Patent Publication Number: US-10782887-B2

Title: Window-based prority tagging of IOPs in a distributed storage system

Description:
RELATED APPLICATIONS 
     This application is related to U.S. application Ser. No. 15/807,035 filed Nov. 8, 2017 and U.S. application Ser. No. 15/806,795 filed Nov. 8, 2017, which are incorporated herein by reference for all purposes. 
     FIELD OF THE INVENTION 
     This invention relates to storing and retrieving information in a distributed storage system. 
     BACKGROUND OF THE INVENTION 
     A provider of data storage may market services with a guaranteed quality of service (QoS). For example, for a higher quality of a service, the provider may charge a higher price. However, in order to implement this approach, input/output operations (IOPs) must be processed in such a way that the guaranteed QoS is met. This requires additional processing, which can increase latency. 
     The system and methods disclosed herein implementing a QoS-based prioritization of IOPs in a distributed storage system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order that the advantages of the invention will be readily understood, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered limiting of its scope, the invention will be described and explained with additional specificity and detail through use of the accompanying drawings, in which: 
         FIG. 1  is a schematic block diagram of a network environment for implementing methods in accordance with an embodiment of the present invention; 
         FIG. 2A  is a process flow diagram of a method for adding IOPs to a queue based on a QoS in accordance with an embodiment of the present invention; 
         FIG. 2B  is a process flow diagram of a method for assigning priorities to IOPs in a queue in accordance with an embodiment of the present invention; 
         FIG. 3  is a schematic diagram illustrating processing of IOPs according to the methods of  FIGS. 2A and 2B  in accordance with an embodiment of the present invention; 
         FIG. 4  is a process flow diagram of a method for transmitting IOPs to a storage node with assigned priorities in accordance with an embodiment of the present invention; 
         FIGS. 5A and 5B  are schematic diagrams illustrating implementation of queues on a storage node in accordance with an embodiment of the present invention; 
         FIG. 6  is a process flow diagram illustrating the selection of IOPs from queues of a storage node in accordance with an embodiment of the present invention; 
         FIG. 7  is a process flow diagram of a method for determining the performance of a storage device of a storage node in accordance with an embodiment of the present invention; 
         FIG. 8  is a process flow diagram of a method for assigning a logical storage volume to a storage node in accordance with an embodiment of the present invention; 
         FIG. 9  is a process flow diagram of a method for reassigning a logical storage volume based on performance of a storage device in accordance with an embodiment of the present invention; 
         FIG. 10  is a process flow diagram of a method for coordinating QoS implementation between primary and clone nodes in accordance with an embodiment of the present invention; 
         FIG. 11  is a process flow diagram of an alternative method for coordinating QoS implementation between primary and clone nodes in accordance with an embodiment of the present invention; 
         FIG. 12  is a schematic block diagram of an example computing device suitable for implementing methods in accordance with embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , the methods disclosed herein may be performed using the illustrated network environment  100 . The network environment  100  includes a storage manager  102  that coordinates the storage of data corresponding to one or more logical storage volumes. In particular, the storage manager  102  may be connected by way of a network  104  to the one or more storage nodes  106 , each storage node having one or more storage devices  108 , e.g. hard disk drives, flash memory, or other persistent or transitory memory. The network  104  may be a local area network (LAN), wide area network (WAN), or any other type of network including wired, fireless, fiber optic, or any other type of network connections. 
     One or more compute nodes  110  are also coupled to the network  104  and host user applications that generate read and write requests with respect to storage volumes managed by the storage manager  102  and stored within the memory devices  108  of the storage nodes  108 . 
     The methods disclosed herein ascribe certain functions to the storage manager  102 , storage nodes  106 , and compute node  110 . The methods disclosed herein are particularly useful for large scale deployment including large amounts of data distributed over many storage nodes  106  and accessed by many compute nodes  110 . However, the methods disclosed herein may also be implemented using a single computer implementing the functions ascribed herein to some or all of the storage manager  102 , storage nodes  106 , and compute node  110 . 
     Referring to  FIGS. 2A and 2B , the illustrated methods provide an approach for managing a queue of IOPs (input/output operations) based on a QoS (quality of service) target for a logical storage volume referenced by the IOPs. Each IOP may be a read command or write command. In some embodiments, each IOP processed according to the methods described below may represent many individual IOPS, e.g., one or more thousands of IOPs. The illustrated method  200  is describe below as being executed by a compute node  110  executing applications that generate IOPs for execution by the storage nodes  106 . However, the illustrated method  200  could be executed by any one of the components  102 ,  106 ,  110  shown in  FIG. 1  or by a combination thereof. 
