Patent Publication Number: US-11048667-B1

Title: Data re-MRU to improve asynchronous data replication performance

Description:
BACKGROUND 
     Field of the Invention 
     This invention relates to systems and methods for improving transfer performance in asynchronous data replication environments. 
     Background of the Invention 
     In asynchronous data replication environments such as z/OS Global Mirror (also referred to as “XRC”) and Global Mirror, data is asynchronously mirrored from a primary storage system to a secondary storage system to maintain two consistent copies of the data. The primary and secondary storage systems may be located at different sites, perhaps hundreds or even thousands of miles away from one another. In the event an outage occurs at the primary storage system, host I/O may be redirected to the secondary storage system, thereby enabling continuous operations. When the outage is corrected or repaired at the primary storage system, host I/O may be redirected back to the primary storage system. 
     In asynchronous data replication environments such as XRC, updated data elements (e.g., tracks) are written to cache of the primary storage system. The updated data elements are recorded in an out-of-sync bitmap (i.e.,  00 S) to indicate that they need to be mirrored to the secondary storage system. Data elements that are written to the primary cache may be destaged to backend storage drives residing on the primary storage system, and eventually demoted. The destage and demotion processes are independent from the asynchronous mirroring process. 
     In certain cases, the primary storage system may destage and demote a data element before the asynchronous mirroring takes place. In such cases, the data element may need to be re-staged to the primary cache so it can then be mirrored to the secondary storage system. In other cases, the data element may be asynchronously mirrored to the secondary storage system before the data element is destaged and demoted from the primary cache. This scenario is preferred, since it only requires mirroring modified portions (e.g., sectors) of the updated data element to the secondary storage system, whereas a scenario that re-stages the data element to the primary cache not only requires moving the data element from the backend storage drives to the primary cache, but also requires mirroring the entire data element (e.g., track) to the secondary storage system. Thus, re-staging and mirroring an unmirrored data element may be significantly less efficient than mirroring the data element prior to its destage and/or demotion. 
     SUMMARY 
     The invention has been developed in response to the present state of the art and, in particular, in response to the problems and needs in the art that have not yet been fully solved by currently available systems and methods. Accordingly, the invention has been developed to improve asynchronous data replication between a primary storage system and a secondary storage system. The features and advantages of the invention will become more fully apparent from the following description and appended claims, or may be learned by practice of the invention as set forth hereinafter. 
     Consistent with the foregoing, a method for improving asynchronous data replication between a primary storage system and a secondary storage system is disclosed. In one embodiment, such a method includes monitoring, in a cache of the primary storage system, unmirrored data elements needing to be mirrored, but that have not yet been mirrored, from the primary storage system to the secondary storage system. The method maintains an LRU list designating an order in which data elements are demoted from the cache. The method determines whether a data element at an LRU end of the LRU list is an unmirrored data element. In the event the data element at the LRU end of the LRU list is an unmirrored data element, the method moves the data element to an MRU end of the LRU list. 
     A corresponding computer program product and system are also disclosed and claimed herein. 
