Patent Publication Number: US-10789004-B2

Title: Method, system and computer program product for managing storage system

Description:
RELATED APPLICATIONS 
     This application claim priority from Chinese Patent Application Number CN 201711031272.8, filed on Oct. 27, 2017 at the State Intellectual Property Office, China, titled “METHOD, SYSTEM AND COMPUTER PROGRAM PRODUCT FOR MANAGING STORAGE SYSTEM” the contents of which is herein incorporated by reference in its entirety. 
     FIELD 
     Various implementations of the present disclosure relate to storage management, and more specifically, to a method and system for managing a storage system (e.g. Redundant Array of Independent Disks (RAID)), as well as a computer program product. 
     BACKGROUND 
     With the development of data storage techniques, various data storage devices now provide users with higher data storage capacity, and also their data access speed has been increased greatly. With the increase of data storage capacity, users also impose greater and greater demands on data reliability and response time of storage systems. So far various data storage systems based on redundant arrays of disks have been developed for improving reliability of data. When one or more disks in a storage system fail(s), data in failed disk(s) can be recovered from other normal disk. 
     Mapped Redundant Array of Independent Disks (mapped RAID) has been developed so far. In the mapped RAID, a disk is a logical concept and may comprise multiple extents in a resource pool. Multiple extents comprised in one logical disk may be distributed over different physical storage devices. Regarding multiple extents in one stripe of the mapped RAID, they may be distributed over different storage devices, so that when a physical storage device where an extent among the multiple extents resides fails, data may be recovered from a physical storage device where other extent resides. 
     During use of the mapped RAID, access levels of different extents differ. This will lead to a difference in usage state between different storage devices where different extents reside, and further devices where some extents reside might become more susceptible to wear. At this point, it becomes a tough issue regarding how to handle a mapping relationship between extents and respective storage devices in a storage device pool. 
     SUMMARY 
     Therefore, it is desirable to develop and implement a technical solution for managing a storage system more easily and reliably. It is desired that the technical solution can be compatible with existing storage systems and manage data in existing storage systems more reliably by changing various configurations of these storage systems. 
     According to one implementation of the present disclosure, there is provided a method for managing a storage system. The method comprises: determining an access level of a target extent comprised in a stripe of a storage system, the access level indicating the possibility that the target extent will be accessed; obtaining a usage state of a target storage device where the target extent resides, the target storage device residing in a storage device pool associated with the storage system; and processing a mapping relationship between the target extent and the target storage device on the basis of the access level and the usage state. 
     According to one implementation of the present disclosure, there is provided a system for managing a storage system, comprising: one or more processors; a memory coupled to at least one processor of the one or more processors; computer program instructions stored in the memory which, when executed by the at least one processor, cause the system to execute a method for managing a storage system. The method comprises: determining an access level of a target extent comprised in a stripe of a storage system, the access level indicating the possibility that the target extent will be accessed; obtaining a usage state of a target storage device where the target extent resides, the target storage device residing in a storage device pool associated with the storage system; and processing a mapping relationship between the target extent and the target storage device on the basis of the access level and the usage state. 
     According to one implementation of the present disclosure, there is provided an apparatus for managing a storage system. The apparatus comprises: a determining module configured to determine an access level of a target extent comprised in a stripe of a storage system, the access level indicating the possibility that the target extent will be accessed; an obtaining module configured to obtain a usage state of a target storage device where the target extent resides, the target storage device residing in a storage device pool associated with the storage system; and a processing module configured to process a mapping relationship between the target extent and the target storage device on the basis of the access level and the usage state. 
     According to one implementation of the present disclosure, there are provided computer program instructions which, when executed by at least one processor, cause the at least one processor to execute a method for managing a storage system as described above. 
     With the technical solution of the present disclosure, storage systems may be managed in a more reliable manner. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       Through the more detailed description in the accompanying drawings, the above and other objects, features and advantages of the implementations of the present invention will become more apparent. Several implementations of the present invention are illustrated schematically and are not intended to limit the present invention. In the drawings: 
         FIG. 1  schematically illustrates a block diagram of an exemplary computer system which is applicable to implement the implementations of the present disclosure; 
         FIGS. 2A and 2B  each schematically illustrates a view of an application environment where the implementations of the present disclosure may be implemented; 
         FIG. 3  schematically illustrates a block diagram of the distribution of extents in a mapped RAID storage system; 
         FIG. 4  schematically illustrates an architecture view for managing a storage system according to one implementation of the present disclosure; 
         FIG. 5  schematically illustrates a flowchart of a method for managing a storage system according to one implementation of the present disclosure; 