     As described below, the QoS for a queue group may be defined using one or more values such as:
         A time window within which the performance for a particular queue group is evaluated.   A MinIOPs value that defines the minimum number of IOPs that must be performed for that queue group within the time window, e.g. 10,000 IOPs/second.   A MaxIOPs value that defines the maximum number of IOPs that are permitted to be performed for that queue group within the time window.       

     Note that “queue group” is used to refer to a grouping of one or more logical storage volumes, or portions of a logical storage volume, having a QoS associated therewith that are collectively managed with respect to the same QoS. A single customer may have multiple queue groups or multiple customers may belong to the same queue group. An association between a logical storage volume, the queue group to which the logical storage volume belongs, and the QoS for that queue group may be stored by the storage manager  102  and propagated to one or both of the compute nodes  110  and storage nodes  106  for use according to the methods disclosed herein. Likewise, the MinIOPs, MaxIOPs, and time window for a queue group may be maintained by the storage manager  102  and propagated to one or both of the compute nodes  110  and storage nodes  106 . 
     Referring specifically to  FIG. 2A , the method  200  may include receiving  202  an IOP (“the subject IOP”) from an application of one or more applications executing on the compute node  110 . The IOP may reference a logical storage volume (“the subject volume”) that belongs to a queue group (“the subject queue group”). The subject IOP may include other information sufficient to execute the IOP according to any approach known in the art, such as an offset within the logical storage volume, operation code (read, write, delete, etc.), size, etc. 
     The method  200  may include evaluating  204  the number of IOPs in a queue of the compute node that both (a) belong to the subject queue group and (b) were added to the queue within the time window from an oldest unexecuted IOP in the queue belonging to the subject queue group. If the number of IOPs meeting conditions (a) and (b) is found  204  to be less than the MaxIOPs for the subject queue group, the subject IOP is added  206  to the queue. Note that each queue group may have its own queue and therefore this queue is evaluated at step  204 . 
     If the number of IOPs meeting conditions (a) and (b) is found  204  to be less than the MaxIOPs value for the subject queue group, then the subject IOP is not added  208  to the queue. As soon as the condition of step  204  is met, the subject IOP will then be added to the queue. 
     In some embodiments, a set of threads may be dedicated to the queue for each queue group. When the number of IOPs for that queue group has exceeded the maximum threshold for a time period, these threads are put to sleep until the end of the time period, so that they do not service any more incoming IOPs. For example, consider a QoS period of 5 seconds and a max IOPs in that period of 100. At the beginning of the period (T 0 ) assume that there are 0 IOPs. If, within 1 second, the threads have processed the allowed 100 IOPs. The thread(s) handling subsequent IOPs will see that the max threshold for that queue group has been reached for that period, and will sleep until the end of the QoS time period (T 0 +5 seconds) before processing the new IOPs for that queue group. In this way a virtual queue is maintained where the IOPs processed by the thread(s) are “in” the queue, while those that have not been are kept “out” of the queue. 
     Referring to  FIG. 2B , the illustrated method  210  may be executed with respect to IOPs in the queue. The method  210  is discussed with reference to the diagram shown in  FIG. 3 . Note that the method  310  is executed with respect to IOPs belonging to the same queue group. References to IOPs, MinIOPs, and MaxIOPs shall be understood in the discussion of  FIG. 2B  and  FIG. 3  to refer to these entities belonging to the queue group that is the subject of the method  200 . Where IOPs from multiple queue groups are stored in the same queue, the method  210  may be executed once for each queue group in the queue. 
     In other embodiments, each queue stores only IOPs from the same queue group and is therefore subject to the method  210  only once, but the method  210  is performed for each queue. 
     The method  300  includes assigning a maximum priority to IOPs to the IOPs in the queue received within the time window from a time of receipt of an oldest unexecuted IOP in the queue up to a total number of MinIOPs. Stated differently, starting at the oldest unexecuted IOP in the queue, the IOPs will be assigned the maximum priority until the number of IOPs assigned the maximum priority is equal to MinIOPs. 
     Those IOPs in the queue received within the time window from a time of receipt of an oldest unexecuted IOP in the queue and are in excess of MinIOPs are assigned a minimum priority that is less than the maximum priority. Stated differently, those IOPs received within the time window but later than those assigned the maximum priority because they are in excess of MinIOPs are assigned the minimum priority. 