    
    
     
       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 high-level block diagram showing one example of a network environment in which systems and methods in accordance with the invention may be implemented; 
         FIG. 2  is a high-level block diagram showing one example of a storage system for use in the network environment of  FIG. 1 ; 
         FIG. 3  is a high-level block diagram showing an example of an asynchronous data replication environment; 
         FIG. 4  is a high-level block diagram showing a regular LRU list that may be maintained for cache in a primary storage system; 
         FIG. 5  is a high-level block diagram showing how the regular LRU list of  FIG. 4  may be used to improve asynchronous data replication performance; 
         FIG. 6  is a flow diagram showing one embodiment of a method for demoting a data element from cache in the environment illustrated in  FIGS. 4 and 5 ; 
         FIG. 7  is a high-level block diagram showing a regular LRU list and a transfer-pending LRU list that may be maintained for cache in a primary storage system; 
         FIG. 8  is a high-level block diagram showing how the regular LRU list and transfer-pending LRU list of  FIG. 7  may be used to improve asynchronous data replication performance; 
         FIG. 9  is a flow diagram showing one embodiment of a method for demoting a data element from cache in the environment illustrated in  FIGS. 7 and 8 ; 
         FIG. 10  is a flow diagram showing one embodiment of a method for demoting unmirrored data elements from cache in the environment illustrated in  FIGS. 7 and 8 ; 
         FIG. 11  is a high-level block diagram showing a regular LRU list, transfer-pending LRU list, and reserved area that may be used to improve asynchronous data replication performance; 
         FIG. 12  is a high-level block diagram showing how the regular LRU list, transfer-pending LRU list, and reserved area of  FIG. 11  may be used to improve asynchronous data replication performance; 
         FIG. 13  is a flow diagram showing one embodiment of a method for demoting a data element from the higher performance portion in the environment illustrated in  FIGS. 11 and 12 ; and 
         FIG. 14  is a flow diagram showing one embodiment of a method for transferring an unmirrored data element from the primary storage system to the secondary storage system in the environment illustrated in  FIGS. 11 and 12 . 
     
    
    
     DETAILED DESCRIPTION 
     It will be readily understood that the components of the present invention, as generally described and illustrated in the Figures herein, could be arranged and designed in a wide variety of different configurations. Thus, the following more detailed description of the embodiments of the invention, as represented in the Figures, is not intended to limit the scope of the invention, as claimed, but is merely representative of certain examples of presently contemplated embodiments in accordance with the invention. The presently described embodiments will be best understood by reference to the drawings, wherein like parts are designated by like numerals throughout. 
     The present invention may be embodied as a system, method, and/or computer program product. The computer program product may include a computer readable storage medium (or media) having computer readable program instructions thereon for causing a processor to carry out aspects of the present invention. 
     The computer readable storage medium may be a tangible device that can retain and store instructions for use by an instruction execution device. The computer readable storage medium may be, for example, but is not limited to, an electronic storage device, a magnetic storage device, an optical storage device, an electromagnetic storage device, a semiconductor storage device, or any suitable combination of the foregoing. A non-exhaustive list of more specific examples of the computer readable storage medium includes the following: a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), a static random access memory (SRAM), a portable compact disc read-only memory (CD-ROM), a digital versatile disk (DVD), a memory stick, a floppy disk, a mechanically encoded device such as punch-cards or raised structures in a groove having instructions recorded thereon, and any suitable combination of the foregoing. A computer readable storage medium, as used herein, is not to be construed as being transitory signals per se, such as radio waves or other freely propagating electromagnetic waves, electromagnetic waves propagating through a waveguide or other transmission media (e.g., light pulses passing through a fiber-optic cable), or electrical signals transmitted through a wire. 
     Computer readable program instructions described herein can be downloaded to respective computing/processing devices from a computer readable storage medium or to an external computer or external storage device via a network, for example, the Internet, a local area network, a wide area network and/or a wireless network. The network may comprise copper transmission cables, optical transmission fibers, wireless transmission, routers, firewalls, switches, gateway computers and/or edge servers. A network adapter card or network interface in each computing/processing device receives computer readable program instructions from the network and forwards the computer readable program instructions for storage in a computer readable storage medium within the respective computing/processing device. 
     Computer readable program instructions for carrying out operations of the present invention may be assembler instructions, instruction-set-architecture (ISA) instructions, machine instructions, machine dependent instructions, microcode, firmware instructions, state-setting data, or either source code or object code written in any combination of one or more programming languages, including an object oriented programming language such as Smalltalk, C++ or the like, and conventional procedural programming languages, such as the “C” programming language or similar programming languages. 
     The computer readable program instructions may execute entirely on a user&#39;s computer, partly on a user&#39;s computer, as a stand-alone software package, partly on a user&#39;s computer and partly on a remote computer, or entirely on a remote computer or server. In the latter scenario, a remote computer may be connected to a user&#39;s computer through any type of network, including a local area network (LAN) or a wide area network (WAN), or the connection may be made to an external computer (for example, through the Internet using an Internet Service Provider). In some embodiments, electronic circuitry including, for example, programmable logic circuitry, field-programmable gate arrays (FPGA), or programmable logic arrays (PLA) may execute the computer readable program instructions by utilizing state information of the computer readable program instructions to personalize the electronic circuitry, in order to perform aspects of the present invention. 
