Patent Publication Number: US-2023137146-A1

Title: Management of flushing working set based on barrier in page descriptor ring

Description:
BACKGROUND 
     Technical Field 
     This application relates to uninterrupted data flushing in storage systems. 
     Description of Related Art 
     A distributed storage system may include a plurality of storage devices to provide data storage to a plurality of hosts. The plurality of storage devices and the plurality of hosts may be situated in the same physical location, or in one or more physically remote locations. The storage devices and the hosts may be connected to one another over one or more computer networks. 
     The storage system may be organized into multiple nodes. 
     SUMMARY OF THE INVENTION 
     One aspect of the current technique is a method for uninterrupted data flushing in a storage system. A barrier in a page descriptor ring is determined to distinguish between a filling flushing work set (FWS) and a frozen FWS. A sequence number of an I/O request is compared to the barrier. A FWS corresponding to the I/O request is identified based on the comparison. The I/O request is committed to the identified FWS. 
     A node may set the barrier in the page descriptor ring. Alternatively, two nodes may negotiate to set the barrier in the page descriptor ring. If the sequence number of the I/O request is less than the barrier, the frozen FWS is selected. If the sequence number of the I/O request is greater than the barrier, the filling FWS is selected. A primary node may send the I/O request to a secondary node for peer commit. The secondary node may determine the sequence number for the I/O request and send the sequence number for the I/O request to the primary node. 
     Another aspect of the current technique is a system, with a processor, for uninterrupted data flushing in a storage system. The processor may be configured to perform any process in conformance with the aspect of the current techniques described above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features and advantages of the present technique will become more apparent from the following detailed description of exemplary embodiments thereof taken in conjunction with the accompanying drawings in which: 
         FIG.  1    depicts an example embodiment of a computer system  10  that may be used in connection with performing the techniques described herein; 
         FIG.  2    is a block diagram depicting exemplary nodes among which the elements of the computer system of  FIG.  1    may be distributed; 
         FIG.  3    is an exemplary diagram showing the page descriptor ring and flushing work sets on one node; 
         FIG.  4    is a schematic diagram showing the flushing work sets across two nodes; 
         FIG.  5    depicts an exemplary pointer that identifies the flushing work set that is receiving data and counters indicating the number of outstanding I/O requests on the flushing work sets; 
         FIG.  6    is an exemplary flow diagram for ensuring data in an I/O request is associated with the filling flushing work set; 
         FIG.  7    is an exemplary flow diagram for determining when flushing for a frozen FWS may begin; 
         FIG.  8    is an exemplary flow diagram for ensuring all nodes have switched their filling flushing work sets before any given node can begin flushing its frozen flushing work set; 
         FIG.  9    is an exemplary schematic diagram showing a problematic situation that can arise when a storage system has multiple nodes; 
         FIG.  10    is an exemplary diagram showing the barrier used for a page descriptor ring in the nodes of the data storage system of  FIG.  1   ; 
         FIGS.  11 - 13    are exemplary flow diagrams of how nodes coordinate logging of data on the flushing work sets; and 
         FIG.  14    is an exemplary diagram of the output of a data storage system employing the techniques of  FIGS.  9 - 14   . 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENT(S) 
     Described below are techniques for uninterrupted data flushing in a storage system. A barrier in a page descriptor ring is determined to distinguish between a filling flushing work set (FWS) and a frozen FWS. A sequence number of an I/O request is compared to the barrier. A FWS corresponding to the I/O request is identified based on the comparison. The I/O request is committed to the identified FWS. 
     A data storage system can use a page descriptor ring to hold data from I/O requests before the data is flushed to storage. In general, the head is a marker that tracks the last entry in the ring that received data, and as new data is added to the ring, the head is incremented accordingly. In many embodiments, the ring is configured such that when the head reaches the end of the ring, the marker is moved to the beginning of the ring. 
     To manage data being entered into the ring and data being flushed from the ring to storage, data structures known as flushing work sets (FWS) may be used. Each FWS may be associated with a contiguous set of entries in the page descriptor ring. The data storage system may add data to one FWS until the FWS is almost full, at which point, the data storage system switches and begins adding data to the other FWS while flushing data in the previous FWS to storage. With respect to these techniques, the FWS receiving data will be called the “filling FWS” while the FWS whose data is being flushed will be called the “frozen FWS”. 
