , a switch control unit  13 , a scheduling and load balancing unit (called hereinafter SLBU)  15 , and a system controller  17 . 
     The system controller  17  is responsible for the overall control of the entire system. It communicates with the computing and storage devices, gathering their request and monitoring status information. This information is distributed to the SLBU, which determines a performance enhancing packet schedule and routing as well as switch configuration. The system controller then distributes the packet schedule, routing information, and stream-oriented switch configuration to the relevant parts of the system. 
     The SLBU is responsible for distributing the communication load between the various available links between the computing and storage devices. It is also responsible for ordering the packet flows through the system. It attempts to minimize frequency of stream-oriented switch reconfigurations. Such reconfigurations generally take time, thereby increasing latency, and decreasing throughput. SLBU is also concerned with providing quality-of-service to the computing and storage devices 
     The packet scheduling unit  19  sends a transmission schedule to the computing and storage devices as well as to the system controller. Based on the information provided by the SLBU, the system controller determines when a switch configuration change is required, and sends a command to the switch control unit to modify the optical switch configuration. Once the optical switch has been reconfigured, the switch control unit informs the rest of the system of the new configuration, and the computing and storage devices commence using the circuits that are available through this new configuration for communicating. 
     A Multiple Fabric Switch for Connecting Computing and Storage Devices 
     In this embodiment of the invention, all devices are connected to one, two, or more interconnects, e.g. an electronic packet switch and an optical switch. The packet switch is used for some of the control and data traffic, while the optical switch is used for the remainder of the traffic. The multiple fabrics are controlled by the system control unit, which receives and issues control and status information via the packet switch and which is responsible for configuring the optical switch. 
     The system decodes the traffic and splits it in two. The control part of the traffic  20  is sent through the packet switch, while the rest of the traffic  22  is routed through the optical switch. A subsection of a system based on this invention with two interconnects is shown in  FIG. 2 , in which one computing device  21  is connected to two storage devices  23 ,  25  both through a packet switch, and a circuit created by an optical switch. In the system state illustrated in  FIG. 2 , the circuit (in the example, an optical path  22 ,  27 ) connects the computing device and Disk A. 
     If the traffic flow changes and it becomes advantageous to create a circuit between the computing device and Disk B, the configuration of the optical switch is changed to the configuration that is shown in  FIG. 3 , where the optical switch implements a circuit (in this example, an optical path  22 ,  31 ) that connects the computing device and Disk B. While the system is in this configuration, the packets that need to be transmitted between the computing device and Disk A are flowing through the packet switch, illustrated by the arrows  20 . 
     While two disks are shown in this example, those skilled in the art will appreciate that any number of storage devices may be used in connection with the herein disclosed invention. Similarly, those skilled in the art will be able to use an optical switch that supports multiple concurrent circuits as opposed to a single circuit shown in  FIGS. 2 and 3 . 
     This embodiment of the invention is controlled by a system controller that is responsible for the overall control of the system. It communicates with the computing and storage devices, gathering their request and status information, and distributing this information to the SLBU. 
     The SLBU is responsible for distributing the communication load between the various available links through the fabrics connecting the computing and storage devices. It recommends to the system controller unit a configuration of the optical switch and the packet switch that divides the traffic between the two fabrics to optimize certain customer selectable criteria such as throughput and delay. 
     The SLBU is also responsible for ordering the packet flows through the system. It attempts to balance the frequency of optical switch reconfigurations (which, generally, take time, thereby increasing latency and decreasing throughput), with optimizing the customer-desired system attributes, e.g. total throughput. The SLBU communicates with the computing and storage devices for the most part via the packet switch. The SLBU sends a transmission schedule to the computing and storage devices, as well as to the system controller. (Alternatively, as shown in  FIG. 4 , SLBU can communicate with splitters/combiners making the switch look like a single-fabric switch from the point of view of the storage and the computer devices.) Based on the information provided by the SLBU, the system controller determines when an optical switch configuration change is required, and sends a command to the switch control unit to modify the optical switch configuration. This is shown in  FIG. 2  by the arrow marked “mirror control.” 
     Additionally, because in many cases a seek operation by the target disk device is required, such a command can be sent to the target via the packet switch in parallel with the reconfiguration of the optical switch. The arrow marked “disk control” illustrates this. Once the optical switch has been reconfigured, as illustrated in  FIG. 3 , the switch control unit informs the rest of the system of the new configuration, and the computing and storage devices commence communicating via this new configuration. 
     An overview of a presently preferred embodiment of the system is shown in  FIG. 4 . Packets sent from computing devices  14  are decoded by respective splitters/combiners  50  (one such device per port) and some control/status information is extracted and forwarded to the load balancer SLBU  15 . The SLBU controls the state of the optical switch  11  (optical interconnect in this example) and instructs the load balancers/splitters which of the packets/streams should be routed through the optical switch, i.e. optical interconnect. The rest of the packets are routed through the packet switch  51  (in this example, the electrical interconnect). Splitters/combiners can be either part of the dual-fabric switch or embedded into computing and storage devices. 
     An example of a transaction in such a system is a read operation by a computing device. In such a case, the computing device issues a request for the data from a storage device. The read request is decoded by the appropriate splitter/combiner. It is immediately forwarded through the packet switch, e.g. electrical interconnect, to the appropriate storage device. At the same time, the information describing the request, e.g. number of bytes to be transmitted and source/destination, is forwarded to the load balancer. While the storage device performs the seek operation, the load balancer instructs the optical switch, e.g. the optical interconnect, to prepare a data path between the appropriate ports. When the path is prepared, it instructs the splitter/combiner responsible for the storage device port to route the data from the storage device through the optical switch. It also instructs the appropriate splitter/combiner on the server side to be ready for data that will shortly arrive from the optical switch. The splitter/combiner receives this data and forwards it to the computing device. 