         FIG. 6  schematically illustrates a block diagram for processing a mapping relationship between a target extent and a target storage device according to one implementation of the present disclosure; 
         FIG. 7  schematically illustrates a block diagram of cyclically arranged sub-extents for storing data and parity according to one implementation of the present disclosure; 
         FIG. 8  schematically illustrates a schematic view of access levels of extents in respective stripes in a storage system according to one implementation of the present disclosure; and 
         FIG. 9  schematically illustrates a block diagram of an apparatus for managing a storage system according to one implementation of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Some preferable implementations will be described in more detail with reference to the accompanying drawings, in which the preferable implementations of the present disclosure have been illustrated. However, the present disclosure can be implemented in various manners, and thus should not be construed to be limited to the implementations disclosed herein. On the contrary, those implementations are provided for the thorough and complete understanding of the present disclosure, and completely conveying the scope of the present disclosure to those skilled in the art. 
       FIG. 1  illustrates an exemplary computer system  100  which is applicable to implement the implementations of the present invention. As illustrated in  FIG. 1 , the computer system  100  may include: CPU (Central Process Unit)  101 , RAM (Random Access Memory)  102 , ROM (Read Only Memory)  103 , System Bus  104 , Hard Drive Controller  105 , Keyboard Controller  106 , Serial Interface Controller  107 , Parallel Interface Controller  108 , Display Controller  109 , Hard Drive  110 , Keyboard  111 , Serial Peripheral Equipment  112 , Parallel Peripheral Equipment  113  and Display  114 . Among above devices, CPU  101 , RAM  102 , ROM  103 , Hard Drive Controller  105 , Keyboard Controller  106 , Serial Interface Controller  107 , Parallel Interface Controller  108  and Display Controller  109  are coupled to the System Bus  104 . Hard Drive  110  is coupled to Hard Drive Controller  105 . Keyboard  111  is coupled to Keyboard Controller  106 . Serial Peripheral Equipment  112  is coupled to Serial Interface Controller  107 . Parallel Peripheral Equipment  113  is coupled to Parallel Interface Controller  108 . And, Display  114  is coupled to Display Controller  109 . It should be understood that the structure as illustrated in  FIG. 1  is only for the exemplary purpose rather than any limitation to the present invention. In some cases, some devices may be added to or removed from the computer system  100  based on specific situations. 
     As will be appreciated by one skilled in the art, aspects of the present invention may be embodied as a system, method or computer program product. Accordingly, aspects of the present invention may take the form of an entirely hardware implementation, an entirely software implementation (including firmware, resident software, micro-code, etc.) or one implementation combining software and hardware aspects that may all generally be referred to herein as a “circuit,” “module” or “system.” Furthermore, aspects of the present invention may take the form of a computer program product embodied in one or more computer readable medium(s) having computer readable program code embodied thereon. 
     Any combination of one or more computer readable medium(s) may be utilized. The computer readable medium may be a computer readable signal medium or a computer readable storage medium. A computer readable storage medium may be, for example, but not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, or device, or any suitable combination of the foregoing. More specific examples (a non-exhaustive list) of the computer readable storage medium would include the following: an electrical connection having one or more wires, 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), an optical fiber, a portable compact disc read-only memory (CD-ROM), an optical storage device, a magnetic storage device, or any suitable combination of the foregoing. In the context of this document, a computer readable storage medium may be any tangible medium that can contain, or store a program for use by or in connection with an instruction execution system, apparatus, or device. 
     A computer readable signal medium may include a propagated data signal with computer readable program code embodied therein, for example, in baseband or as part of a carrier wave. Such a propagated signal may take any of a variety of forms, including, but not limited to, electro-magnetic, optical, or any suitable combination thereof. A computer readable signal medium may be any computer readable medium that is not a computer readable storage medium and that can communicate, propagate, or transport a program for use by or in connection with an instruction execution system, apparatus, or device. 
     Program code embodied on a computer readable medium may be transmitted using any appropriate medium, including but not limited to wireless, wireline, optical fiber cable, RF, etc., or any suitable combination of the foregoing. 
     Computer program code for carrying out operations for aspects of the present invention may be written in any combination of one or more programming languages, including an object oriented programming language such as Java, Smalltalk, C++ or the like and conventional procedural programming languages, such as the “C” programming language or similar programming languages. The program code may execute entirely on the user&#39;s computer, partly on the user&#39;s computer, as a stand-alone software package, partly on the user&#39;s computer and partly on a remote computer or entirely on the remote computer or server. In the latter scenario, the remote computer may be connected to the 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). 
     Aspects of the present invention are described below with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems) and computer program products according to implementations 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, can be implemented by computer program instructions. These computer 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 program instructions may also be stored in a computer readable medium that can direct a computer, other programmable data processing apparatus, or other devices to function in a particular manner, such that the instructions stored in the computer readable medium produce an article of manufacture including instructions which implement the function/act specified in the flowchart and/or block diagram block or blocks. 