     Challenges arise when switching the filling FWS from one FWS to the other, and when ensuring that multiple nodes associate the data with the correct FWS. For example, on a single node, I/O requests may still be in-flight when the data storage system determines that the filling FWS is approaching its full capacity and I/O requests should be directed to the other FWS. The data storage system may stop processing incoming I/O requests until the switch has been completed. However, this pause would create a queue of I/O requests and thereby degrade the performance of the storage system. Furthermore, as the storage system switches back and forth between FWSs, I/O requests would exhibit a sinusoidal latency as requests build up in the queue, are processed, and build up again. 
     Additionally, when a storage system has two nodes, one node recognizes the need to switch FWSs before the other; the node switches its own filling FWS, communicates the switch to the other node, and receives confirmation that the other node has made the switch. There is a delay between the two nodes making the switch, during which both nodes continue receiving I/O requests. 
     The techniques described herein ensure that the nodes can continue receiving and processing I/O requests when the filling FWS is being switched, both on a single node and across nodes, and that the nodes assign I/O requests to FWSs in a consistent manner. In at least some implementations in accordance with the techniques as described herein, one or more of the following advantages can be provided: improved performance due to uninterrupted processing of I/O requests and maximum effective use of the page descriptor ring. 
       FIG.  1    depicts an example embodiment of a computer system  10  that may be used in connection with performing the techniques described herein. The system  10  includes one or more data storage systems  12  connected to server or hosts  14   a - 14   n  through communication medium  18 . The system  10  also includes a management system  16  connected to one or more data storage systems  12  through communication medium  20 . In this embodiment of the system  10 , the management system  16 , and the N servers or hosts  14   a - 14   n  may access the data storage systems  12 , for example, in performing input/output (I/O) operations, data requests, and other operations. The communication medium  18  may be any one or more of a variety of networks or other type of communication connections as known to those skilled in the art. Each of the communication mediums  18  and  20  may be a network connection, bus, and/or other type of data link, such as a hardwire or other connections known in the art. For example, the communication medium  18  may be the Internet, an intranet, network or other wireless or other hardwired connection(s) by which the hosts  14   a - 14   n  may access and communicate with the data storage systems  12 , and may also communicate with other components (not shown) that may be included in the system  10 . In one embodiment, the communication medium  20  may be a LAN connection and the communication medium  18  may be an iSCSI, Fibre Channel, Serial Attached SCSI, or Fibre Channel over Ethernet connection. 
     Each of the hosts  14   a - 14   n  and the data storage systems  12  included in the system  10  may be connected to the communication medium  18  by any one of a variety of connections as may be provided and supported in accordance with the type of communication medium  18 . Similarly, the management system  16  may be connected to the communication medium  20  by any one of variety of connections in accordance with the type of communication medium  20 . The processors included in the hosts  14   a - 14   n  and management system  16  may be any one of a variety of proprietary or commercially available single or multi-processor system, or other type of commercially available processor able to support traffic in accordance with any embodiments described herein. 
     It should be noted that the particular examples of the hardware and software that may be included in the data storage systems  12  are described herein in more detail, and may vary with each particular embodiment. Each of the hosts  14   a - 14   n , the management system  16  and data storage systems  12  may all be located at the same physical site, or, alternatively, may also be located in different physical locations. In connection with communication mediums  18  and  20 , a variety of different communication protocols may be used such as SCSI, Fibre Channel, iSCSI, and the like. Some or all of the connections by which the hosts  14   a - 14   n , management system  16 , and data storage systems  12  may be connected to their respective communication medium  18 ,  20  may pass through other communication devices, such as switching equipment that may exist such as a phone line, a repeater, a multiplexer or even a satellite. In one embodiment, the hosts  14   a - 14   n  may communicate with the data storage systems  12  over an iSCSI or a Fibre Channel connection and the management system  16  may communicate with the data storage systems  12  over a separate network connection using TCP/IP. It should be noted that although  FIG.  1    illustrates communications between the hosts  14   a - 14   n  and data storage systems  12  being over a first communication medium  18 , and communications between the management system  16  and the data storage systems  12  being over a second different communication medium  20 , other embodiments may use the same connection. The particular type and number of communication mediums and/or connections may vary in accordance with particulars of each embodiment. 