     A similar procedure is executed for computing device writes to the storage devices. In general, the SLBU monitors the data streams that are passing through the system. It also monitors the overall throughput and the data transfers that are in progress, redirecting some of the transfers to pass through the optical switch, e.g. optical interconnect. The rest of the data is forwarded through the packet switch, e.g. electrical interconnect. Observe that only meta-data, such as request descriptions, are passed through the SLBU. The actual data flows through the two interconnects. 
     In some cases, it is advantageous to use buffering to aggregate multiple data write requests, as well as to use data caching and predictive data read-ahead to improve performance by increasing the amount of data that can be transferred through the optical switch between reconfigurations. Using two network adapters in each server can eliminate the need for splitters/combiners on the server side. In this case, one of these adapters is connected to the packet switch, e.g. electrical interconnect, and the load balancer, e.g. using Gigabit Ethernet, while the other adapter is connected to the optical switch, e.g. optical interconnect, using optical interface cards such as Fibre Channel. In this case, it is possible to use a server-side driver, e.g. a software package, that collects global data about the requests and sends it to the load balancer, e.g. through Gigabit Ethernet. 
     Reconfiguration of optical switches can often require a non-negligible amount of time. To improve performance, one embodiment uses more than one optical switch. In this case, the SLBU instructs the splitters/combiners to forward data through interconnects that are already reconfigured, while reconfiguring other interconnects at the same time. This causes reconfiguration intervals of one optical switch, e.g. in the case of an optical interconnect, the interval of time when the optical interconnect is being reconfigured and hence is unavailable for forwarding packets, to overlap the intervals of time when the other interconnect, or interconnects, are already reconfigured and hence can forward the packets. To improve performance, the system supports the connection of a single server to several ports, e.g. by using several network adapters, and the connection of a single storage device to several ports. To reduce cost, the system supports the connection of several servers and/or storage devices to a single port. 
     A Prediction System Based on an Adaptive Statistical System Model 
     Computing device requests can be predicted with some degree of accuracy. For example, if a computing device has read the first seven sectors of a 10-sector file in quick succession, it is it quite likely that the computing device will soon request the remaining three sectors. This embodiment of the invention provides a performance enhancement system in the form of a transaction prediction system that predicts future behavior of the computing and storage devices based on prior transactions and known behavior patterns, e.g. the read example discussed above. The system builds and continually adapts a statistical model of the system, for example using a Hidden Markov Model, HMM. The system improves from time to time the statistical model and uses it to enhance the performance of the herein disclosed optical switch by making predictions about the future transactions of the system. 
     For example, consider a system as shown in  FIG. 2 , where the computer has read 300 sectors from file X, which contains 350 sectors and resides on Disk A, and then issues a read request for two sectors from file Y, that reside on Disk B. Without a prediction mechanism, the system may choose to reconfigure the optical switch to create a path between the computing device and Disk B. However, it is quite likely that the computing device would soon request the remaining 50 sectors of file X. Therefore, in most cases, better overall throughput and lower latency would be achieved by routing the sectors from file Y through the packet switch, and leaving the optical switch circuit established between storage the computing device and Disk A. 
     The decision of when to reconfigure the optical switch and how to reconfigure it can be made using, for example, a Markov Decision Process, MDP. In this case, the maintained statistical model is used to predict the expected benefit, e.g. performance improvement, of each possible reconfiguration (or no reconfiguration) at every step and to choose the reconfiguration (or a sequence of reconfigurations) that achieves significant improvement in the sense of mathematical expectation. When preparing the reconfiguration at each step, the system can also take into account possible errors in the predictions of the statistical model, as well as the statistical confidence in the predictions. Performance can also be enhanced by using the adaptive statistical model to predict reads and writes, therefore pre-positioning the disk heads, thereby decreasing latency, and enhancing overall system throughput. In the case where this prediction is implemented in the context of the optical switch, and the actual system behavior differs from the predicted behavior, the packets that have not been predicted can be either temporary buffered in the system and/or routed though the packet switch, thus not incurring the latency penalty that may be imposed by a required reconfiguration of the optical switch. 
     Dynamically Rearranging Data Stored on the Storage Devices 
     Using statistical analysis tools such as those mentioned above, another embodiment of the invention detects instances where the data are distributed among the storage devices in a less than optimal way. For example, consider the case where a large video file that is usually accessed sequentially by a single computing device at a time is split among several storage devices connected to several different ports on the system. In this case, it might be advantageous to relocate this file to a single storage device or to plurality of storage devices attached to a single port. Such relocation reduces the number of necessary reconfigurations of the optical switch that are needed for optimum access to the file. 
     It is important to note that there is a tradeoff in such relocation. While the file is split among several storage devices, it is theoretically possible to access it concurrently from several different computing devices, increasing the overall throughput. On the other hand, this (split) configuration might require extra reconfigurations of the optical switch. Based on the adaptive statistical access model mentioned above, the SLBU uses a decision policy, e.g. based on Markov Decision Process, to analyze these advantages and disadvantages and decides whether to relocate the file and where to relocate it. Note that the use of a file here is an example. A similar approach can be used for relocating sectors, blocks, or regions of data. The approach also can be applied if the storage device uses virtualization to hide its actual physical storage architecture/configuration. 
     Although the invention is described herein with reference to the preferred embodiment, one skilled in the art will readily appreciate that other applications may be substituted for those set forth herein without departing from the spirit and scope of the present invention. Accordingly, the invention should only be limited by the claims included below.