Abstract:
A method and system for data storage provides a digital fast-write storage device, a plurality of digital slow-write storage devices, and a controller. The digital fast-write storage device might be a solid state drive. The digital slow-write storage devices might be conventional rotational media drives. Typically, read operations are directed to the fast-write storage device. The slow-write storage devices provide redundancy by mirroring the contents of the high-speed storage device. Data on the slow-write storage devices is organized in stripes, allowing data to be written in parallel. The number of slow-write storage devices can be chosen to compensate for the speed differential on write operations. In some embodiments, the controller will represent the storage system as a virtual disk drive.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of data storage, and, more particularly, to mirroring data between storage devices having different write speeds 
     BACKGROUND OF THE INVENTION 
     Storage virtualization inserts a logical abstraction layer or facade between one or more computer systems and one or more physical storage devices. With storage virtualization, a computer can address storage through a virtual disk (VDisk), which responds to the computer as if it were a physical disk (PDisk). A VDisk may be configured using a plurality of physical storage devices and techniques to provide redundancy and improve performance. 
     Virtualization is often performed within a storage area network (SAN), allowing a pool of storage devices with a storage system to be shared by a number of host computers. Hosts are computers running application software, such as software that performs input or output (IO) operations using a database. Connectivity of devices within many modern SANs is implemented using Fibre Channel technology. Ideally, virtualization is implemented in a way that minimizes manual configuration of the relationship between the logical representation of the storage as one or more VDisks, and the implementation of the storage using physical devices. Tasks such as backing up, adding a new physical storage device, and handling failover in the case of an error condition should be handled as automatically as possible. 
     In effect, a VDisk is a facade that allows a set of PDisks, or more generally a set of portions of PDisks, to imitate a single PDisk. Virtualization techniques for configuring the PDisks behind the VDisk facade can improve performance and reliability compared to the more traditional approach of a disk drive directly connected to a single computer system. Standard virtualization techniques include mirroring, striping, concatenation, and writing parity information. 
     Mirroring involves maintaining two or more separate copies of data. Strictly speaking, mirroring involves maintaining copies of the contents of an extent, either a real extent or a virtual extent. An extent is a set of consecutively addressed units of storage (e.g., bytes or words). The copies are maintained on an ongoing basis over a period of time. During that time, the data within the mirrored extent might change, but the mirroring relationship will be maintained such that the copies change correspondingly. When we say herein that data is being mirrored, it should be understood to mean that an extent containing data is being mirrored. 
     Typically, the copies are located on distinct disks that, for purposes of security or disaster recover, are sometimes remote from each other, in different areas of a building, different buildings, or different cities. Mirroring provides redundancy. If a device containing one copy, or a portion of a copy, suffers a failure of functionality (e.g., a mechanical or electrical problem), then that device can be serviced or removed while one or more of the other copies is used to provide storage and access to existing data. Mirroring can also be used to improve read performance. Given copies of data on drives A and B, then a read request can be satisfied by reading, in parallel, a portion of the data from A and a different portion of the data from B. Alternatively, a read request can be sent to both A and B. The request is satisfied from either A or B, whichever returns the required data first. If A returns the data first then the request to B can either be cancelled, or the request to B can be allowed to proceed, but the results will be ignored. Mirroring can be performed synchronously or asynchronously. 
     Striping involves splitting data into smaller pieces, called “stripes.” Logically sequential data stripes are written to separate storage devices, in a round-robin fashion. For example, suppose a file or dataset were regarded as consisting of 6 contiguous parts of equal size, numbered 1 to 6. Striping these across 3 drives would typically be implemented with parts 1 and 4 as stripes on the first drive; parts 2 and 5 as stripes on the second drive; and parts 3 and 6 as stripes on the third drive. Striping improves performance on conventional hard disks because data does not need to be written sequentially by a single drive, but instead can be written in parallel by several drives. In the example just described, stripes 1, 2, and 3 could be written in parallel. Striping can reduce reliability, however, because failure of any one of the devices holding stripes will render the data in that entire copy unrecoverable. To avoid this, striping and mirroring are often combined. 
     Writing of parity information is an alternative to mirroring for recovery of data upon failure. In parity redundancy, redundant data is typically calculated from several areas (e.g., 2, 4, or 8 different areas) of the storage system and then stored in one area of the storage system. The size of the redundant storage area is less than the remaining storage area used to store the original data. 
     A Redundant Array of Independent (or Inexpensive) Disks (RAID) describes several levels of storage architectures that employ the above techniques. For example, a RAID 0 architecture is a striped disk array that is configured without any redundancy. Since RAID 0 is not a redundant architecture, it is often omitted from a discussion of RAID systems. A RAID 1 architecture involves storage disks configured according to mirror redundancy. Original data is stored on one set of disks and duplicate copies of the data are maintained on separate disks. Conventionally, a RAID 1 configuration has an extent that fills all the disks involved in the mirroring. In practice, mirroring sometimes only utilizes a fraction of a disk, such as a single partition, with the remainder being used for other purposes. The RAID 2 through RAID 5 architectures each involve parity-type redundant storage. RAID 10 is simply a combination of RAID 0 (striping) and RAID 1 (mirroring). This RAID type allows a single array to be striped over two or more PDisks with the stripes also mirrored over two or more PDisks. 
     Concatenation involves combining two or more disks, or disk partitions, so that the combination behaves as if it were a single disk. Not explicitly part of the RAID levels, concatenation is a virtualization technique to increase storage capacity behind the VDisk facade. 