     Each of the hosts  14   a - 14   n  may perform different types of data operations in accordance with different types of tasks. In the embodiment of  FIG.  1   , any one of the hosts  14   a - 14   n  may issue a data request to the data storage systems  12  to perform a data operation. For example, an application executing on one of the hosts  14   a - 14   n  may perform a read or write operation resulting in one or more data requests to the data storage systems  12 . 
     The management system  16  may be used in connection with management of the data storage systems  12 . The management system  16  may include hardware and/or software components. The management system  16  may include one or more computer processors connected to one or more I/O devices such as, for example, a display or other output device, and an input device such as, for example, a keyboard, mouse, and the like. The management system  16  may, for example, display information about a current storage volume configuration, provision resources for a data storage system  12 , and the like. 
     Each of the data storage systems  12  may include one or more data storage devices  17   a - 17   n . Unless noted otherwise, data storage devices  17   a - 17   n  may be used interchangeably herein to refer to hard disk drive, solid state drives, and/or other known storage devices. One or more data storage devices  17   a - 17   n  may be manufactured by one or more different vendors. Each of the data storage systems included in  12  may be inter-connected (not shown). Additionally, the data storage systems  12  may also be connected to the hosts  14   a - 14   n  through any one or more communication connections that may vary with each particular embodiment. The type of communication connection used may vary with certain system parameters and requirements, such as those related to bandwidth and throughput required in accordance with a rate of I/O requests as may be issued by the hosts  14   a - 14   n , for example, to the data storage systems  12 . It should be noted that each of the data storage systems  12  may operate stand-alone, or may also be included as part of a storage area network (SAN) that includes, for example, other components such as other data storage systems  12 . The particular data storage systems  12  and examples as described herein for purposes of illustration should not be construed as a limitation. Other types of commercially available data storage systems  12 , as well as processors and hardware controlling access to these particular devices, may also be included in an embodiment. 
     In such an embodiment in which element  12  of  FIG.  1    is implemented using one or more data storage systems  12 , each of the data storage systems  12  may include code thereon for performing the techniques as described herein. 
     Servers or hosts, such as  14   a - 14   n , provide data and access control information through channels on the communication medium  18  to the data storage systems  12 , and the data storage systems  12  may also provide data to the host systems  14   a - 14   n  also through the channels  18 . The hosts  14   a - 14   n  may not address the disk drives of the data storage systems  12  directly, but rather access to data may be provided to one or more hosts  14   a - 14   n  from what the hosts  14   a - 14   n  view as a plurality of logical devices or logical volumes (LVs). The LVs may or may not correspond to the actual disk drives. For example, one or more LVs may reside on a single physical disk drive. Data in a single data storage system  12  may be accessed by multiple hosts  14   a - 14   n  allowing the hosts  14   a - 14   n  to share the data residing therein. An LV or LUN (logical unit number) may be used to refer to the foregoing logically defined devices or volumes. 
     The data storage system  12  may be a single unitary data storage system, such as single data storage array, including two storage processors  114 A,  114 B or computer processing units. Techniques herein may be more generally use in connection with any one or more data storage system  12  each including a different number of storage processors  114  than as illustrated herein. The data storage system  12  may include a data storage array  116 , including a plurality of data storage devices  17   a - 17   n  and two storage processors  114 A,  114 B. The storage processors  114 A,  114 B may include a central processing unit (CPU) and memory and ports (not shown) for communicating with one or more hosts  14   a - 14   n . The storage processors  114 A,  114 B may be communicatively coupled via a communication medium such as storage processor bus  19 . The storage processors  114 A,  114 B may be included in the data storage system  12  for processing requests and commands. In connection with performing techniques herein, an embodiment of the data storage system  12  may include multiple storage processors  114  including more than two storage processors as described. Additionally, the two storage processors  114 A,  114 B may be used in connection with failover processing when communicating with the management system  16 . Client software on the management system  16  may be used in connection with performing data storage system management by issuing commands to the data storage system  12  and/or receiving responses from the data storage system  12  over connection  20 . In one embodiment, the management system  16  may be a laptop or desktop computer system. 