     Note that the minimum priority and maximum priority may be specific to the queue group that is the subject of the method  210 . For example, a queue group with higher priority hay have higher maximum and minimum priorities than a lower priority queue group. In some embodiments, the maximum and priorities function as a queue group identifier, i.e. each has a unique value that identifies the queue group to which an IOP belongs when tagged with the maximum or minimum priority. In some embodiments, the minimum priority will be a value near zero whereas the maximum priority may be a value on the order of a thousand or more. For example, for queue group 3, the maximum priority is 1003 and the minimum priority is 3. For queue group 2, the maximum priority is 1002 and the minimum priority is 2, and so on for each queue group. 
     Referring to  FIG. 3 , IOPs that are not queued may be stored in a separate queue  300  until they can be added to the queue referenced with respect to  FIGS. 2A and 2B . Each IOP may include such information as a volume identifier  304  referring to a logical storage volume, address  306  within the logical storage volume, and payload data  308  in the case of a write command or size or range of addresses in the case of a read or delete command. 
     IOPs are added to the queue  302  in the order received, with the top IOPs  310  at the top of the queue being oldest in the illustrated example. A time  312  that the IOP was added to the queue  302  may be stored for each IOP  310 . The time  312  may also be a time the IOP was received from an application to account for delays in adding the IOP  310  to the queue  302  according to the method  200 . 
     Portion  314  of the queue  302  indicates the portion of the queue containing IOPs  310  received within the time window from the last unexecuted IOP  310 . Portion  316  indicates the range of IOPs  310  assigned the maximum priority  318  and will be in number less than or equal to MinIOPs. Portion  320  includes the IOPs  310  that are within the time window from the last unexecuted IOP  310  but in excess of MinIOPs. These IOPs are assigned a minimum priority Those IOPs that are outside of the time window are not assigned a priority. The total number  324  of IOPs  310  in the queue  302  is constrained to be less than MaxIOPs according to the method  200 . 
     In the diagram of  FIG. 3 , only IOPs for the queue group that is the subject of the method  210  are shown. However, in practice, IOPs from other queue groups may be intermingled in the queue  302 . In other embodiments, each queue group may have its own queue. 
     Referring again to  FIG. 2B , the method  210  may further include evaluating  216  whether acknowledgment of completion of an IOP from the queue  302  has been received. If so, that IOP is removed  218  from the queue  302 . IOPs  310  may be transmitted from the queue  302  in the order received prior to receiving acknowledgments and may be sent in blocks or individually at a predetermined rate or based on capacity of the storage node to which the IOPs  310  are transmitted. 
     If an IOP  310  in the queue is found  220  to be unexecuted after a time period equal to the time window for the queue group to which it belongs, then an alert may be generated  222 . In some embodiments, priority of IOPs within that queue group may be increased in order to avoid failing to meet the QoS for that queue group. 
     Note that steps  212  and  214  may be executed repeatedly, such as periodically according to a fixed period or for every N IOPs that is acknowledged, where N may be a value equal to one or a larger integer. Accordingly, the minimum priorities  322  may be changed to the maximum priorities  318  as IOPs are acknowledged and removed from the queue  302  and the time window moves forward in time. 
     Referring to  FIG. 4 , IOPs  310  from the queue  302  are transmitted to one or more storage nodes  106 , such as a storage node storing a logical storage volume reference by each IOP  310 . As discussed above, IOPs  310  may remain in the queue  302  until acknowledgement of completion of the IOPs  310  are received. 
     In the illustrated example  400 , IOPs  310  are selected from the queue  302  and tagged  402  with information such as an identifier of the queue group to which the IOP  310  belongs and the priority  322 ,  318  of the IOP  310 . The tagged IOPs are then transmitted  404  to the storage node storing a logical storage volume reference by the tagged IOP. 
     This storage node then adds  406  the tagged IOP to one of a plurality of queues corresponding to its queue group and priority. IOPs are then selected  408  from the plurality of queues and executed according to the priorities of the plurality of queues. 
     Referring to  FIGS. 5A and 5B , a storage node  106  may maintain three types of queues: a user queue  502 , a clone queue  504 , and a garbage collection queue  506 . Note that although three types of queues are listed here, any number of queues, e.g. four or more, could be implemented with their own priorities. IOPs could then be addressed to these queues and processed according to their priorities in the same manner as for the three queues discussed below. The user queue stores IOPs received from user applications executing on compute nodes  110 . The clone queue  504  stores IOPs received from other storage nodes that are used to update replicas of a primary copy of a logical storage volume. The garbage collection queue  506  stores IOPs generated as part of a garbage collection process, i.e. IOPs copying valid data to new areas of storage from a former area of storage having a high concentration of invalid data so that the former areas of storage may be freed for storing new data. 
