Patent Publication Number: US-7225118-B2

Title: Global data placement

Description:
FIELD OF THE INVENTION 
     The invention is generally related to networks or systems. More particularly, the invention is related to allocating resources in a network or system. 
     BACKGROUND 
     More and more, the emerging model of enterprise computing is one where compute and storage resources are distributed globally. To derive most benefit from the investment in the infrastructure, resources are preferred to be consolidated into one pool. Users may then be able to just run an application on the pool, without needing to consider how and where the resources were actually derived from. However, to make best use of the available resources, the system should be able to make efficient allocation decisions, such as deciding where an application is run, where some database is stored, or how much bandwidth is allocated on some network for one application. 
     Users may generate workloads where each workload is an application with computational and storage requirements. Each of these workloads may be assigned to a compute server to perform the required computations. The data that the workloads access may also be assigned storage servers from which the compute server accesses the data. 
     However, costs are incurred in such environments. For instance, the cost of running a workload on a server could be measured in the amount of time the application used the server, the cost of locating a piece of data on a storage server could be measured in the amount of storage space used, or the cost of using a network link could be measured in the amount of bandwidth that was consumed. The problem then becomes matching workloads with the appropriate resources to minimize costs. 
     One approach to allocating resources is the storage configuration approach, as described by Alvarez et al., “Minerva: An Automated Resource Provisioning Tool for Large-Scale Storage Systems,”  ACM Transactions on Computer Systems,  2001 and Anderson et al., “Hippodrome: Running Circles Around the Storage Administrator,”  Conference on File and Storage Technologies,  2002. The storage configuration approach involves placing data onto storage devices subject to capacity and utilization constraints, while minimizing the cost of the storage devices. 
     However, the storage configuration approach assumes computation to be local to the storage and is separately assigned. In particular, there is no network latency between computation and storage. Thus, the storage configuration approach is not suitable when modeling behavior of a system whose resources are distributed. 
     There have been other works that attempt to solve variants of the data placement problems such as the file assignment approach, web object placement approach, and the web cache placement approach. However, these approaches had many deficiencies such as inability to explore load-balancing issues, computational inefficiency, lack of provable quality solution and/or performance, and the like. 
     SUMMARY OF THE INVENTION 
     According to an embodiment of the present invention, a method of global data placement may include assigning one or more workloads to one or more compute servers and controlling flow of the one or more workloads wherein each workload flows to one compute server. 
     According to another embodiment of the present invention, a method of global data placement includes assigning one or more workloads; assigning one or more workload nodes such that each of one or more workloads is connected to at least one of the one or more workload nodes; assigning one or more compute server nodes connected to the one or more workload nodes such that a flow from each workload node passes through an edge to a single compute server node, assigning one or more storage server nodes such that each compute server node is connected to at least one of the one or more storage server nodes; and assigning one or more store nodes such that each of one or more storage server nodes is connected to at least one of the one or more store nodes. 
     According to yet another embodiment of the present invention, a method of allocating resources in a network includes modeling a source and a sink for each data stream of the network; modeling intermediate nodes including one or more workload nodes, one or more compute server nodes, and one or more storage server nodes such at each workload node is connected to only one of the one or more compute server nodes and such that each compute server node is connected to at least one of the one or more storage server nodes; connecting the source for each data stream to at least one of the one or more workload nodes; and connecting the source for each data stream to at least one of the one or more storage server nodes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       Features of the present invention will become known from the following description with reference to the drawings, in which: 
         FIG. 1  is a block diagram of an exemplary system where multi-commodity flow modeling may be used according to an embodiment of the present invention; 
         FIG. 2  is a block diagram of an exemplary graph illustrating the concept of flow conservation; 
         FIG. 3  is a block diagram of an exemplary graph illustrating the concept of global data placement in multi-commodity flow modeling approach according to an embodiment of the present invention; 
         FIG. 4  is a block diagram of an exemplary graph illustrating use of control commodities according to an embodiment of the present invention; 
         FIG. 5  is a block diagram of an exemplary graph illustrating modeling flow from workload to compute servers according to an embodiment of the present invention; 
         FIG. 6  is a block diagram of an exemplary graph illustrating modeling flow from compute servers to storage servers according to an embodiment of the present invention; and 
         FIG. 7  is a block diagram of another exemplary graph illustrating modeling flow from compute servers to storage servers according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     For simplicity and illustrative purposes, the principles of the present invention are described by referring mainly to exemplary embodiments thereof. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, the present invention may be practiced without limitation to these specific details. In other instances, well known methods and structure have not been described in detail so as not to unnecessarily obscure the present invention. 
     In an embodiment of the present invention, the data placement problem is modeled as a mixed-integer minimum-cost multi-commodity flow problem. In this approach, the problem of allocating resources in a global system is modeled as a multi-commodity network flow problem, which is then solved. The flow of the different commodities on the network indicates how different resources are allocated. 
     The limited resources in the system impose natural constraints on the solution. For instance, the computation resources on the server are bounded, the storage space on storage servers is bounded, and the link bandwidth is also bounded. 
     Thus, in an embodiment of the present invention under this modeling approach, the inputs to the model are (a) stores, which are data chunks characterized by size; (b) workloads, characterized by a server requirement such as computation and memory; (c) compute servers providing computation and memory resources; (d) storage servers providing storage resources; and (e) an arbitrary network connecting compute servers and storage servers. Note that a particular store may be replicated on multiple storage servers, and the number of replicas of every store (each of which may be different), may be specified as part of the input. 
     The solution to the global data problem is a mapping of workloads to storage servers, stores to storage servers, and paths for different workloads to access their data, while obeying constraints such as computation, memory, and bandwidth constraints of the network and the servers. The output is a solution that minimizes aggregate costs. As noted previously, costs include compute cost, storage cost, transmission cost and the like. Costs may also be defined so that finding shortest path, minimizing resource usage, maximizing performance. 
     Overview of GDP as a Multi-commodity Flow Problem 
     The multi-commodity network flow problem may be viewed as a network flow formulation with multiple commodities. On each edge (a connection between any two nodes) of the graph, a flow may be defined. In other words, a flow is a value for each commodity on each edge of the graph. Each commodity has its own source and sink and honors its own flow conservation constraints. In other words, at each intermediate node (other than a source or a sink), the incoming flow equals the outgoing flow. Examples of intermediate nodes include the workload nodes, compute server nodes and storage server nodes. The flow conservation may be described by the following equation: 
                         ∑   j     ⁢     x   ij   k       -       ∑   j     ⁢     x   ji   k         =     b   i   k             (   1   )               
where
 