     The particular data storage system  12  as described in this embodiment, or a particular device thereof, such as a disk, should not be construed as a limitation. Other types of commercially available data storage systems  12 , as well as processors and hardware controlling access to these particular devices, may also be included in an embodiment. 
     In some arrangements, the data storage system  12  provides block-based storage by storing the data in blocks of logical storage units (LUNs) or volumes and addressing the blocks using logical block addresses (LBAs). In other arrangements, the data storage system  12  provides file-based storage by storing data as files of a file system and locating file data using inode structures. In yet other arrangements, the data storage system  12  stores LUNs and file systems, stores file systems within LUNs, and so on. 
     The two storage processors  114 A,  114 B (also referred to herein as “SP”) may control the operation of the data storage system  12 . The processors may be configured to process requests as may be received from the hosts  14   a - 14   n , other data storage systems  12 , management system  16 , and other components connected thereto. Each of the storage processors  114 A,  114 B may process received requests and operate independently and concurrently with respect to the other processor. With respect to data storage management requests, operations, and the like, as may be received from a client, such as the management system  16  of  FIG.  1    in connection with the techniques herein, the client may interact with a designated one of the two storage processors  114 A,  114 B. Upon the occurrence of failure of one the storage processors  114 A,  114 B, the other remaining storage processors  114 A,  114 B may handle all processing typically performed by both storage processors  114 A. 
       FIG.  2    is a block diagram depicting exemplary nodes  205   a ,  205   b ,  205   c  (individually and collectively, “ 205 ”) among which the elements of the storage system  12  may be distributed. Although  FIG.  2    depicts three nodes  205   a ,  205   b ,  205   c , various embodiments of the invention may include any number of nodes. The nodes  205  may form a cluster. Each node  205  may receive I/O requests, and communicate with one another to ensure that the data on the nodes  205  are consistent with one another. 
       FIG.  3    is an exemplary diagram showing the page descriptor ring  305  and FWSs  320   a ,  320   b  (individually and collectively, “ 320 ”) on one node  205   a . The page descriptor ring  305  includes multiple entries  310   a ,  310   b ,  310   n  (“ 310 ”), each corresponding to a page of data. The head  330  is a marker that points to the next entry  310  available for receiving data. In some embodiments, the head  330  is represented as an offset within the page descriptor  305 , or a sequence ID number. When the node  205   a  receives new data to store, the data is stored in the entry  310  corresponding to the head  330  and the head  330  is advanced to the next entry  310 . As shown, each FWS  320  corresponds to a different set of entries  310  in the page ring descriptor  305 . When the node  205   a  determines that the filling FWS  320  should be switched, e.g., from FWS  320   a  to FWS  320   b , the node  205   a  continues to add data to the ring  305 . FWS  320   a  becomes the frozen FWS  320  whose associated entries in the ring  305  are flushed to storage, while FWS  320   b  becomes the filling FWS and receives incoming data. 
       FIG.  4    is a schematic diagram showing the FWSs  320   a ,  320   b ,  320   a ′,  320   b ′ across two nodes  205   a ,  205   b . The FWSs  320 ,  320 ′ across the nodes  205  mirror one another; the frozen FWSs  320   a ,  320   a ′ are associated with the same data (i.e., the same entries  310  in the page descriptor ring  305 ), as are the filling FWSs  320   b ,  320   b ′. When the filling FWSs  320   b ,  320   b ′ are receiving incoming data from I/O requests, both nodes  205   a ,  205   b  flush data associated with the frozen FWSs  320   a ,  320   a ′ to storage. 