     Aspects of the present invention are described herein with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and/or block diagrams, and combinations of blocks in the flowchart illustrations and/or block diagrams, may be implemented by computer readable program instructions. 
     These computer readable program instructions may be provided to a processor of a general purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks. These computer readable program instructions may also be stored in a computer readable storage medium that can direct a computer, a programmable data processing apparatus, and/or other devices to function in a particular manner, such that the computer readable storage medium having instructions stored therein comprises an article of manufacture including instructions which implement aspects of the function/act specified in the flowchart and/or block diagram block or blocks. 
     The computer readable program instructions may also be loaded onto a computer, other programmable data processing apparatus, or other device to cause a series of operational steps to be performed on the computer, other programmable apparatus, or other device to produce a computer-implemented process, such that the instructions which execute on the computer, other programmable apparatus, or other device implement the functions/acts specified in the flowchart and/or block diagram block or blocks. 
     Referring to  FIG. 1 , one example of a network environment  100  is illustrated. The network environment  100  is presented to show one example of an environment where embodiments of the invention may operate. The network environment  100  is presented only by way of example and not limitation. Indeed, the systems and methods disclosed herein may be applicable to a wide variety of different network environments in addition to the network environment  100  shown. 
     As shown, the network environment  100  includes one or more computers  102 ,  106  interconnected by a network  104 . The network  104  may include, for example, a local-area-network (LAN)  104 , a wide-area-network (WAN)  104 , the Internet  104 , an intranet  104 , or the like. In certain embodiments, the computers  102 ,  106  may include both client computers  102  and server computers  106  (also referred to herein as “hosts”  106  or “host systems”  106 ). In general, the client computers  102  initiate communication sessions, whereas the server computers  106  wait for and respond to requests from the client computers  102 . In certain embodiments, the computers  102  and/or servers  106  may connect to one or more internal or external direct-attached storage systems  112  (e.g., arrays of hard-disk drives, solid-state drives, tape drives, etc.). These computers  102 ,  106  and direct-attached storage systems  112  may communicate using protocols such as ATA, SATA, SCSI, SAS, Fibre Channel, or the like. 
     The network environment  100  may, in certain embodiments, include a storage network  108  behind the servers  106 , such as a storage-area-network (SAN)  108  or a LAN  108  (e.g., when using network-attached storage). This network  108  may connect the servers  106  to one or more storage systems  110 , such as arrays  110   a  of hard-disk drives or solid-state drives, tape libraries  110   b , individual hard-disk drives  110   c  or solid-state drives  110   c , tape drives  110   d , CD-ROM libraries, or the like. To access a storage system  110 , a host system  106  may communicate over physical connections from one or more ports on the host  106  to one or more ports on the storage system  110 . A connection may be through a switch, fabric, direct connection, or the like. In certain embodiments, the servers  106  and storage systems  110  may communicate using a networking standard such as Fibre Channel (FC) or iSCSI. 
     Referring to  FIG. 2 , one embodiment of a storage system  110   a  containing an array of storage drives  204  (e.g., hard-disk drives and/or solid-state drives) is illustrated. As shown, the storage system  110   a  includes a storage controller  200 , one or more switches  202 , and one or more storage drives  204  such as hard disk drives and/or solid-state drives (such as flash-memory-based drives). The storage controller  200  may enable one or more hosts  106  (e.g., open system and/or mainframe servers  106 ) to access data in the one or more storage drives  204 . The storage drives  204  may, in certain embodiments, be configured in RAID arrays of various RAID levels to provide desired levels of I/O performance and/or data redundancy. Logical volumes  302  (as shown in  FIG. 3 ) may be carved from these RAID arrays. 