               ∑   j     ⁢     x   ij   k           
represents the sum of outgoing flow of commodity k from node i,
 
               ∑   j     ⁢     x   ji   k           
represents the sum of incoming flow of commodity k into node i, and b i   k  represents the net flow of commodity k generated by node i. For the intermediate nodes, the net flow b i   k  generated is zero indicating that the sum of the outgoing flow
 
               ∑   j     ⁢     x   ij   k           
is equivalent to the sum of incoming flow
 
     
       
         
           
             
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       FIG. 2  is a block diagram of an exemplary graph  200  illustrating the concept of flow conservation described above. The graph  200  includes a source  210  (denoted by “s”), sink  220  (denoted by “t”), and intermediate nodes as exemplified by node  230 . The flow conservation described by equation (1) simply indicates that flow on edge A 2  into the node  230  equals the sum of flow on edges B 2  and C 2  out of the node  230 . 
     In the multi-commodity flow modeling, each edge may have a capacity constraint. The edge constraint may generally be described as follows: 
     
       
         
           
             
               
                 
                   
                     
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                     U 
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     where w ij   k  represents a weight of the commodity k flowing through edge from node i to node j (or edge ij), x ij   k  represents the amount flow of commodity k through the edge ij, and U ij  represents the upper limit on capacity of the same edge. The weight represents the amount of capacity needed for each unit of the commodity. Equation (2) states that the sum of weighted commodity flows may not exceed the capacity of the edge. 
     In addition to edge capacity constraints, each commodity may have its own individual capacity constraint. For example, in  FIG. 2 , it may be that only a certain range of commodity from the source node  210  may flow through edge A 1 . Such individual constraint may be described by the following equation:
 
 l   ij   k   ≦x   ij   k   ≦u   ij   k   (3)
 
where l ij   k  and u ij   k  represent lower and upper bounds on the capacity of the commodity k through the edge ij.
 
     The goal of solving the multi-commodity flow model then becomes an effort to minimize cost. This may be expressed by the following equation: 
                   min   ⁡     (       ∑   k     ⁢       ∑   ij     ⁢       C   ij   k     ⁢     x   ij   k           )             (   4   )               
where C ij   k  represents a cost of flow of commodity k between nodes i and j. The minimizing cost as shown in equation (4) is subject to the constraints defined by equations (1), (2), and (3).
 