     The computer program instructions may also be loaded onto a computer, other programmable data processing apparatus, or other devices to cause a series of operational steps to be performed on the computer, other programmable apparatus or other devices to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide processes for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks. 
     In a storage system such as Redundant Array of Independent Disks (RAID), multiple storage devices (e.g. disks) may be combined into an array of disks. By providing redundant storage devices, reliability of an entire disk group is caused to significantly exceed a single storage device. RAID may offer various advantages over a single storage device, for example, enhancing data integrity, enhancing fault tolerance, increasing throughput or capacity, etc. There exist a number of RAID standards, such as RAID-1, RAID-2, RAID-3, RAID-4, RAID-5, RAID-6, RAID-10, RAID-50, etc. For more details about RAID levels, those skilled in the art may refer to https://en.wikipedia.org/wiki/Standard_RAID_levels and https://en.wikipedia.org/wiki/Nested_RAID_levels, etc. 
       FIG. 2A  schematically illustrates a schematic view  200 A of a structure of RAID according to one technical solution. In this figure, working principles of RAID are illustrated by taking RAID-5 (4D+1P, where 4D represents that 4 storage devices are included in the storage system for storing data, and 1P represents that 1 storage device is included in the storage system for storing parity) that consists of five independent storage devices ( 210 ,  212 ,  214 ,  216  and  218 ) as an example. It should be noted although five storage devices are schematically shown in  FIG. 2A , in other implementations more or less storage devices may be comprised according to different versions of RAID. Moreover, although in  FIG. 2A  there are shown stripes  220 ,  222  and  224 , in other examples the RAID system may further comprise different numbers of stripes. 
     In RAID, one stripe crosses multiple physical storage devices (for example, the stripe  220  crosses storage the devices  210 ,  212 ,  214 ,  216  and  218 ). The stripe may be simply construed as a storage area among multiple storage devices which satisfies a given address range. Data stored in the stripe  220  comprises multiple parts: a data block D 00  stored in the storage device  210 , a data block D 01  stored in the storage device  212 , a data block D 02  stored in the storage device  214 , a data block D 03  stored in the storage device  216 , and a data block P 0  stored in the storage device  218 . In this example, data blocks D 00 , D 01 , D 02  and D 03  are stored data, and data block P 0  is a P parity of the stored data. 
     The mode of storing data in other stripes  222  and  224  is similar to that in the stripe  220 , and the difference is that the parity about other data block may be stored in other storage device than the storage device  218 . In this way, when one of the multiple storage devices  210 ,  212 ,  214 ,  216  and  218  fails, data in the failing device may be recovered from other normal storage devices. 
       FIG. 2B  schematically illustrates a schematic view  220 B of rebuilding process of RAID according to one technical solution. As shown in  FIG. 2B , when one storage device (e.g. the storage device  218  shown in shades) fails, data may be recovered from the other storage devices  210 ,  212 ,  214  and  216  that operate normally. At this point, a new standby storage device  218 B may be added to RAID to replace the storage device  218 . In this way, recovered data may be written to  218 B, and system rebuilding may be realized. 
     While a RAID-5 storage system comprising 5 storage devices (among which 4 storage devices are used for storing data and 1 storage device is used for storing parity) has been described with reference to  FIGS. 2A and 2B , according to definition of other RAID levels, there may further exist a storage system comprising other number of storage devices. On the basis of definition of RAID 6, for example, two storage devices may be used to store parity P and Q respectively. For another example, according to definition of triple-parity RAID, three storage devices may be used to store parity P, Q and R respectively. 
       FIG. 3  schematically shows a block diagram  300  of the distribution of extents in a mapped RAID storage system. This figure illustrates multiple storage devices  310 ,  320 ,  330 ,  340 , . . . ,  350 ,  360 . Each storage device may comprise multiple extents, wherein an extent shown with blank pattern  360  represents a free extent, an extent shown with stripe pattern  362  represents an extent for a first stripe, and an extent shown with shade pattern  364  represents an extent for a second stripe. At this point, extents  312 ,  322 ,  332 ,  342  and  352  for the first stripe are for storing data blocks D 00 , D 01 , D 02 , D 03  and parity P 0  of the first stripe respectively; extents  324 ,  334 ,  344 ,  366  and  314  for the second stripe are for storing blocks D 10 , D 11 , D 12 , D 13  and parity P 1  of the second stripe respectively. 
     Note in  FIG. 3  the 4+1 R5 RAID storage system is taken as an example to illustrate how extents in various stripes are uniformly distributed over multiple storage systems in the resource pool. When RAID based on other level is used, those skilled in the art may implement concrete details based on the above described principles. For example, in the 4+2 RAID-6 storage system, the storage pool may at least comprise 6 storage devices. 6 extents in each stripe may be uniformly distributed over multiple storage devices so as to ensure a load balance between the multiple storage devices. 
     Note in the 4D+1P storage system, data access levels may differ among 5 extents in one stripe. For example, regarding 4 extents that store data, when a write is performed to any extent that stores data, the write needs to be further performed to the extent that stores parity. Therefore, the extent storing parity is more susceptible to be worn, which further reduces the reliability of the storage system and increases the risk that the storage system fails. At this point, how to guarantee the reliability of the storage system becomes a technical problem. 