     In selected embodiments, the storage controller  200  includes one or more servers  206 . The storage controller  200  may also include host adapters  208  and device adapters  210  to connect the storage controller  200  to host devices  106  and storage drives  204 , respectively. During normal operation (when both servers  206  are operational), the servers  206  may manage I/O to different logical subsystems (LSSs) within the enterprise storage system  110   a . For example, in certain configurations, a first server  206   a  may handle I/O to even LSSs, while a second server  206   b  may handle I/O to odd LSSs. These servers  206   a ,  206   b  may provide redundancy to ensure that data is always available to connected hosts  106 . Thus, when one server  206   a  fails, the other server  206   b  may pick up the I/O load of the failed server  206   a  to ensure that I/O is able to continue between the hosts  106  and the storage drives  204 . This process may be referred to as a “failover.” 
     In selected embodiments, each server  206  includes one or more processors  212  and memory  214 . The memory  214  may include volatile memory (e.g., RAM) as well as non-volatile memory (e.g., ROM, EPROM, EEPROM, flash memory, local disk drives, local solid state drives etc.). The volatile and non-volatile memory may, in certain embodiments, store software modules that run on the processor(s)  212  and are used to access data in the storage drives  204 . These software modules may manage all read and write requests to logical volumes  302  in the storage drives  204 . 
     In selected embodiments, the memory  214  includes a cache  218 , such as a DRAM cache  218 . Whenever a host  106  (e.g., an open system or mainframe server  106 ) performs a read operation, the server  206  that performs the read may fetch data from the storages drives  204  and save it in its cache  218  in the event it is required again. If the data is requested again by a host  106 , the server  206  may fetch the data from the cache  218  instead of fetching it from the storage drives  204 , saving both time and resources. Similarly, when a host  106  performs a write, the server  106  that receives the write request may store the write in its cache  218 , and destage the write to the storage drives  204  at a later time. When a write is stored in a cache  218 , the write may also be stored in non-volatile storage (NVS)  220  of the opposite server  206  so that the write can be recovered by the opposite server  206  in the event the first server  206  fails. 
     One example of a storage system  110   a  having an architecture similar to that illustrated in  FIG. 2  is the IBM DS8000® enterprise storage system. The DS8000® is a high-performance, high-capacity storage controller providing disk and solid-state storage that is designed to support continuous operations. Nevertheless, the systems and methods disclosed herein are not limited to the IBM DS8000® enterprise storage system, but may be implemented in any comparable or analogous storage system or group of storage systems, regardless of the manufacturer, product name, or components or component names associated with the system. Any storage system that could benefit from one or more embodiments of the invention is deemed to fall within the scope of the invention. Thus, the IBM DS8000® is presented only by way of example and is not intended to be limiting. 
     Referring to  FIG. 3 , in certain embodiments the host systems  106  and storage systems  110   a  described in  FIGS. 1 and 2  may be used in a data replication environment, such as an asynchronous data replication environment  300 . As previously mentioned, in asynchronous data replication environments such as z/OS Global Mirror (also referred to hereinafter as “XRC”) and Global Mirror, data is mirrored from a primary storage system  304   a  to a secondary storage system  304   b  to maintain two consistent copies of the data. The primary and secondary storage systems  304   a ,  304   b  may each be a storage system  110   a  such as that illustrated in  FIG. 2 . The primary and secondary storage systems  304   a ,  304   b  may be located at different sites, perhaps hundreds or even thousands of miles away from one another. In the event an outage occurs at the primary site, host I/O may be redirected to the secondary storage system  304   b , thereby enabling continuous operations. When the outage is corrected or repaired at the primary site, host I/O may be redirected back to the primary storage system  304   a.    