     As previously explained, switching the filling FWS  320  from one FWS  320   a  to the other  320   b  poses a number of challenges. When a node  205   a  determines that the filling FWS  320   b  is reaching capacity, the node  205   a  already has I/O requests whose data has not yet been stored in the page descriptor ring  305 , and the node  205   a  will continue to receive I/O requests while the switching is the occurring. One technique would be to identify the last I/O request for the current filling FWS  320   b , queue subsequent I/O requests until data for all I/O requests preceding and including the identified I/O request have been added to the page descriptor ring  305 , switch the filling FWS from  320   b  to  320   a , and resume servicing I/O requests, beginning with requests in the queue. However, this approach would increase latency periodically whenever the nodes  205   a ,  205   b  switch FWSs  320  and thereby degrade performance. 
     An exemplary solution uses a pointer  505  that identifies the FWS  320  serving as the filling FWS  320  and counters  515   a ,  515   b  that track the number of I/O requests, for their respective FWSs  320   a ,  320   b , that have not yet been logged in the page descriptor ring  305 , as shown in  FIG.  5   . In this embodiment, the counters  515   a ,  515   b  pertain to the I/O requests for the entire FWSs  320 , but in other embodiments, an FWS  320  may have multiple counters  515 , each associated with a different processor  114  in the data storage system  12 . 
     When a node  205   a  determines that the filling FWS  320  is nearing its capacity, and/or that the frozen FWS  320  has finished flushing its data to storage, the node  205   a  may switch FWSs  320  by setting the pointer  505  to the desired FWS  320 . After the switch occurs, all subsequent I/O requests are directed to the new filling FWS  320 . 
       FIG.  6    is an exemplary flow diagram  600  for ensuring data in an I/O request is associated with the filling FWS  320 . When a node  205  receives an I/O request, the node  205  saves a copy of the pointer  505  to the filling FWS  320  (step  605 ). The counter  515  for pending I/O requests to the FWS  320  referenced in the copy of the pointer  505  is incremented (step  610 ). The saved copy of the pointer  505  is then compared to the pointer  505  to the filling FWS  320  (step  615 ), to determine whether the filling FWS  320  has recently changed. If the pointers are not the same, the I/O request will not be logged to the FWS  320  referenced by the saved copy of the pointer  505 , as that FWS  320  has since been frozen. Thus, the counter  515  for the FWS in the saved copy of the pointer  505  is decremented, and control returns to step  605 , where a new copy of the pointer  505  to the filling FWS  320  is saved to get the most current identification of the filling FWS  320 . 
     If the saved copy of the pointer  505  matches the pointer  505  for the filling FWS  320  itself, then the I/O request is processed for this filling FWS  320 . The I/O request is committed to the filling FWS  320  (step  625 ), referenced by both the saved copy of the pointer  505  and the pointer  505 . Consequently, data for the I/O request is logged in the page descriptor ring  305 . To ensure the I/O request is also committed on peer nodes, the node  205  sends the I/O request and the identity of the filling FWS  320  to each peer node (step  630 ). Because the I/O request has been logged in the page descriptor ring  305 , the counter  515  for the FWS  320  in the saved copy of the pointer  505  is decremented (step  635 ). 
     To demonstrate this method in more detail, consider an example in which the filling FWS  320  is FWS  320   a , and the node  205  switches the filling FWS  320  to FWS  320   b . When the node  205   a  receives an I/O request, the node  205   a  saves a copy of the pointer  505  to the filling FWS  320 . Thus, the node  205   a  stores a pointer to FWS  320   a . To indicate that an I/O request for FWS  320   a  is in flight, the node  205   a  increments the counter  515   a  for FWS  320   a . Because the filling FWS  320  may have changed while the counter  515   a  was being accessed and incremented, the node  205  compares the saved pointer to the pointer  505  for the filling FWS  320 . 
     If the filling FWS  320  has not been switched, then data for the I/O request will be logged for the FWS  320   a . The node  205   a  identifies the head  330  of the page descriptor ring  305 , stores the data in the corresponding entry  310 , and advances the head  330 . Furthermore, to ensure other nodes  205   b ,  205   c  remain consistent with the node  205   a , the node  205   a  sends the I/O request to them, while also identifying the FWS  320   s  that should be associated with the data. In turn, the other nodes  205   b ,  205   c  associate the data with the FWS  320   a ′,  320   a ″ corresponding to the FWS  320   a  of the node  205   a . After the other nodes  205   b ,  205   c  have confirmed logging of the I/O request, the node  205   a  decrements the counter  515   a  for FWS  320   a  because one of the I/O requests for the FWS  320   a  has been completed. 