     To solve the foregoing drawbacks, implementations of the present disclosure provide a method, device and program produce for managing a storage system. Specifically, according to one implementation of the present disclosure, there is provided a technical solution for managing a storage system.  FIG. 4  schematically shows an architecture view  400  for managing a storage system according to one implementation of the present disclosure. In this figure, depicted are multiple extents  312 ,  322 ,  332 ,  342  and  352  in one stripe  220  in the storage system (4D+1P). Based on principles of mapped RAID, these extents come from storage devices  310 ,  320 ,  330 ,  340 ,  350  and  360  comprised in a storage device pool  410  respectively. 
     The extents  312 ,  322 ,  332  and  342  are used for storing data D 00 , D 01 , D 02  and D 03  respectively, and the extent  352  is used for storing parity P 0 . According to one implementation of the present disclosure, it can be learned by determining an access level of each extent in the stripe  220  that the access level of the extent  352  is much higher than those of other extents because the extent  352  is used for storing parity P 0 . Further, the usage state of the storage device  350  where the extent  352  resides may be determined. If it is found the storage device  350  is worn to a great degree, when the extent  352  is faced with many access operations, the storage device  350  where the extent  352  resides is confronted with a huge risk. Once the storage device  350  breaks down, the reliability of the storage system will be reduced. 
     At this point, how to handle a mapping relationship between a target extent and a target storage device may be determined on the basis of the access level and usage state of the extent  352 . Since the extent  352  has a higher access level and the storage system  350  is worn to a great degree, a further extent may be selected from other storage device (e.g., the storage device  360 ) in the storage device pool  410  to replace the extent  352 . 
     With reference to  FIG. 5 , a detailed description is presented below to concrete steps of a method according to one implementation of the present disclosure.  FIG. 5  schematically shows a flowchart of a method  500  for managing a storage system according to one implementation of the present disclosure. As depicted, in block  510 , an access level of a target extent comprised in a stripe of the storage system is determined, here the access level indicates the possibility that the target extent will be accessed. It will be appreciated that the access level is an indicator indicative of access frequency, and it is not limited in which way the access level is represented. Through the context of the present disclosure, the access level may be represented with different units. For example, the access level may be represented by an access count per second or may be represented in the form of a relative value. Continuing the example of the 4D+1P storage system shown in  FIG. 4 , suppose the access level of a data extent is represented by a unit of 1, since every time a write is performed to any of the 4 data extents, data is written to the parity extent, the access level of the parity extent may be presented by 4. In other words, the parity extent has an access level 4 times as large as the data extent. 
     In block  520 , usage state of a target storage device where the target extent resides is obtained, the target storage device residing in a storage device pool associated with the storage system. Here the usage state may represent a wear level of a target storage device where the target extent resides. For example, regarding an SSD storage device, the wear level may be directly read from the SSD storage device. Continuing the example shown in  FIG. 4 , the usage state associated with the extent  352  is the wear level of the storage device  350 , and the usage state associated with the extent  312  is the wear level of the storage device  310 . 
     In block  530 , a mapping relationship between the target extent and the target storage device is handled on the basis of the access level and the usage state. At this point, the mapping relationship may be handled according to the access level and the usage state. If the access level is higher and the usage state indicates the wear level of the storage device is high too, then the mapping relationship may be modified (e.g. an extent in other storage device having a lower wear level may be selected to replace the target extent so as to update the mapped storage system). If the access level is lower and the usage state indicates the wear level of the storage device is low too, then the existing mapping relationship may be maintained. 
       FIG. 6  schematically shows a block diagram  600  for handling a mapping relationship between a target extent and a target storage device according to one implementation of the present disclosure. As depicted, a stripe  220  comprises multiple extents, among which extents  312 ,  322 ,  332  and  342  are used for storing data blocks D 00 , D 01 , D 02  and D 03  respectively and an extent  352  is used for storing parity P 0 . The usage state of a storage device where each extent resides is shown at the bottom of  FIG. 6 . When represented as the wear level, the usage state may be denoted by a percentage or other value (e.g. time, access count, etc.). 
     Specifically, the wear level of a storage device where the extent  312  resides is 10%, which means the wear of the current storage device amounts to 10%. Suppose access operations to the storage device are substantially uniform, when the wear is measured by time, if the storage device has been used for 1 month, then 10% may represent the storage device has a life of 10 months and may be used for 9 more months in future. As depicted, usage states of storage devices where the extents  312 ,  322 ,  332 ,  342  and  352  reside are 10%, 15%, 12%, 20% and 50% respectively. 
     According to one implementation of the present disclosure, the stripe comprises a set of extents having a first type (e.g. data) or a second type (e.g. parity) extents (the extents  312 ,  322 ,  332 ,  342  and  352 ). The access level of the target extent may be determined on the basis of the type of the target extent. Continuing the example in  FIG. 6 , where the extents  312 ,  322 ,  332  and  342  are used for storing data and the extent  352  is used for storing parity, since the access frequency of parity is much higher than that of data, it may be determined that the extent  352  is a parity type extent and further has a higher access level. 
     In the example of  FIG. 6 , it is determined that the extent  352  is a parity type extent and the wear level of a storage device where the extent resides is 50%. Since 50% denotes a high wear level, at this point it may be determined that the extent  352  is faced with a potential risk and may be replaced by an extent in other storage device having a lower wear level (e.g. 50% below). 