     Each queue type has a probability  508  associated therewith indicating the probability that an IOP will be selected from a queue of a give type  502 ,  504 ,  506 . In general, the user queue will have higher probability  508  then the clone queue  504  and the clone queue has higher probability than the garbage collection queue  506 . In this manner, original IOPs and replication IOPs will be given higher priority than garbage collection IOPs. 
     Referring to  FIG. 5B , the user queue  502  may be divided into a set  510  of high priority queues and a set  512  of low priority queues. Each high priority queue  514  in the set  514  corresponds to a particular queue group. Accordingly, each IOP referencing a queue group and having the maximum priority for that queue group will be added to the queue  514  for that queue group and executed in the order in which it was received (first in first out (FIFO). Each queue  514  has a probability  516  associated with it that corresponds to the priority of the queue group for the each queue. Accordingly, higher priority queues will have higher probabilities  516 . 
     In a like manner, each low priority queue  518  in the set  512  corresponds to a particular queue group. Accordingly, each IOP referencing a queue group and having the minimum priority for that queue group will be added to the queue  518  for that queue group and executed in the order in which it was received (first in first out (FIFO). 
     As noted above with respect to the method  210 , the priorities of IOPs may change as IOPs are executed and the time window moves forward in time. As this occurs, the compute node  110  may transmit updated priorities for IOPs that are already stored in the low priority queue  518 . These IOPs may then be moved to the high priority queue  514  in response to the updated priority. It is unlikely, but in some instances an update may change the priority of an IOP from the maximum priority to the minimum priority. Accordingly, the IOP would be moved to the low priority queue  518  from the high priority queue. 
     In use, when the user queue  502  is selected, one of the queues  514  will be selected based on the probabilities  516 . If the queue  514  is empty, then an IOP from the low priority queue  518  corresponding to the selected high priority queue  514  (belonging to the same queue group) will be executed. 
     In some embodiments, each of the clone queue  504  and the garbage collection queue is similarly divided into high and low priority queues  514 ,  518  and corresponding probabilities  516  for each queue group. The probabilities  516  may be the same or different for each type  502 - 506  of queue. 
       FIG. 6  illustrates one method  600  for selecting among the types of queues  502 - 506  and among the high priority queues  514 . In the method  600 , probabilities  508  and probabilities  516  are represented by a range of values such that the ranges for probabilities  508  do not overlap one another and the ranges for probabilities  516  do not overlap one another. To implement a higher probability for a given probability  508 ,  516 , the range of possible values for it is increased. 
     The method  600  includes generating  602  a first token and selecting  604  a queue type ( 502 - 506 ) having a range of values including the first token. The first token may be generated using a random, e.g., pseudo random, number generator. The random number generate may generate numbers with a uniform probability distribution within a minimum (e.g., 0) and maximum value, the ranges of values assigned to the types of queues  502 - 506  may be non-overlapping and completely cover the range of values between the minimum and maximum values. 
     The method  600  includes generating  606  a second token and selecting  608  a queue  514  having a range of values including the second token. Stated differently, a queue group may be selected, which has a corresponding high priority queue  514  and a low priority queue  518  The first token may be generated using a random, e.g., pseudo random, number generator in the same manner as for step  602 . 
     If the queue  514  selected at step  608  if found  610  to include at least one IOP, then the oldest IOP in the selected queue  514  is executed  612 . 
     If not, and the low priority queue  518  corresponding to the same queue group as the queue  514  is found  614  to include at least one IOP, then the oldest IOP in the low priority queue  518  is executed  616 . 
     The IOP executed at step  612  or  616  is removed from the corresponding queue  514 ,  518  in which it was stored and the method repeats at step  602 . 
     Referring to  FIG. 7 , logical storage volumes, or parts thereof, and replicas of logical storage volumes, or parts thereof, may be assigned to storage nodes based on performance (e.g., IOPs/s) and storage capacity (gigabytes GB, terabytes (TB), etc.). 
     The method  700  illustrates an approach for determining the performance of a storage device  108  of a storage node  106 . The method  700  may be executed for each storage device  108  (“the subject device”) of the storage node  106  (“the subject node”). The combined, e.g. summed, performances of the storage devices  108  of the subject node indicate the performance of the subject node. 
     The method  700  includes selecting  700  an initial value for “Max Pending.” This may be a manual selection or based on prior assessments of the performance of the subject device. 
     The method  700  then includes sending  704  a number of IOPs equal to max pending to the subject device. These IOPs may be selected from queues according to the approach of  FIGS. 4 through 5A and 5B  or some other approach. 