     However, if the filling FWS  320  has been switched to FWS  320   b  in the time the counter  515   a  was being accessed and incremented, the saved pointer to FWS  320   a  will not match the pointer  505  for the filling FWS  320 . Thus, data for the I/O request will not be associated with FWS  320   a , and the node  205  decrements the counter  515   a  for FWS  320   a . The node  205  again saves a copy of the pointer  505  to the filling FWS  320 , which is now FWS  320   b . Because the saved pointer now matches the pointer  505  for the filling FWS  320  (i.e., FWS  320   b ), the node  205   a  proceeds to log the I/O request with the FWS  320   b . Because the pointer  505  for the filling FWS  320  is checked twice before an I/O request is logged, the techniques described herein prevent an I/O request from being associated with an erroneous FWS  320 . 
       FIG.  7    is an exemplary flow diagram  700  for determining when flushing for a frozen FWS  320  may begin (i.e., when all I/O requests for the frozen FWS  320  have been logged in the page descriptor ring  305 ). When a node  205  determines that the filling FWS  320  is nearing its capacity, and/or the frozen FWS  320  has been flushed, the node  205  atomically switches the pointer  505  for the filling FWS from one FWS  320   a  to another  320   b  (step  705 ). In some embodiments, the node  205  sets the pointer  505 , and any I/O requests that have not yet been logged and those that are subsequently received are logged with the FWS  320  newly referenced by the pointer  505 . 
     The previous filling FWS  320  is saved as the frozen FWS  320  (step  710 ). The counter  515  associated with the frozen FWS  320  indicates the number of I/O requests for the now-frozen FWS  320  that have not yet been logged. Any given I/O request may still be in the process of being committed to the page descriptor ring  305  on the node  205 , the ring  305 ′ on other nodes  205 , or both. As described above, the counter  515  is decremented each time an I/O request is logged on the node  205  and its peers  205 . The counter  515  for the now-frozen FWS  320  may be compared to zero (step  715 ). If the counter  515  is non-zero, the counter  515  is rechecked after a predetermined interval of time, such as such 100 ms (step  720 ). In this manner, by monitoring the counter  515 , the node  205  can determine when all I/O requests associated with the now-frozen FWS  320  have been logged in the page descriptor ring  305 , such that flushing of the frozen FWS  320  can begin. 
     The filling and frozen FWSs  320  must be consistent across the nodes  205 , with respect to both the I/O requests that the nodes  205  receive and the FWS  320  that is being filled at any given time. Because any given I/O request arrives at one node  205  before it is propagated to other nodes  205  and the nodes  205  will receive disparate volumes of I/O requests, one node  205  will need to switch FWSs  320  before the others  205 . However, a node  205  cannot begin flushing data from a FWS  320  to storage until all nodes  205  have switched their filling FWSs  320 . 
       FIG.  8    is an exemplary flow diagram  800  for ensuring all nodes have switched their filling FWSs  320  before any given node  205  can begin flushing its frozen FWS  320 . The node  205  that first initiates its switching atomically switches its pointer  505  for the filling FWS  320  to another FWS  320  (step  805 ). The node  205  instructs its peer node  205  to switch its filling FWS (step  810 ). The peer node  205  initiate its own switch of FWSs  320 , and confirms to the node  205  that its switch has been completed. The requesting node  205  receives a message from the peer node  205  that its filling FWS  320  has been switched (step  815 ). After the requesting node  205  receives such messages from every other node  205  in the data storage system  12 , the requesting node  205  begins flushing data from its frozen FWS  320  to storage (step  820 ). 