     An approach to a global data placement (“GDP”) problem under the multi-commodity flow model taken by an embodiment of the present invention may generally be described as follows: constructing a network and commodities corresponding to the problem instance, solving it using multi-commodity flow algorithms, and using the way that commodities flow in the constructed network to determine assignment of workloads to computation servers, determine the assignment of data replicas to storage servers, and determine the data access paths for the workloads. 
     The graph may be viewed as a layered network that includes sources, compute server nodes, storage nodes, and sinks as the layers. The multi-commodity flow graph may be enhanced with additional edges and nodes to accommodate additional constraints.  FIG. 3  is a block diagram of an exemplary graph  300  illustrating this concept, which includes store source nodes  310  (denoted by “s”), store sink nodes  320  (denoted by “t”), workload nodes  330  (denoted by “w”), compute server nodes  340  (denoted by “c”), and storage server nodes  350  (denoted by “ss”). 
     The edge capacity constraints may be used to enforce the constraints in the system. These could be, for example, the sum of computational requirements for workloads directed to a compute server may not exceed the total computational capacity of the compute server. As another example, the sum of storage requirements for a storage server may not exceed the storage capacity of the storage server. 
     Control Flows 
     In conventional flow formulations, flows may be split among different edges. For example, referring back to  FIG. 3 , the flow of commodity X may be split to the first and second workload nodes  330 . While not shown, the flow from any of the workload nodes  230  may be split to flow to both of the compute notes  340 . In addition, there is no standard way of grouping flows of multiple commodities together so that if one commodity flows on a edge then the rest of the commodities in the group also flow on the same edge. 
     There are situations in which it is desirable to keep the flow from splitting. While not exhaustive, the situations include: (1) mapping each workload to exactly one compute server node; (2) not exceeding a specified number of replicas per store; and (3) replicating write flow to a store to all replicas of that store. In an embodiment of the present invention, a concept of control commodities is introduced to keep the flows from splitting. 
     A control commodity is a fictitious commodity used to enforce restriction on the flow of regular commodities. The control commodities may be used to select an edge among many in a network and channel all related flows along the chosen edge.  FIG. 4  is a block diagram of an exemplary graph  400  illustrating the use of control commodities. As shown, the graph  400  includes store source nodes  410  (denoted by “s”), workload nodes  430  (denoted by “w”), and compute server nodes  440  (denoted by “c”). The graph  400  also includes a control commodity source  460  and sink  465  (both denoted by “cf”). The relatively thin lines indicate the control commodity flows and the relatively thick lines indicate other commodity flows. 
     The network is constructed such that when the control commodity flows on an edge, it consumes all the capacity of the edge. This may be done, for instance, by specifying that the control commodity flow is integral (0/1) and specifying the weight of that commodity on the edge to be equal to the capacity of the edge. 
     In an embodiment of the present invention, control commodities have zero cost associated with them. Thus, through the use of control commodities, the desired constraints are enforced without affecting the cost. 
     Allocating Resources on Compute Server Nodes 
     As noted above, in an embodiment of the multi-commodity flow approach, it is desired to map each workload to exactly one compute server node, i.e. direct all workload flow from a workload node to exactly one compute server node. For example, referring back to  FIG. 4 , it may be desired to have the entire workflow from the first workload node  430  (first from the top) go to the first compute server node  440  (again, first from the top) along an edge A 4 . The manner in which this may be enforced is as follows. Along the edge B 4  (indicated by a relatively thin line connecting the top workload node  430  to the second compute server node  440 ), the control commodity flow on this edge is specified to consume the entire capacity of the edge. Because the edge B 4  is entirely consumed by the control commodity flow, any other flow from the top workload node  430  must be on edge A 4 . 
     This may be generalized as follows. If there are n compute server nodes, a particular workload node may have n edges connecting that particular workload node with each of the n compute server nodes. In all but one of the edges (n−1 edges), a control flow may be specified to consume the entire capacity of each of the edges; this may be done by specifying the weight of the control flow to be equal to the capacity of the edge. Thus, the control flow may be used to block all but one of the edges (n−1 edges). Then only the remaining desired edge may carry the workflow commodity. This is repeated for each workload node to prevent splitting of flow of commodities from each workload node. 
     It is also desirable to ensure that the compute resource bounds of computation servers is not exceeded. Towards this end, a binary workload-cap flow may be defined for each workload. The binary workload-cap flow is 0/1 to indicate whether a workload is assigned to a compute server or not. This ensures that only the edges incident on all the compute server nodes carries this commodity. 
     Once the assignments are completed, whether the computation constraint of a compute server node is satisfied or not may be checked by adding up computation requirements for all the workloads assigned to the same compute server node. The same control commodity as above can be used to ensure that this commodity flows along with the other commodities of the corresponding workload. 
     As a result of the above use of control flow, the flow of a binary valued workload-cap commodity, {tilde over (y)} ij   k  will be such that {tilde over (y)} ij   k  is 1 if the store flows associated with that workload (aggregated over all the store commodities that the workload accesses) is non-zero, and {tilde over (y)} ij   k  is 0 otherwise. 
     The workload-cap flow can now be used to enforce the constraint that the total compute capacity at the compute server is bounded. This is done by writing the edge constraint of equation (2) for the appropriate nodes as: 
                       ∑   k     ⁢         w   ~     ij   k     ⁢       y   ~     ij   k         ≤     U   c             (   5   )               
where {tilde over (w)} ij   k  represents the capacity required by the workload k and U c  represents the total compute capacity of the server c. A similar setup can also be used to ensure that the total memory capacity at the compute server is not exceeded.
 