     The method  700  may further include counting  706  a number of acknowledgments received during a latency period, i.e. within a latency period from at time of sending of the first IOP sent at step  704 . The latency period may be an operator specified value. A large latency period means adaptation to changes in the performance of the subject device will be slower. A shorter period adds more overhead processing but results in more accurate tracking of performance. In general, the latency period should be many multiples (e.g., at least four times) the latency of the subject device. A latency period of 2 ms to 500 ms has been found to be adequate for most applications. 
     If the count of step  706  is found  708  to be larger than or equal to max pending, then the value of max pending is increased  710  and the method repeats from step  704 . In some embodiments, max pending is initially set to a small value. Accordingly, the increases of step  710  may be large, e.g. doubling of the former value of max pending. Other increments may be used and may be constant or a function of the former value of max pending, e.g. the increment amount may be a fixed value or increase or decrease with increase in the value of max pending. 
     If the count of step  706  is found  712  to be smaller than max pending, then the value of max pending is decreased  714  and the method repeats from step  704 . In some embodiments, max pending is decreased more gradually at step  714  then it is increased at step  716 . Accordingly, the decrement amount or function that computes the new value of max pending may result in a much smaller decrease than the corresponding increase for the same prior value of max pending at step  710 , e.g. less than half of the value of the corresponding increase, less than 10 percent of the corresponding increase, or some other percentage. 
     The performance as adjusted at step  710  or  714  for each storage device  108  may be reported  716  to the storage manager  102  for purposes of assigning logical storage volumes to storage nodes and storage devices  108  of storage nodes  106 . At step  716 , usage of each storage device  108  of the storage node may also be reported  176 , i.e. the amount of physical storage space that is currently storing data and not available to be overwritten. Step  716  may be performed for each iteration of the method  700  or less frequently. Usage and performance may be reported separately and independently from one another and at different update intervals. 
       FIG. 8  illustrates a method  800  that may be executed by the storage manager  102  to allocate logical storage volumes, or portions thereof, to storage nodes  106  and storage devices  108  of storage nodes  106 . 
     The method  800  includes receiving  802  a request for storage that includes both a storage requirement (“the capacity requirement”) and a quality of service (QoS) requirement (“the performance requirement”). 
     The method  800  may include evaluating whether a storage device  108  of one of the storage nodes  106  has both performance and capacity sufficient to meet the performance requirement and the capacity requirement. The capacity and performance of the storage device may be as reported  716  according to the method  700 . As used herein with respect to the method  800 , “capacity” is a portion of the total storage capacity of a device  108  that is available to be written or overwritten, i.e. is not currently storing data that is not available to be overwritten. As used herein with respect to the method  800 , “performance” is a portion of the total performance of a device  108  that is not currently used, i.e. based on current measurements of throughput of the device  108  within some window preceding the current time, the device  108  is available to process additional IOPs at a rate equal to the “performance” before the total performance of the device  108  is fully used. Total performance may refer to the performance reported by the device  108  at step  716  of the method  700 . 
     If so, then the method  800  may include allocating  806  the storage request to a smallest capacity device  108  meeting the condition of step  804 . Allocating a storage request to a storage device  108  may include notifying the storage node  106  hosting the storage device, generating a logical storage volume for the storage request, and executing IOPs by the hosting storage node  106  with respect to the logical storage volume using the storage device  108  to which the storage request was allocated. 
     If no device  108  is found  804  to have both the performance and capacity to meet the performance and capacity requirements, the method  800  may include evaluating  808  whether a device  108  meets the performance requirement but not the capacity requirement. If so, and usage of that device  108  is found  810  to be below a threshold percentage of the capacity of the device  108 , then the storage request may be allocated  812  to that device  108 . Where multiple devices  108  meet the condition of step  808 , the device  108  selected may be the smallest capacity device  108  meeting the condition of step  808 . 
     If multiple devices are found to match the capacity and performance requirements, then a device from among these devices that most closely matches the requirements may be selected. For example, if the requirement is for 100 GB@10000 IOPS and there are two devices—D 1  with 200 GB@20000 IOPS and D 2  with 150 GB@15000 IOPS we will pick D 2 . In some embodiments, if D 1  has 200 GB@15000 IOPS and D 2  has 150 GB@20000 IOPs, D 2  will be selected according to a preference to select the lowest capacity device from among the multiple devices that meet the requirements. In some embodiments, the lowest performance device may be selected from among the multiple devices that meet the requirements when specified by a configuration parameter. 
     Where a device  108  meeting the condition of step  804  is not found and a device  108  meeting the condition of step  808  is selected, usage of the selected device  108  may be evaluated  810  periodically. In the event that the usage of the selected device  108  exceeds the threshold percentage of the total capacity of the selected device  108 , one or more logical storage volumes allocated to the selected device may be reassigned, such as by executing the method  800  for the one or more logical storage volumes. 