     Having multiple nodes  205  poses additional complications. Although nodes  205  coordinate the switching of their filling FWSs  320 , the nodes  205  will not switch at exactly the same time. During the delay between one node  205  switching its filling FWS  320  and its peer nodes  205  accomplishing the same objective, the peer nodes  205  continue receiving I/O requests that they would normally continue associating with their current filling FWS  320 . In this manner, the peer nodes  205  would log data with a FWS  320  that has already been frozen on another node  205 . Furthermore, the data might be logged on page descriptor rings  305  on different nodes  205  in different orders, resulting in a situation such as that depicted in  FIG.  9   . Moreover, certain I/O requests may have data dependencies such that it would be advantageous for the data to be logged with the same FWS  320  so as to be flushed to storage together. If dependent data is logged to different FWSs  320 , data may not be available when it is needed because the storage system  12  must wait for the new filling FWS  320  to fill before it will be frozen and flushed. 
     To remedy these issues, the data storage system  12  uses a barrier  1050  in the page descriptor rings  305  for the nodes  205 , as shown in  FIG.  10   . The barrier  1050  may correspond to a sequence number for the I/O requests. After the barrier  1050  is set, all entries  310  prior to the barrier  1050  in the page descriptor ring  305  will be associated with the frozen FWS  320 , whereas entries  310  thereafter will be associated with the filling FWS  320 . The nodes  205  communicate with one another regarding the sequence number for I/O requests, the barrier  1050 , and FWS  320  to which any given I/O request should be committed. 
     When a node  205  switches its filling FWS  320 , the node  205  sets a barrier  1050  in the page descriptor ring  305  to distinguish data to be associated with the different FWSs  320 . The node  205  synchronizes the barrier  1050  with the other nodes  205 . In some embodiments, the node  205  negotiates the barrier  1050  with the peer nodes  205 . However, barrier setting may happen at any point of I/O committing and thus may not be available during particular steps of any given I/O commit process. 
     For illustrative purposes, in the following example, the node  205  that initiates switching of the FWS  320  is the primary node  205   a , and the peer nodes are secondary nodes  205   b ,  205   c , etc. When the primary node  205   a  receives an I/O request, the primary node  205   a  determines whether a barrier  1050  has been set. If no barrier has been set, the primary node  205   a  increases the counter  515  for the FWS  320   a  being frozen; otherwise, the counter  515  for the new filling FWS  320   b  is incremented. 
     The primary node  205   a  sends the I/O request and the barrier  1050  to a secondary node  205   b  to be committed. The secondary node  205   b  determines the sequence number for the I/O request, and compares this value with the barrier  1050 . If the sequence number is lower than the barrier  1050 , the secondary node  205   b  associates the data with the frozen FWS  320   a ′, and if the sequence number is higher, the secondary node  205   b  selects the filling FWS  320   b ′. The secondary node  205   b  commits the I/O request to the selected FWS  320  and confirms the commitment to the primary node  205   a . The confirmation may identify the FWS  320  chosen by the secondary node  205   b . The primary node  205   a  then logs the I/O request with the FWS  320  identified by the secondary node  205   b , and decrements the counter  515  that it previously incremented. 
     When the secondary node  205   b  receives an I/O request, the secondary node  205   b  sends the I/O request to the primary node  205   a  to be committed. The primary node  205   a  determines the sequence number for the I/O request, and compares this value with the barrier  1050  that it has set. If the sequence number is lower than the barrier  1050 , the primary node  205  logs the data with the frozen FWS  320   a , and if the sequence number is higher, the primary node  205  selects the filling FWS  320   b . The primary node  205   a  commit the I/O request to the selected FWS  320  and confirms the commitment to the secondary node  205   b , identifying the FWS  320  to which the secondary node  205   b  should commit the I/O request. The secondary node  205   b  then logs the I/O request with the FWS  320  identified by the primary node  205   a , and decrements the counter  515  that it previously incremented. 
       FIGS.  11 - 13    are exemplary flow diagrams  1100 - 1300  of how nodes coordinate logging of data on the flushing work sets. With respect to  FIG.  11   , the data storage system determines the barrier in the page descriptor ring between the filling and frozen FWSs (step  1105 ). As described above, the primary node  205   a  may select the barrier, or the primary node  205   a  may negotiate the barrier with one or more secondary nodes  205   b . The sequence number of an I/O request may be compared to the barrier (step  1110 ). The FWS  320  corresponding to the I/O request may be identified based on the comparison (step  1115 ). If the sequence number is lower than the barrier, then the I/O request is logged with the frozen FWS  320 ; otherwise, the I/O request is logged with the filling FWS  320 . Then, the I/O request is committed to the identified FWS  320  (step  1120 ). 