       FIG. 3  is a high-level block diagram showing an asynchronous data replication environment such as XRC. Using XRC, updated data elements (e.g., tracks) are written to cache  218  of the primary storage system  304   a . The updated data elements are recorded in an out-of-sync bitmap (i.e.,  00 S) to indicate that they need to be mirrored to the secondary storage system  304   b . Data elements that are written to the primary cache  218  may be destaged to backend storage drives  204  (i.e., volumes  302   a ) residing on the primary storage system  304   a , and eventually demoted from the primary cache  218 . The destage and demotion processes are independent from the asynchronous mirroring process. In certain cases, the primary storage system  304   a  may destage and demote a data element before the asynchronous mirroring takes place. In such cases, the data element may need to be re-staged to the primary cache  218  so it can be mirrored to the secondary storage system  304   b.    
     In other cases, the data element may be asynchronously mirrored to the secondary storage system  304   b  before the data element is destaged and demoted from the primary cache  218 . This scenario is preferred, since it only requires mirroring modified portions (e.g., sectors) of the updated data element to the secondary storage system  304   b , whereas a scenario that re-stages the data element to the primary cache  218  not only requires moving the data element from the backend storage drives  204  to the primary cache  218 , but also requires mirroring the entire data element (e.g., track) to the secondary storage system  304   b . Thus, re-staging and mirroring an unmirrored data element may be significantly less efficient than mirroring the data element prior to its destage and/or demotion. 
     Referring to  FIG. 4 , in certain embodiments, functionality may be provided within the primary storage system  304   a  to improve asynchronous data replication between a primary storage system  304   a  and a secondary storage system  304   b . More specifically, the functionality may retain more unmirrored data elements in the primary cache  218  so that the unmirrored data elements do not have to be retrieved from backend storage drives  204  prior to being mirrored to the secondary storage system  304   b . For the purposes of this disclosure, “unmirrored data elements” are defined to include data elements that need to be mirrored, but that have not yet been mirrored, from the primary storage system  304   a  to the secondary storage system  304   b . For example, data elements that belong to mirroring relationships and have been modified on the primary storage system  304   a , but have not yet been mirrored to the secondary storage system  304   b , may be considered “unmirrored data elements.” 
     As shown in  FIG. 4 , in certain embodiments, in order to improve asynchronous mirroring performance between a primary storage system  304   a  and a secondary storage system  304   b , a regular LRU list  400  may be maintained for a cache  218  residing on the primary storage system  304   a . This regular LRU list  400  may designate an order in which data elements are demoted from the primary cache  218 . The primary cache  218  may contain unmirrored data elements as well as data elements that do not need to be mirrored from the primary storage system  304   a  to the secondary storage system  304   b  (e.g., unmodified data elements or data elements that do not participate in mirroring relationships with the secondary storage system  304   b ). 
     Referring to  FIG. 5 , while continuing to refer generally to  FIG. 4 , as data elements are demoted from the regular LRU list  400 , systems and methods in accordance with the invention may determine whether the data elements at the LRU end of the regular LRU list  400  are unmirrored data elements. If so, the systems and methods may move the unmirrored data elements back to a most recently used (MRU) end of the regular LRU list  400  (hereinafter referred to as “re-MRUing”) instead of demoting them, thereby providing the unmirrored data elements additional time in the primary cache  218  before they are demoted. This, in turn, provides additional time to asynchronously mirror the unmirrored data elements from the primary cache  218  to the secondary storage system  304   b  without needing to restage the unmirrored data elements from backend storage drives  204 . 
     In certain embodiments, unmirrored data elements may only be moved to the MRU end of the regular LRU list  400  a certain number of times before they are demoted from the cache  218 . For example, in one embodiment, unmirrored data elements may only be re-inserted at the MRU end of the regular LRU list  400  a single time, not counting times the unmirrored data elements are added to the MRU end as a result of other accesses (e.g., reads and writes) not related to mirroring. Unmirrored data elements may be flagged to indicate that they have been re-added to the MRU end so that they are not re-added again. If an unmirrored data element is encountered at the LRU end of the regular LRU list  400  that has already been previously re-MRUed, the unmirrored data element may be demoted like other data elements. 
       FIG. 6  shows one embodiment of a method  600  for demoting a data element from the primary cache  218  using the techniques illustrated and described in association with  FIGS. 4 and 5 . As shown, the method  600  determines  602  whether a demotion is needed to clear space in the cache  218 . If so, the method  600  determines  604  whether a number of unmirrored data elements in the cache  218  is above a threshold. 