     Specifically, the performance and capacity requirements of the logical storage volumes created upon allocation  812  may be used to select a different device according to the method  800  in the same manner as for an original storage request received at step  802 . However, actual data written to the logical storage volume may be taken into account, i.e. allocating to a device  108  such that storing the data written to the logical volume would cause the usage of the device to exceed the threshold percentage may be avoided. 
     If no device  108  meets the condition of steps  804  and  808 , the method  800  may include evaluating  814  whether a device  108  is available that has a capacity meeting the capacity requirement but does not have performance meeting the performance requirement, if so, the storage request may be allocated  816  to the highest performance device  108  meeting the capacity requirement. 
     If no device  108  meets the conditions of steps  804 ,  808 , and  814 , the storage request may be allocated  818  to a highest performance disk that may not meet the capacity requirement. In some embodiments, if no disk meets the requirements of steps  804 ,  808 , and  814 , the storage request may remain unallocated and an alert may be generated indicating that the storage request cannot be allocated unless more storage devices  108  are added to the distributed storage system. 
     Referring to  FIG. 9 , after a storage request is allocated to a device  108 , the method  900  may be executed by the storage node  106  hosting that device  108 . The method  900  may include monitoring  902  performance of the device (see  FIG. 7 ). If the performance of the device  108  is found  904  to fall below a required performance, e.g. a sum of the performance requirements of storage requests allocated to the device, then one or more storage requests previously allocated to the storage device may be reallocated  906 , such as according to the method  800 , to one or more different devices  108 . The remaining performance and capacity of the storage device, as increased due to reallocation of one or more storage requests, may then be returned  908  to a pool of available devices  108  for processing according to the method  800 . 
     In some embodiments, steps  810 ,  812  of the method  800  may be periodically executed by the storage node  106  for each device  108  in order to ensure that the usage of the device  108  remains below its total capacity. If not, one or more storage requests allocated to the device may be reallocated and the performance and capacity of the device that is thereby freed up may be returned to a pool of available devices  108  for allocation according to the method  800 . 
     Referring to  FIG. 10 , data written to a primary copy of each logical storage volume may also be written to one or more clone storage volumes. In some embodiments, QoS limits may also be enforced with respect to IOPs performed on the clone storage volumes. For purposes of the method  1000  of  FIG. 10  a primary node is a node that stores all or part of a primary copy of a logical storage volume and a clone node is a node that stores all or part of a clone of the logical storage volume. A storage node  106  may function as a primary node for one or more logical storage volume and as a clone node for one or more other logical storage volumes. 
     The method  1000  may include receiving  1002  an original IOP on the primary node, such as from an application executing on a compute node  110 . A priority may be assigned  1004  to the original IOP on the primary node, such as according to the approach describe above with respect to  FIGS. 4 through 6 . Alternatively, any other approach known in the art for implementing a QoS guarantee may be used. 
     The method  1000  may further include executing  1006  the original IOP on the primary node according to the priority. For example, the original IOP, along with other IOPs, may be added to one or more queues according to priority and executed with respect to one or more storage devices  108  of the primary node. In particular, the original IOPs may be executed in an order that indicates their priority, with higher priority IOPs being more likely to be executed than lower priority IOPs. An example approach for implementing this is described above with respect to  FIGS. 4 through 6 . 
     The method  1000  may further include transmitting  1008  a clone of the original IOP to one or more clone node along with the priority determined at step  104 . Each clone node will then execute  1010  the clone IOP along with other IOPs received by the clone node according to the priority and the priorities of the other IOPs. In particular, the IOPs may be executed by the clone node in an order that indicates their priority, with higher priority IOPs being more likely to be executed than lower priority IOPs (e.g., according to the approach of  FIGS. 4 through 6 ). The clone IOP is executed on the clone node with respect to the clone of the logical storage volume referenced by the original IOP of step  1002 . For example, the clone IOP may include a reference to the clone storage volume or may be inferred to refer to the clone storage volume from a reference to the logical storage volume. 
     The clone node may transmit acknowledgment of execution of the clone IOP to the primary node. Once the original IOP is executed  1006  on the primary node and acknowledgment is received from all clone nodes, the primary node may acknowledge  1012  execution of the IOP to a source of the IOP received at step  1002 , e.g., the compute node  110  that generated the IOP of step  1002 . 
     Note that each node may operate as both a primary node and a secondary node. Accordingly, the primary node may perform the functions of the method  1000  of the primary node with respect to one or more IOPs while also performing the functions of the clone node with respect to one or more IOPs. Accordingly, both original IOPs and clone IOPs may be executed in an order according to the priorities assigned to them at step  104  according to the method  1000 . 