     With respect to  FIG.  12   , the primary node  205   a  receives an I/O request. The primary node  205   a  logs the request (step  1205 ) and sends the request and the barrier to the secondary node  205   b . The secondary node  205   b  determines a sequence number for the I/O request. The secondary node  205   b  identifies the FWS  320  for the I/O request based on the barrier (step  1210 ). The secondary node  205   b  commits the I/O request to the identified FWS  320  (step  1215 ) and confirms the commitment to the primary node  205   a . The commitment may identify the FWS associated with the I/O request. The primary node  205   a  receives confirmation of the commitment (step  1120 ), and uses the identification of the FWS  320  from the secondary node  205   b  to commit the I/O request to the identified FWS  320  (step  1225 ). 
     With respect to  FIG.  13   , the secondary node  205   b  receives an I/O request and logs it (step  1305 ). The secondary node  205   b  sends the I/O request to the primary node  205   a . The primary node  205   a  determines a sequence number for the I/O request, and compares this number against a barrier to identify the FWS  320  to which the request shall be committed (step  1310 ). The primary node  205   a  commits the I/O request to this FWS  320  (step  1315 ). The primary node  205   a  confirms the commitment to the secondary node  205   b , including the identity of the associated FWS  320 . The secondary node  205   b  receives the confirmation of the commitment and commits the I/O request to the FWS  320  identified by the primary node  205   a  (step  1325 ). Using such techniques, primary and secondary nodes  205   a ,  205   b  may achieve consistent associations between data and FWSs  320 , as depicted in  FIG.  14   . 
     In some situations, one node  205  may go offline and need to reboot. When the node  205  reboots, the node  205  may communicate with a surviving node regarding the contents of its FWSs  320 . The rebooting node  205  may obtain the barrier from the surviving node  205  and use this barrier to reconstruct the FWSs  320 . The rebooting node  205  may compare the sequence ID numbers of the I/O requests it obtains from the surviving node  205  to the barrier, and sort the I/O requests between the FWSs  320  accordingly. 
     It should again be emphasized that the implementations described above are provided by way of illustration, and should not be construed as limiting the present invention to any specific embodiment or group of embodiments. For example, the invention can be implemented in other types of systems, using different arrangements of processing devices and processing operations. Also, message formats and communication protocols utilized may be varied in alternative embodiments. Moreover, various simplifying assumptions made above in the course of describing the illustrative embodiments should also be viewed as exemplary rather than as requirements or limitations of the invention. Numerous alternative embodiments within the scope of the appended claims will be readily apparent to those skilled in the art. 
     Furthermore, as will be appreciated by one skilled in the art, the present disclosure may be embodied as a method, system, or computer program product. Accordingly, the present disclosure may take the form of an entirely hardware embodiment, an entirely software embodiment (including firmware, resident software, micro-code, etc.) or an embodiment combining software and hardware aspects that may all generally be referred to herein as a “circuit,” “module” or “system.” Furthermore, the present disclosure may take the form of a computer program product on a computer-usable storage medium having computer-usable program code embodied in the medium. 
     The flowchart and block diagrams in the figures illustrate the architecture, functionality, and operation of possible implementations of systems, methods and computer program products according to various embodiments of the present disclosure. In this regard, each block in the flowchart or block diagrams may represent a module, segment, or portion of code, which comprises one or more executable instructions for implementing the specified logical function(s). It should also be noted that, in some alternative implementations, the functions noted in the block may occur out of the order noted in the figures. For example, two blocks shown in succession may, in fact, be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved. It will also be noted that each block of the block diagrams and/or flowchart illustration, and combinations of blocks in the block diagrams and/or flowchart illustration, can be implemented by special purpose hardware-based systems that perform the specified functions or acts, or combinations of special purpose hardware and computer instructions. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the disclosure. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising”, when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     While the invention has been disclosed in connection with preferred embodiments shown and described in detail, their modifications and improvements thereon will become readily apparent to those skilled in the art. Accordingly, the spirit and scope of the present invention should be limited only by the following claims.