     In general, if unmirrored data elements occupy more than a designated amount (e.g., twenty five percent) of the cache  218 , unmirrored data elements may be demoted as opposed to re-MRUed if they are encountered at the LRU end of the regular LRU list  400 . On the other hand, if unmirrored data elements occupy less than the threshold, the unmirrored data elements may be re-MRUed if not previously re-MRUed. Thus, at step  604 , the method  600  may branch in one of two different directions depending on whether a number of unmirrored data elements in the cache  218  is above or below a designated threshold. 
     If, at step  604 , a number of unmirrored data elements in the cache  218  is above the designated threshold, the method  600  demotes  612 , from the cache  218 , the data element at the LRU end of the regular LRU list  400 , regardless of whether the data element is an unmirrored data element. If, on the other hand, a number of unmirrored data elements in the cache  218  is at or below the threshold, the method  600  determines  606  whether the data element at the LRU end of the regular LRU list  400  is an unmirrored data element. If not, the method  600  demotes  612  the data element. 
     If, at step  606 , the data element at the LRU end of the regular LRU list  400  is an unmirrored data element, the method  600  determines  608  whether the unmirrored data element has been previously re-MRUed. If so, the method  600  demotes  612  the unmirrored data element from the cache  218 . If the unmirrored data element has not been previously re-MRUed, the method  600  determines  610  whether a re-MRU threshold has been reached. For example, if a re-MRU threshold is five hundred and a number of unmirrored data elements that have been re-MRUed during a cache demotion process has exceeded the re-MRU threshold, an unmirrored data element may be demoted  612  from the cache  218  regardless of its unmirrored status. If the re-MRU threshold has not been reached, the method  600  re-MRUes the unmirrored data element by reinserting it at the MRU end of the regular LRU list  400 . 
       FIG. 7  shows an alternative embodiment of an environment that may be used to improve asynchronous data replication between a primary storage system  304   a  and a secondary storage system  304   b . As shown, instead of using just a regular LRU list  400 , the illustrated environment includes a regular LRU list  400  and a transfer-pending LRU list  700  that are used in association with a cache  218 . The regular LRU list  400  designates an order in which data elements are demoted from the cache  218 . The transfer-pending LRU list  700 , by contrast, is a list of unmirrored data elements that will be demoted from the cache  218  after they have been transferred to the secondary storage system  304   b.    
     Referring to  FIG. 8 , while continuing to refer generally to  FIG. 7 , when space needs to be cleared in the cache  218 , a data element may be analyzed at an LRU end of the regular LRU list  400 . If the data element is an unmirrored data element, the unmirrored data element may be moved from the regular LRU list  400  to the transfer-pending LRU list  700  where it may reside until its mirrored to the secondary storage system  304   b . On the other hand, if the data element at the LRU end of the regular LRU list  400  is not an unmirrored data element, the data element may be demoted to clear space in the cache  218 . 
     When unmirrored data elements in the transfer-pending LRU list  700  are transferred from the cache  218  to the secondary storage system  304   b , the unmirrored data elements may be removed from the transfer-pending LRU list  700  and demoted from the cache  218 . By contrast, if an unmirrored data element in the regular LRU list  400  is transferred to the secondary storage system  304   b , the data element may remain in its current position in the regular LRU list  400  and be demoted in due course. If a cache hit (other than a mirror transfer to the secondary storage system  304   b ) occurs to an unmirrored data element in the transfer-pending LRU list  700  or the regular LRU list  400 , the data element may be removed from its current positions on the transfer-pending LRU list  700  or regular LRU list  400  and moved to the MRU end of the regular LRU list  400 . 