     According to one implementation of the present disclosure, the stripe comprises multiple sub-stripes, a sub-stripe among the multiple sub-stripes comprising a set of sub-extents consisting of respective sub-extents in extents of the set of extents, a sub-extent in the set of sub-extents having a first type or a second type. In the mapped storage system, an extent from one storage device may be a large extent, and one stripe may further comprise multiple sub-stripes. At this point, the sub-stripe may be construed as a stripe having smaller granularity. 
       FIG. 7  schematically shows a block diagram  700  of cyclically arranged sub-extents for storing parity according to one implementation of the present disclosure. As depicted, a stripe  710  comprises 8 sub-stripes across multiple extents. The first row of extents in the stripe  710  represents a sub-stripe  720  (4 data extents and 1 parity extent), and each row of extents under the stripe  720  represents one sub-stripe. 
     It will be appreciated that the arrangement mode of extents in each sub-stripe may be arbitrary. For example, in each sub-stripe data may be stored at positions 0 to 3, and parity may be stored at position 4. Alternatively or additionally, different types of information may be stored at different positions. For example, with reference to the second sub-stripe in the stripe  710 , parity is stored at position 0, and data is stored at following positions 1 to 4. Further, in stripes subsequent to the stripe  720 , different types of extents may be cyclically arranged in a direction as shown by arrow  730 . For example, parity may be stored at position 0 in the second sub-stripe, at position 1 in the third sub-stripe, at position 2 in the fourth sub-stripe, . . . , at position 0 in the seventh sub-stripe, at position 1 at the eighth sub-stripe, and so on and so forth. 
     Where one stripe comprises multiple sub-stripes, the access level may be determined on the basis of the type of one of more sub-stripes comprised in the target extent. Specifically, in the example shown in  FIG. 7 , respective sub-extents at position 0 reside in the same storage device, at which point the access level of an extent at position 0 may be determined on the basis of the type of each sub-extent at position 0. 
     According to one implementation of the present disclosure, a proportion of sub-extents having second type in the target extent may be determined; subsequently, the access level may be determined on the basis of the proportion. Take an extent at position 0 in  FIG. 7  as an example. This extent comprises 8 sub-extents, among which 6 sub-extents are used for storing data and 2 sub-extents are used for storing parity. At this point, a proportion of the sub-extents for storing parity to all the sub-extents in the extent is 2/8=25%. Therefore, the access level may be indicated by the determined proportion. 
     Likewise, the access level of an extent at other position may be determined. For example, an extent at position 1 comprises 2 sub-extents for storing parity, so its access level is 2/8=25%. An extent at position 2 comprises 1 sub-extent for storing parity, so its access level is ⅛=12.5%. An extent at position 3 comprises 1 sub-extent for storing parity, so its access level is ⅛=12.5%. An extent at position 4 comprises 2 sub-extents for storing parity, so its access level is 2/8=25%. According to one implementation of the present disclosure, in response to the access level being higher than an access level of a further extent other than the target extent in the stripe, the target extent is mapped to a further storage device in the storage device pool other than a storage device where the stripe resides. The higher the access level, the more frequently the stripe will be accessed, and the more likely the stripe will be damaged. Therefore, preferentially an extent having a higher access level may be remapped to other more reliable device in the storage device pool  410 . 
     An extent having a higher access level (e.g. extents at positions 0, 1 and 4 in  FIG. 7 ) may be selected from extents, and then the selected extent may be judged whether the wear degree of a storage device where the extent resides puts the extent at potential risk. Take an extent at position 4 as an example, whose access level is relatively high (25%). Suppose the wear level of a storage device where the extent resides is already as high as 50%, then it may be considered that more access to the extent will cause more wear to the storage device, so remapping may be performed. 
     Note since the remapped storage system is required to keep properties of mapped RAID, it should be ensured that the remapped extent is from a storage device other than storage devices where the extents associated with the current stripe reside. Suppose extents at positions 0 to 3 come from storage devices 0 to 3 respectively, and an extent at position 4 comes from storage device 4. During remapping, the extent at position 4 may be replaced by an extent in a further storage device other than storage devices 0 to 4 in the storage device pool. 
     According to one implementation of the present disclosure, when sub-extents in the stripe are cyclically arranged in order of the first type and the second type, sub-extents having the second type are regularly arranged (for example, a position of a parity sub-extent in the previous sub-stripe is moved backward by one position). At this point, the access level may be determined on the basis of the number of the set of sub-stripes, the number of the sub-extents having the first type, the number of the sub-extents having the second type as well as a start position of the sub-extents having the second type. 
     Note while the situation where one stripe comprises 8 sub-stripes has been shown in the context of a 4D+1P storage system, according to one implementation of the present disclosure, the storage system may further be of other type, e.g. a 3D+1P, 4D+1P+1Q or 4D+1P+1Q+1R storage system. To generally represent the respective numbers of data extents and parity extents in one stripe, the number of data extents may be denoted by W, the number of parity extents may be denoted by M, and the width of an entire stripe may be denoted by width. 
     Let i (i is an integer between 0 and width−1) denote the ith position in a stripe, and let n denote the nth stripe in the storage system. At this point, the access level ParityPercentage(n, i) of an extent at the ith position in the nth stripe in the storage system may be determined from formula group 1. 
     