       FIG. 11  illustrates an alternative approach for implementing QoS constraints across a primary node and one or more clone nodes. The method  1100  may include receiving  1002  an original IOP, assigning  1004  a priority to it, and executing  1006  the original IOP according to the priority in the same manner as for the method  1000 . 
     However, in the method  1100 , a clone IOP corresponding to the original IOP is transmitted  1102  to the clone node prior to assigning  1004  a priority to it. In this manner, latency is reduced since the QoS algorithm does not need to complete before the clone node receives the clone IOP. The clone node then assigns  1104  a priority to the clone IOP. Assigning a priority may take into account loading of the clone node, i.e. other IOPs that remain to be executed. In particular, where the approach of  FIGS. 4 through 6  is implemented, IOPs will be selected according to a locally executed QoS approach that balances execution among multiple queues and takes into account actual throughput and loading of the clone node. 
     The clone node executes  1010  the clone IOP according to the priority of step  1104 , which may be in the same manner as described above with respect to  1010  of the method  1000 . In particular, the order in which IOPs are selected for execution may be performed according to their priority, with higher priority IOPs being more likely to be executed than low priority IOPs. 
     As for the method  1000 , clone nodes acknowledge completion of the clone IOPs to the primary node. Once the original IOP completes on the primary node and acknowledgments are received for all of the clone IOPs, the primary node acknowledges  1012  completion of the IOP received at step  1002 . 
       FIG. 12  is a block diagram illustrating an example computing device  1200 . Computing device  1200  may be used to perform various procedures, such as those discussed herein. The storage manager  102 , storage nodes  106 , and compute nodes  110  may have some or all of the attributes of the computing device  1200 . 
     Computing device  1200  includes one or more processor(s)  1202 , one or more memory device(s)  1204 , one or more interface(s)  1206 , one or more mass storage device(s)  1208 , one or more Input/output (I/O) device(s)  1210 , and a display device  1230  all of which are coupled to a bus  1212 . Processor(s)  1202  include one or more processors or controllers that execute instructions stored in memory device(s)  1204  and/or mass storage device(s)  1208 . Processor(s)  1202  may also include various types of computer-readable media, such as cache memory. 
     Memory device(s)  1204  include various computer-readable media, such as volatile memory (e.g., random access memory (RAM)  1214 ) and/or nonvolatile memory (e.g., read-only memory (ROM)  1216 ). Memory device(s)  1204  may also include rewritable ROM, such as Flash memory. 
     Mass storage device(s)  1208  include various computer readable media, such as magnetic tapes, magnetic disks, optical disks, solid-state memory (e.g., Flash memory), and so forth. As shown in  FIG. 12 , a particular mass storage device is a hard disk drive  1224 . Various drives may also be included in mass storage device(s)  1208  to enable reading from and/or writing to the various computer readable media. Mass storage device(s)  1208  include removable media  1226  and/or non-removable media. 
     I/O device(s)  1210  include various devices that allow data and/or other information to be input to or retrieved from computing device  1200 . Example I/O device(s)  1210  include cursor control devices, keyboards, keypads, microphones, monitors or other display devices, speakers, printers, network interface cards, modems, lenses, CCDs or other image capture devices, and the like. 
     Display device  1230  includes any type of device capable of displaying information to one or more users of computing device  1200 . Examples of display device  1230  include a monitor, display terminal, video projection device, and the like. 
     Interface(s)  1206  include various interfaces that allow computing device  1200  to interact with other systems, devices, or computing environments. Example interface(s)  1206  include any number of different network interfaces  1220 , such as interfaces to local area networks (LANs), wide area networks (WANs), wireless networks, and the Internet. Other interface(s) include user interface  1218  and peripheral device interface  1222 . The interface(s)  1206  may also include one or more peripheral interfaces such as interfaces for printers, pointing devices (mice, track pad, etc.), keyboards, and the like. 
     Bus  1212  allows processor(s)  1202 , memory device(s)  1204 , interface(s)  1206 , mass storage device(s)  1208 , I/O device(s)  1210 , and display device  1230  to communicate with one another, as well as other devices or components coupled to bus  1212 . Bus  1212  represents one or more of several types of bus structures, such as a system bus, PCI bus, IEEE 1394 bus, USB bus, and so forth. 
     For purposes of illustration, programs and other executable program components are shown herein as discrete blocks, although it is understood that such programs and components may reside at various times in different storage components of computing device  1200 , and are executed by processor(s)  1202 . Alternatively, the systems and procedures described herein can be implemented in hardware, or a combination of hardware, software, and/or firmware. For example, one or more application specific integrated circuits (ASICs) can be programmed to carry out one or more of the systems and procedures described herein. 