       FIG. 9  shows one embodiment of a method  900  that may be executed when space needs to be cleared in the cache  218  of the environment illustrated in  FIGS. 7 and 8 . As shown in  FIG. 9 , the method  900  initially determines  902  whether a demotion is needed to clear space in the cache  218 . If so, the method  900  determines  904  whether a data element at the LRU end of the regular LRU list  400  is an unmirrored data element. If not, the method  900  demotes  906 , from the cache  218 , the data element at the LRU end of the regular LRU list  400 . If the data element is an unmirrored data element, the method  900  transfers  908  the data element to the MRU end of the transfer-pending LRU list  700 . 
     When clearing space in the cache  218  using the regular LRU list  400 , if too many unmirrored data elements are encountered at the LRU end of the regular LRU list  400 , unmirrored data elements may be demoted from the cache  218  regardless of their unmirrored status.  FIG. 10  shows one embodiment of a method  1000  that may be executed in such a situation. As shown in  FIG. 10 , if, at step  1002 , a specified number of unmirrored data elements are encountered at the LRU end of the regular LRU list  400  when demoting data elements from the cache  218 , the method  1000  demotes  1004 , from the cache  218 , a specified number of unmirrored data elements from the LRU end of the transfer-pending LRU list  700 . As an example, if five hundred unmirrored data elements are encountered at the LRU end of the regular LRU list  400  during a cache demotion process, the method  1000  demotes  1004 , from the cache  218 , five hundred unmirrored data elements from the LRU end of the transfer-pending LRU list  700 . These numbers are presented by way of example and not limitation. 
     In an alternative embodiment, step  1002  of  FIG. 10  may determine whether a size of the transfer-pending LRU list  700  exceeds a threshold. If so, the method  1000  may demote  1004 , from the cache  218 , a specified number of unmirrored data elements from the LRU end of the transfer-pending LRU list  700 . Alternatively, if a size of the transfer-pending LRU list  700  exceeds a threshold, the method  1000  may attempt to transfer a designated number of data elements from the cache  218  to the secondary storage system  304   b  and then demote the data elements from the cache  218 . If transfer is not possible due to bandwidth or link issues, the method  1000  may simply demote  1004  a specified number of unmirrored data elements from the LRU end of the transfer-pending LRU list  700 , or demote data elements from the LRU end of the transfer-pending LRU list  700  until the size of the transfer-pending LRU list  700  falls below the threshold. 
       FIG. 11  shows yet another embodiment of an environment that may be used to improve asynchronous data replication between a primary storage system  304   a  and a secondary storage system  304   b . In the illustrated embodiment, the cache  218  is a heterogeneous cache  218  comprising a higher performance portion  218   a  and a lower performance portion  218   b . In one embodiment, the higher performance portion  218   a  is made up of DRAM memory and the lower performance portion  218   b  is made up of flash memory such as storage class memory (SCM). Within the lower performance portion  218   b , an area  1100  may be reserved for unmirrored data elements that are waiting to be transferred to the secondary storage system  304   b . A regular LRU list  400  may be maintained for the higher performance portion  218   a  and may indicate an order in which data elements are demoted from the higher performance portion  218   a . A transfer-pending LRU list  700  may be maintained for the reserved area  1100  and may indicate an order in which unmirrored data elements are demoted from the reserved area  1100  when space needs to be cleared therein. 
     Referring to  FIG. 12 , while continuing to refer generally to  FIG. 11 , when space needs to be cleared in the higher performance portion  218   a , a data element at the LRU end of the regular LRU list  400  is checked to determine if it is an unmirrored data element. If the data element is an unmirrored data element, the data element is transferred from the higher performance portion  218   a  to the reserved area  1100  within the lower performance portion  218   b . The unmirrored data element may also be removed from a directory (hash table) associated with the higher performance portion  218   a  and added to a directory associated with the reserved area  1100 . Furthermore, the unmirrored data element may be removed from the LRU end of the regular LRU list  400  and added to the MRU end of the transfer-pending LRU list  700 . 