     In an embodiment of the present invention, it may be desired to transform a multi-commodity network graph into another equivalent multi-commodity network graph. Certain transformations allow the constraints be defined in a manner that is easier to solve.  FIG. 5  is a block diagram of an exemplary graph  500  illustrating modeling flow from workload to compute servers (storage servers are dealt with in  FIG. 6 ). The graph  500  is much like graph  400  of  FIG. 4  except that each compute server node  440  has been split into compute server nodes  542  (level 1),  544  (level 2) and  546  (level 3). 
     It should be noted that other number of levels are possible. In an embodiment of the present invention, from each workload node  430 , only one first level compute server node  542  may be assigned through the use of the control flow from the control flow source  460 . 
     Transforming the system in such a manner allows the multiple constraints specified originally on a node or an edge as a series of single constraints on the new edges. In this instance, it may be that the computation capacity constraint may be moved on the edges between nodes  542  and  544  (between levels 1 and 2) and the memory constraint may be moved on the edges between nodes  544  and  546  (between levels 2 and 3). This allows complex problems to be broken down to sub-problems and thus simplifying the computation. In addition, this allows the constraints to be expressed on the edges, which is typically required in order for the problem to be solved using multi-commodity flow algorithms. 
       FIG. 6  is a block diagram of an exemplary graph  600  illustrating modeling flow from compute servers to storage servers according to an embodiment of the present invention. The graph  600  may be viewed as a continuation of the graph  500  of  FIG. 5 . Graph  600  may include compute server nodes  546  (like the third level nodes in  FIG. 5 ), storage server nodes  650  and store sinks  620 . 
     Allocating Resources on Storage Servers 
     Like the compute server nodes, storage server nodes may also be split into multiple levels. This is shown in  FIG. 7 . As shown, each compute server  546  may have flows to one or more storage servers  650  (split into storage server nodes  752 ,  754 , and  756 ), which in turn may have flows to one or more stores  720 . Each store  720  may have a pre-defined, possibly different, number of replicas and each storage server  650  contains at most one replica of each store. For example, if a store  720  has m replicas, then m units of flow may be associated with the store-cap commodity  722  (described below) corresponding to the store  720 . The control flow  750  may be defined to bind the m units of the store-cap flow and the stream for the store  720  together to exactly m storage servers  650 . At most, one unit of store-cap flow may be allowed to flow to each storage server  650 . 
     Similar to the situation with computation servers, it is desirable to ensure that the storage resource bounds of the storage servers is not exceeded. Towards this end, a binary store-cap flow may be defined for each store. The binary store-cap flow is 0/1 to indicate whether a replica is assigned to a storage server or not. Writing a constraint that uses all the store-cap flows, each multiplied with the corresponding store&#39;s size, will then enforce the constraint that the total storage capacity at the storage node is limited. 
     As a result of the above use of control flow, the flow of the binary valued store-cap commodity, {tilde over (z)} ij   k  will be such that {tilde over (z)} ij   k  is 1 if the store flow associated with that store is non-zero, and {tilde over (z)} ij   k  is 0 otherwise. 
     The store-cap flow can now be used to enforce the constraint that the total storage capacity at the storage server is bounded. This is done by writing the edge constraint of equation (2) for the appropriate nodes as: 
                       ∑   k     ⁢         w   ~     ij   k     ⁢       z   ~     ij   k         ≤     U   s             (   6   )               
where {tilde over (w)} ij   k  represents the capacity required by the store k and U s  represents the total storage capacity of the server s.
 