       
         
           
             
               
                 
                   
                     
                       ParityPercentage 
                       ⁡ 
                       
                         ( 
                         
                           n 
                           , 
                           i 
                         
                         ) 
                       
                     
                     = 
                     
                       
                         floor 
                         ( 
                         
                           
                             
                               DE 
                               P 
                             
                             - 
                             
                               p 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               0 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               
                                 index 
                                 ⁡ 
                                 
                                   ( 
                                   
                                     n 
                                     , 
                                     i 
                                   
                                   ) 
                                 
                               
                             
                             + 
                             
                               ( 
                               
                                 width 
                                 - 
                                 1 
                               
                               ) 
                             
                           
                           width 
                         
                         ) 
                       
                       / 
                       
                         ( 
                         
                           DE 
                           P 
                         
                         ) 
                       
                     
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     
                       p 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       0 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         index 
                         ⁡ 
                         
                           ( 
                           
                             n 
                             , 
                             i 
                           
                           ) 
                         
                       
                     
                     = 
                     
                       
                         ( 
                         
                           width 
                           - 
                           
                             
                               ( 
                               
                                 
                                   n 
                                   * 
                                   
                                     DE 
                                     P 
                                   
                                 
                                 - 
                                 
                                   ( 
                                   
                                     i 
                                     + 
                                     1 
                                   
                                   ) 
                                 
                               
                               ) 
                             
                             ⁢ 
                             % 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             width 
                           
                         
                         ) 
                       
                       ⁢ 
                       % 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       width 
                     
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     width 
                     = 
                     
                       ( 
                       
                         W 
                         + 
                         M 
                       
                       ) 
                     
                   
                 
               
               
                 
                   formula 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   group 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   1 
                 
               
             
           
         
       
     