     In the above disclosure, reference has been made to the accompanying drawings, which form a part hereof, and in which is shown by way of illustration specific implementations in which the disclosure may be practiced. It is understood that other implementations may be utilized and structural changes may be made without departing from the scope of the present disclosure. References in the specification to “one embodiment,” “an embodiment,” “an example embodiment,” etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to affect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described. 
     Implementations of the systems, devices, and methods disclosed herein may comprise or utilize a special purpose or general-purpose computer including computer hardware, such as, for example, one or more processors and system memory, as discussed herein. Implementations within the scope of the present disclosure may also include physical and other computer-readable media for carrying or storing computer-executable instructions and/or data structures. Such computer-readable media can be any available media that can be accessed by a general purpose or special purpose computer system. Computer-readable media that store computer-executable instructions are computer storage media (devices). Computer-readable media that carry computer-executable instructions are transmission media. Thus, by way of example, and not limitation, implementations of the disclosure can comprise at least two distinctly different kinds of computer-readable media: computer storage media (devices) and transmission media. 
     Computer storage media (devices) includes RAM, ROM, EEPROM, CD-ROM, solid state drives (“SSDs”) (e.g., based on RAM), Flash memory, phase-change memory (“PCM”), other types of memory, other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other medium which can be used to store desired program code means in the form of computer-executable instructions or data structures and which can be accessed by a general purpose or special purpose computer. 
     An implementation of the devices, systems, and methods disclosed herein may communicate over a computer network. A “network” is defined as one or more data links that enable the transport of electronic data between computer systems and/or modules and/or other electronic devices. When information is transferred or provided over a network or another communications connection (either hardwired, wireless, or a combination of hardwired or wireless) to a computer, the computer properly views the connection as a transmission medium. Transmissions media can include a network and/or data links, which can be used to carry desired program code means in the form of computer-executable instructions or data structures and which can be accessed by a general purpose or special purpose computer. Combinations of the above should also be included within the scope of computer-readable media. 
     Computer-executable instructions comprise, for example, instructions and data which, when executed at a processor, cause a general purpose computer, special purpose computer, or special purpose processing device to perform a certain function or group of functions. The computer executable instructions may be, for example, binaries, intermediate format instructions such as assembly language, or even source code. Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the described features or acts described above. Rather, the described features and acts are disclosed as example forms of implementing the claims. 
     Those skilled in the art will appreciate that the disclosure may be practiced in network computing environments with many types of computer system configurations, including, an in-dash vehicle computer, personal computers, desktop computers, laptop computers, message processors, hand-held devices, multi-processor systems, microprocessor-based or programmable consumer electronics, network PCs, minicomputers, mainframe computers, mobile telephones, PDAs, tablets, pagers, routers, switches, various storage devices, and the like. The disclosure may also be practiced in distributed system environments where local and remote computer systems, which are linked (either by hardwired data links, wireless data links, or by a combination of hardwired and wireless data links) through a network, both perform tasks. In a distributed system environment, program modules may be located in both local and remote memory storage devices. 
     Further, where appropriate, functions described herein can be performed in one or more of: hardware, software, firmware, digital components, or analog components. For example, one or more application specific integrated circuits (ASICs) can be programmed to carry out one or more of the systems and procedures described herein. Certain terms are used throughout the description and claims to refer to particular system components. As one skilled in the art will appreciate, components may be referred to by different names. This document does not intend to distinguish between components that differ in name, but not function. 
     It should be noted that the sensor embodiments discussed above may comprise computer hardware, software, firmware, or any combination thereof to perform at least a portion of their functions. For example, a sensor may include computer code configured to be executed in one or more processors, and may include hardware logic/electrical circuitry controlled by the computer code. These example devices are provided herein purposes of illustration, and are not intended to be limiting. Embodiments of the present disclosure may be implemented in further types of devices, as would be known to persons skilled in the relevant art(s). 
     At least some embodiments of the disclosure have been directed to computer program products comprising such logic (e.g., in the form of software) stored on any computer useable medium. Such software, when executed in one or more data processing devices, causes a device to operate as described herein. 
     While various embodiments of the present disclosure have been described above, it should be understood that they have been presented by way of example only, and not limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail can be made therein without departing from the spirit and scope of the disclosure. Thus, the breadth and scope of the present disclosure should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents. The foregoing description has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. Further, it should be noted that any or all of the aforementioned alternate implementations may be used in any combination desired to form additional hybrid implementations of the disclosure.