     However, in graph  700 , control flow is used in a manner different than illustrated in  FIGS. 4 and 5 . Here, the flow control from a flow control source  780  may be used to block an edge from one of the storage server nodes  756  to a store  720 . In this way, the number of replicas may be controlled. For example, if the number of storage nodes is n and the number of replicas to place is r, then by ensuring that n-r units of control flow originates from the storage nodes, (n-r) edges may be blocked to guarantee that the number of replicas is only r. This also ensures that there is no more than one replica of a store on the same storage node. 
     Reads and Writes 
     Reads and writes may be handled differently. In the case of reads, it may be acceptable for a workload to get its data from more than one replica, and possibly getting the data at different rates from different replicas. In the case of writes, it may be necessary to send every write to every replica, indicating that the workload may generate the same write rate to each replica. In an embodiment of the present invention, control flows may also be used to enforce these requirements of reads and writes. 
     In the case of reads, the entire store may be represented by one commodity; not one commodity per replica. If the sum total of the read rates from all the workloads for a particular store is R, then it may be sufficient that the workloads be drawn, in any proportion, from any of the r replicas for that store to total R. 
     Each store may have a pre-determined number of replicas, which may be different for different stores. In other words, if r x  represents the number of replicas for a store s x , r x  is not determined as a solution to the GDP problem. Rather, r x  is provided as an input. In an embodiment of the present invention, the solution to the GDP problem is such that if any flow rate is drawn from a particular storage node, a replica is guaranteed to be located at the storage node, i.e., there is storage space allocated there for that replica. In addition, it is ensured that there is only one replica per store at a storage node. 
     For each store that a workload reads from, a flow, corresponding to the rate at which the workload accesses that store, is constructed. All such flows, from the stores that the workload reads, are made to flow to the same computation node using the control-flow that was used to assign the workload to exactly one computation node. This is used to ensure that all the read traffic that the workloads assigned to a computation node generate, all originate from the same computation node. 
     On the storage server side, it may not be necessary to distinguish the individual flows to the same store from different workloads: they are all for the same commodity. Also, it may not be necessary to group multiple stores accessed by the same commodity, as done in the compute server side. This ensures that a) that the stores accessed by one workload may be located at different storage servers, and b) the data from one store that one workload accesses may be derived piecemeal from the different replicas of that store. 
     As discussed before, by using a control-flow it is ensured there are only the predefined number of replicas of a particular store. The same control-flow can also be used to ensure that there is flow for a particular store (corresponding to the reads generated by workloads) only if there is a replica located at that storage node. 
     In case of writes, the write load generated by a workload to a particular store may need to go to each of its replicas so that all replicas of a store are consistent. Thus, for writes, a commodity may be defined corresponding to each replica of a store. Thus, if a store has r replicas, then there are r commodities for that store. Note that the number of these commodities is independent of the number of workloads, and only depends on the number of replicas of that store. In this instance, if a workload generates a write load of w units, then w units of each of the r kinds of commodities flow along with the commodities that correspond to the reads of that workload. In this manner, if a workload is assigned to a computation node, all the read and write traffic that the workload generates appear to originate from the same computation node (the one where the workload is assigned to). 
     Now at the storage nodes, the following should be ensured: a) all the write flow to a particular replica ends up at the storage node where the replica is located, and b) there are no more than one replica of a store on the same node. This may be accomplished by first ensuring that, at a storage node, there is write flow for a particular store only if there is a replica located there: this is done using the same control flow which was used to ensure that the reads went to only nodes where the store was located. Then it may be ensured that for each store, the total write flow at each storage node is at most W, where W is the sum total of the write traffic generated by all the workloads to that store: this is done using a simple constraint on the incoming edge to the storage node. Then it may be ensured that all the write flow for a particular replica ends up at exactly one storage node: this is ensured using another control-flow in a manner similar to that was used to ensure that all the workload was assigned to exactly one compute server node. 
     Solving GDP 
     Once the GDP problems are modeled using the multi-commodity flow approach, very good algorithms may be utilized to solve the problems. For example, branch and bound algorithms may be used. Approximation algorithms, such as randomized rounding and integer packing, may be used as well. The multi-commodity flow approach to modeling and solving GDP problems allows provable quality solutions to be achieved. 
     The terms and descriptions used herein are set forth by way of illustration only and are not meant as limitations. Those skilled in the art will recognize that these and many other variations are possible within the spirit and scope of the invention as defined in the following claims, and their equivalents, in which all terms are to be understood in their broadest sense unless otherwise indicated.