     In formula group 1, floor denotes a round down operation, DE denotes the size of an extent, P denotes the size of a sub-extent, p0index(n,i) denotes the start position of a parity sub-extent in the nth sub-stripe, width denotes the width of the stripe, W denotes the number of data extents, and M denotes the number of parity extents. 
     Note in formula group 1, how to determine the access level has been described by taking an example where the parity sub-extents start from the last position among multiple positions. According to one implementation of the present disclosure, when the parity sub-extents start from other position, the access level may be determined from other formula. According to one implementation of the present disclosure, the parity may loop using different step sizes. At this point, those skilled in the art may calculate an offset value of each parity sub-extent at a position on the basis of a concrete step size and further determine a corresponding access level on the basis of the offset value. 
     Using the formulas as shown above, in the 4D+1P storage system, when parity sub-extents in the first sub-stripe start from the last position, the determined percentage of each extent in the storage system is as shown in  FIG. 8 , which illustrates a schematic view  800  of access levels of extents in each stripe in a storage system according to one implementation of the present disclosure. 
     In the storage system shown in  FIG. 8 , comprised are 8 stripes (stripe 0 to stripe 7). As depicted, the value at an intersection of stripe and position denotes the percentage of parity sub-extents in a corresponding extent. Specifically, stripe 0 corresponds to the stripe  710  in  FIG. 7 , and parity sub-extents in the extent at position 0 take a percentage of 25% and parity sub-extents in the extent at position 1 take a percentage of 25%. For another example, regarding stripe 2, parity sub-extents in the extent at position 3 take a percentage of 12.50%. Based on the percentages in  FIG. 8 , extents with an access level of 25% may be processed first, and then extents with an access level of 12.50% may be processed later. According to one implementation of the present disclosure, extents may be ranked in decreasing order by percentage, and those with a higher percentage may be processed first. 
     According to one implementation of the present disclosure, in response to the usage state satisfying a first predefined condition, the target extent is mapped to the further storage device. In other words, whether to perform a remapping operation may be determined on the basis whether the usage state satisfies a predefined condition or not. For example, if the access level indicates the target extent has a high access frequency, whereas the usage state of a storage device where the target extent resides shows the storage device is almost new, then at this point there is no need to perform a remapping operation immediately. For another example, if the access level indicates the target extent has a high access frequency, and the usage state of a storage device where the target extent resides shows the storage device has a high wear level, then remapping may be performed. 
     According to one implementation of the present disclosure, a free extent may be replaced with a further extent in a storage device with a usage state better than that of the target device in the storage device pool  410 . In other words, the usage state of the replacing extent is better than that of the target device. In this way, since the storage system is built using extents in a storage device with a better and more reliably usage state, it may be guaranteed the storage system has a higher reliability. 
     Concrete details of handling the mapping relationship on the basis of an access level in the storage system have been described above. It will be appreciated the current access level is a “static” access level that is determined on the basis of the type of each extent in a stripe in the storage system. According to one implementation of the present disclosure, the future possibility of each extent in the stripe may further be predicted from historical access frequency of each stripe in the storage system. 
     According to one implementation of the present disclosure, first an access history of the stripe may be determined, and when the history satisfies a second predefined condition, the mapping relationship between the target extent and the target storage device is processed on the basis of the access level and the usage state. In this way, the possibility that a stripe will be accessed in future may be estimated on the basis of the access history of the stripe. 
     According to one implementation of the present disclosure, the history may be described in various manners. For example, the history may be described as the count that the stripe has been accessed in a previous predefined period of time. Alternatively or additionally, the history may be described as the amount of accessed data or using other method. If the history indicates a stripe (or the target extent itself) where the target extent resides has been accessed frequently (e.g. access frequency is higher than a predefined value), then the target extent will be processed first. For another example, if the history indicates a stripe (or the target extent itself) where the target extent resides has not been accessed for a long time, then the target extent may be processed later. 
     According to one implementation of the present disclosure, the access level may be determined on the basis of both the percentage and the historical frequency. For example, the access level may be determined on the basis of a product or weighted sum of both the percentage and the historical frequency. Suppose the first target extent has a percentage of 12.5% and a historical frequency of 10 times/second, and the second target extent has a percentage of 25% and a historical frequency of 1 time/second. At this point, the access level for the first target extent may be denoted as 12.5×10=1.25, and the access level for the second target extent may be denoted as 25%×1=0.25. Since 1.25&gt;0.25, the first target extent may be processed preferentially. 
     According to one implementation of the present disclosure, where the respective extents have similar or identical access levels, an extent in a storage device with a worse usage state may be processed preferentially. For example, usage states of storage devices where multiple extents in stripes in the storage system reside may further be ranked. With reference to concrete values of the access levels shown in  FIG. 8 , extents with an access level of 25% may be processed preferentially in a bad-to-good order of usage state. 
     According to one implementation of the present disclosure, the target storage device is a solid-state storage device, and the obtaining a usage state of the target storage device comprises: reading a wear level of the solid-state storage device as the usage state. It will be appreciated the wear level is an effective index describing the life of the solid-state storage device. Where the storage device pool is constructed on the basis of solid-state storage devices, the wear level may be directly read from the solid-state storage device as the usage state. 
     Various implementations implementing the method of the present invention have been described above with reference to the accompanying drawings. Those skilled in the art may understand the method may be implemented in software, hardware or a combination of software and hardware. Moreover, those skilled in the art may understand by implementing various steps in the above method in software, hardware or a combination of software and hardware, there may be provided an apparatus based on the same invention concept. Even if the apparatus has the same hardware structure as a general-purpose processing device, the functionality of software contained therein makes the apparatus manifest distinguishing properties from the general-purpose processing device, thereby forming an apparatus of the various implementations of the present invention. The apparatus described in the present invention comprises several means or modules, which are configured to execute corresponding steps. Upon reading this specification, those skilled in the art may understand how to write a program for implementing actions performed by these means or modules. Since the apparatus is based on the same invention concept as the method, the same or corresponding implementation details are also applicable to means or modules corresponding to the method. As a detailed and complete description has been presented above, it might be ignored below. 
       FIG. 9  schematically shows a block diagram of an apparatus  900  for managing storage according to one implementation of the present disclosure. The apparatus  900  comprises: a determining module  910  configured to determine an access level of a target extent comprised in a stripe of a storage system, the access level indicating the possibility that the target extent will be accessed; an obtaining module  920  configured to obtain a usage state of a target storage device where the target extent resides, the target storage device residing in a storage device pool associated with the storage system; and a processing module  930  configured to process a mapping relationship between the target extent and the target storage device on the basis of the access level and the usage state. 
     According to one implementation of the present disclosure, the stripe comprises a set of extents having a first type or a second type, and the determining module  910  is further configured to determine the access level of the target extent on the basis of the type of the target extent. 
     According to one implementation of the present disclosure, the stripe comprises multiple sub-stripes, a sub-stripe among the multiple sub-stripes comprising a set of sub-extents comprising respective sub-extents in respective extents in the set of extents, a sub-extent in the set of sub-extents having a first type or a second type. The determining module  910  is further configured to determine the access level on the basis of the type of one or more sub-extents comprised in the target extent. 
     According to one implementation of the present disclosure, the determining module  910  is further configured to: determine a proportion of sub-extents having the second type in the target extent to the target extent; and determine the access level on the basis of the proportion. 