     Referring to  FIG. 13 , when space needs to be cleared in the higher performance portion  218   a , a method  1300  may be used to demote data elements from the higher performance portion  218   a . As shown, the method  1300  initially determines  1302  whether a demotion is needed from the higher performance portion  218   a . If so, the method  1300  determines  1304  whether the data element at the LRU end of the regular LRU list  400  is an unmirrored data element. In not, the method  1300  demotes  1308 , from the higher performance portion  218   a , the data element that is at the LRU end of the regular LRU list  400 . 
     If, at step  1304 , the data element at the LRU end of the regular LRU list  400  is an unmirrored data element, the method  1300  determines  1306  whether space is available in the reserved area  1100 . If not, the method  1300  demotes  1310 , from the reserved area  1100 , the unmirrored data element that is at the LRU end of the transfer-pending LRU list  700 . This will clear space in the reserved area  1100  to receive a new unmirrored data element. The method  1300  then moves  1312  the unmirrored data element from the higher performance portion  218   a  to the reserved area  1100 , moves  1312  the unmirrored data element from the LRU end of the regular LRU list  400  to the MRU end of the transfer-pending LRU list  700 , and moves the unmirrored data element from the directory associated with the higher performance portion  218   a  to the directory associated with the reserved area  1100 . If, at step  1306 , space is already available in the reserved area  1100 , the method  1300  performs each of the steps  1312  without clearing space in the reserved area  1100 . 
     Referring to  FIG. 14 , when asynchronous data replication functionality mirrors an unmirrored data element to the secondary storage system  304   b , a method  1400  may be performed to locate and mirror the unmirrored data element. As shown, the method  1400  determines  1402  whether it is time to mirror an unmirrored data element to the secondary storage system  304   b . If so, the method  1400  determines  1404  whether the unmirrored data element is in the higher performance portion  218   a . If so, the method  1400  transfers  1410  the unmirrored data element from the higher performance portion  218   a  to the secondary storage system  304   b.    
     If the unmirrored data element is not in the higher performance portion  218   a , the method  1400  determines  1406  whether the unmirrored data element is in the reserved area  1100 . If so, the method  1400  transfers  1412  the unmirrored data element from the reserved area  1100  to the secondary storage system  304   b  and removes the unmirrored data element from the reserved area  1100  (which may include removing the unmirrored data element from the transfer-pending LRU list  700  and the directory associated with the reserved area  1100 ). If the unmirrored data element is not in the higher performance portion  218   a  or the reserved area  1100 , the method  1400  stages  1408  the unmirrored data element from backend storage drives  204  to the higher performance portion  218   a , transfers  1408  the unmirrored data element from the higher performance portion  218   a  to the secondary storage system  304   b , and then demotes  1408  the data element from the higher performance portion  218   a.    
     The various techniques illustrated in  FIGS. 4 through 14  are advantageous in that they retain, as much as possible, unmirrored data elements in a cache  218  until the unmirrored data elements can be mirrored from the primary storage system  304   a  to the secondary storage system  304   b . This reduces overhead associated with staging unmirrored data elements from backend storage drives  204  to the cache  218 . This also increases efficiency in that mirroring directly from the cache  218  to the secondary storage system  304   b  only requires mirroring modified portions of the data elements (e.g., modified sectors of tracks) to the secondary storage system  304   b , whereas staging the data element back to the primary cache  218  also requires mirroring the entire data element (e.g., the entire track) to the secondary storage system  304   b . The disclosed techniques are also advantageous in that they do not starve other data elements from the cache  218 . That is, if unmirrored data elements occupy too much of the cache  218 , the disclosed techniques may demote unmirrored data elements from the cache  218  regardless of whether the unmirrored data elements have been mirrored to the secondary storage system  304   b . Thus, the disclosed techniques balance LRU order with improved retention of unmirrored data elements. 
     The flowcharts and block diagrams in the Figures illustrate the architecture, functionality, and operation of possible implementations of systems, methods, and computer-usable media according to various embodiments of the present invention. In this regard, each block in the flowcharts 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 illustrations, and combinations of blocks in the block diagrams and/or flowchart illustrations, may be implemented by special purpose hardware-based systems that perform the specified functions or acts, or combinations of special purpose hardware and computer instructions.