     According to one implementation of the present disclosure, sub-extents in the stripe are cyclically arranged in order of the first type and the second type. The determining module  910  is further configured to: determine the access level on the basis of the number of the set of sub-stripes, the number of the sub-extents having the first type, the number of the sub-extents having the second type as well as a start position of the sub-extents having the second type. 
     According to one implementation of the present disclosure, the processing module  930  is further configured to: in response to the access level being higher than an access level of a further extent other than the target extent in the stripe, map the target extent to a further storage device in the storage device pool other than a storage device where an extent in the stripe resides. 
     According to one implementation of the present disclosure, the processing module  930  is further configured to: in response to the usage state satisfying a first predefined condition, map the target extent to the further storage device. 
     According to one implementation of the present disclosure, a usage state of the further storage device is better than the usage state of the target device. 
     According to one implementation of the present disclosure, the determining module  910  is further configured to determine an access history of the stripe; and the processing module  930  is further configured to: in response to the history satisfying a second predefined condition, process a mapping relationship between the target extent and the target storage device on the basis of the access level and the usage state. 
     According to one implementation of the present disclosure, the target storage device is a solid-state storage device. The obtaining module  920  is further configured to read a wear level of the solid-state storage device as the usage state. 
     According to one implementation of the present disclosure, there is provided a system for managing a storage system, comprising: one or more processors; a memory coupled to at least one processor of the one or more processors; computer program instructions stored in the memory which, when executed by the at least one processor, cause the system to execute a method for managing a storage system. The method comprises: determining an access level of a target extent comprised in a stripe of a storage system, the access level indicating the possibility that the target extent will be accessed; obtaining a usage state of a target storage device where the target extent resides, the target storage device residing in a storage device pool associated with the storage system; and processing a mapping relationship between the target extent and the target storage device on the basis of the access level and the usage state. 
     According to one implementation of the present disclosure, the stripe comprises a set of extents having a first type or a second type, and the determining an access level comprises: determining the access level of the target extent on the basis of a type of the target extent. 
     According to one implementation of the present disclosure, the stripe comprises multiple sub-stripes, a sub-stripe among the multiple sub-stripes comprising a set of sub-extents comprising a sub-extent in each extent in the set of extents, a sub-extent in the set of sub-extents having a first type or a second type. 
     According to one implementation of the present disclosure, the determining an access level of a target extent comprises: determining the access level on the basis of the type of one or more sub-extents comprised in the target extent. 
     According to one implementation of the present disclosure, the determining an access level comprises: determining a proportion of sub-extents having the second type in the target extent to the target extent; and determining the access level on the basis of the proportion. 
     According to one implementation of the present disclosure, sub-extents in the stripe are cyclically arranged in order of the first type and the second type. 
     According to one implementation of the present disclosure, the determining a proportion comprises: determining the access level on the basis of the number of the set of sub-stripes, the number of the sub-extents having the first type, the number of the sub-extents having the second type as well as a start position of the sub-extents having the second type. 
     According to one implementation of the present disclosure, the processing a mapping relationship between the target extent and the target storage device comprises: in response to the access level being higher than an access level of a further extent other than the target extent in the stripe, mapping the target extent to a further storage device in the storage device pool other than a storage device where an extent in the stripe resides. 
     According to one implementation of the present disclosure, in response to the usage state satisfying a first predefined condition, the target extent is mapped to the further storage device. 
     According to one implementation of the present disclosure, a usage state of the further storage device is better than the usage state of the target device. 
     According to one implementation of the present disclosure, an access history of the stripe is determined. 
     According to one implementation of the present disclosure, the processing a mapping relationship between the target extent and the target storage device further comprises: in response to the history satisfying a second predefined condition, processing a mapping relationship between the target extent and the target storage device on the basis of the access level and the usage state. 
     According to one implementation of the present disclosure, the target storage device is a solid-state storage device. 
     According to one implementation of the present disclosure, the obtaining a usage state of the target storage device comprises: reading a wear level of the solid-state storage device as the usage state. 
     In one implementation of the present disclosure, there is provided a computer program product tangibly stored on a non-transient computer readable medium and comprising machine executable instructions which, when executed, cause the machine to execute a method for managing a storage system. The method comprises: determining an access level of a target extent comprised in a stripe of a storage system, the access level indicating the possibility that the target extent will be accessed; obtaining a usage state of a target storage device where the target extent resides, the target storage device residing in a storage device pool associated with the storage system; and processing a mapping relationship between the target extent and the target storage device on the basis of the access level and the usage state. 
     According to one implementation of the present disclosure, the stripe comprises a set of extents having a first type or a second type, and the determining an access level comprises: determining the access level of the target extent on the basis of the type of the target extent. 
     According to one implementation of the present disclosure, the stripe comprises multiple sub-stripes, a sub-stripe among the multiple sub-stripes comprising a set of sub-extents comprising respective sub-extents in respective extents in the set of extents, a sub-extent in the set of sub-extents having a first type or a second type, wherein the determining the access level of the target extent on the basis of the type of the target extent comprises: determining the access level on the basis of the type of one or more sub-extents comprised in the target extent. 
     According to one implementation of the present disclosure, the determining the access level on the basis of the type of one or more sub-extents comprised in the target extent comprises: determining a proportion of sub-extents having the second type in the target extent to the target extent; and determining the access level on the basis of the proportion. 
     According to one implementation of the present disclosure, sub-extents in the stripe are cyclically arranged in order of the first type and the second type, and the determining a proportion comprises: determining the access level on the basis of the number of the set of sub-stripes, the number of the sub-extents having the first type, the number of the sub-extents having the second type as well as a start position of the sub-extents having the second type. 
     According to one implementation of the present disclosure, the processing a mapping relationship between the target extent and the target storage device comprises: in response to the access level being higher than an access level of a further extent other than the target extent in the stripe, mapping the target extent to a further storage device in the storage device pool other than a storage device where an extent in the stripe resides. 
     According to one implementation of the present disclosure, in response to the usage state satisfying a first predefined condition, the target extent is mapped to the further storage device. 
     According to one implementation of the present disclosure, a usage state of the further storage device is better than the usage state of the target device. 
     According to one implementation of the present disclosure, an access history of the stripe is determined; and the processing a mapping relationship between the target extent and the target storage device further comprises: in response to the history satisfying a second predefined condition, processing a mapping relationship between the target extent and the target storage device on the basis of the access level and the usage state. 
     According to one implementation of the present disclosure, the target storage device is a solid-state storage device, and the obtaining a usage state of the target storage device comprises: reading a wear level of the solid-state storage device as the usage state. 
     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 implementations of the present invention. 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 illustrated 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 descriptions of the various implementations of the present invention have been presented for purposes of illustration, but are not intended to be exhaustive or limited to the implementations disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described implementations. The terminology used herein was chosen to best explain the principles of the implementations, the practical application or technical improvement over technologies found in the marketplace, or to enable others of ordinary skill in the art to understand the implementations disclosed herein.