     Virtualization can be implemented in any of three storage system levels—in the hosts, in the storage devices, or in a network device operating as an intermediary between hosts and storage devices. Each of these approaches has pros and cons that are well known to practitioners of the art. 
     Various types of storage devices are used in current data processing systems. A typical system may include one or more large capacity tape units and/or disk drives (magnetic, optical, or semiconductor) connected to the systems through respective control units for storing data. High-speed, reliable data storage and file serving is a must for any large computing system. Virtualization, implemented in whole or in part as one or more RAIDs, is a preferred method of providing high-speed, reliable data storage and file serving. 
     A VDisk is usually represented to the host by the storage system as a logical unit number (LUN) or as a mass storage device. Typically, a VDisk is simply the logical combination of one or more RAIDs. 
     Because a VDisk emulates the behavior of a PDisk, virtualization can be done hierarchically. An example of this principle, VDisk mirroring is a critical component of virtualized storage systems. The concept is to create a separate RAID that is used to duplicate an existing RAID. As already described, mirroring allows data recovery and access via the mirrored system when a serious event disables the entire primary system, or even just the primary VDisk (or RAID) if all of the RAIDs are in the same system. In VDisk mirroring, the copies will have the same size but can otherwise have very different virtual configurations, such as different types of RAIDs. For example, a VDisk containing two 200 gigabyte (200G) RAID 5 arrays may be mirrored to a VDisk that contains one 400G RAID 10 array. 
     Solid state drives (SSDs), sometimes called solid state disks, are a major advance in storage system technology. An SSD is a data storage device that uses non-volatile memory such as flash, or volatile memory, such as SDRAM, to store data. The SSD can replace a conventional rotational media hard drive (RMD), which has spinning platters. There are a number of advantages of SSDs in comparison to traditional RMDs, including much faster read and write times, better mechanical reliability, much greater IO capacity, an extremely low latency, and zero seek time. A typical RMD may have an input/output (IO) capacity of 200 random IO operations per second, while a typical DRAM SSD may have an IO capacity of 20,000 random IOs per second. This speed improvement of nominally two orders of magnitude is offset, however, by a cost of SSD storage that, at today&#39;s prices, is roughly two orders of magnitude higher than RMD storage. 
     The invention encompasses any situation in which a device with a fast write speed, or input/output capacity for writes, is mirrored to a plurality of devices with lower write speeds. For example, a DRAM SSD is about 20% faster for reading than a Flash SSD, but may be 10 to 20 times faster for writing. Currently, the cost per gigabyte of DRAM SSD is roughly 16 times that of Flash SSD. Some embodiments of the invention include a DRAM SSD that is mirrored to a plurality of Flash SSDs, across which the data is striped. Even with technologies that do not exist today, the approach of the invention will allow discrepancies in write speeds between types of mirroring devices to be compensated for by striping across a plurality of slower devices. 
     SUMMARY OF THE INVENTION 
     A storage system employing only SSDs for mirroring would provide excellent read and write performance. This is true whether the mirroring is traditional RAID mirroring, or VDisk mirroring. But the cost of maintaining two or more copies of data, each copy on a separate SSD, would be high. Because of their high speed, striping across SSDs is rarely used. 
     Anything that can be done to reduce the cost per gigabyte of storage while improving the performance of a mirrored pair over an approach based solely on RMDs can have a dramatic effect on applications for which the storage system is the primary bottleneck for throughput or responsiveness. 
     The inventor has realized that a mirroring solution having much of the performance advantage of mirroring between two SSDs can be obtained by mirroring data between an SSD and a relatively large number of significantly slower RMDs, striping data from the SSD across the RMDs. Under normal circumstances, all read operations can be done by accessing the SSD. The same inventive concept applies more generally, whenever a device having a fast write speed is mirrored to a plurality of devices, across which the data is striped, having a slower write speed. 
     The large number of RMDs utilized in striping the mirror copy will allow substantial parallelization of write operations, in effect recovering much of the performance advantage that would be achieved by having two SSDs mirror each other. A synchronous write operation requires completion by all the RMDs involved in striping. In an exemplary system employing fifty RMDs, a write to the RMDs would complete in about twice the time the corresponding write would require on the SSD. Initial purchase of so many RMDs would itself be costly. On the other hand, the RMDs are legacy equipment that a company might have available already, which might be complemented by a new SSD to dramatically improve mirror performance. In some embodiments of the invention, the combined write speed of the slow-write storage devices (e.g., RMDs) is no less than 25% that of the fast-write storage device (e.g., SSD) for writing the mirror copy, while the fast-write storage device is at least 10 times as fast in write operations as an individual slow-write storage device. 
     The entire SSD might require mirroring, but it is also possible that a mirroring scheme might apply to just some portion of the SSD. The SSD itself might be subdivided into regions, such as partitions analogously to partitions on an RMD. Every SSD has some smallest addressable unit of storage, which we will refer to as a data storage cell. The data storage cell might be a single byte, or two or more bytes in length. It is easiest to conceptualize a “region” containing data to be mirrored as consisting of contiguous storage cells arranged sequentially. Because the hardware characteristics of an SSD are similar to computer memory, a “region” of data to be mirrored might consist of two or more discontinuous extents, possibly connected by pointers. Henceforth in this document, mirroring the SSD should be understood to mean mirroring all of the SSD or the portion of the SSD involved in the particular configuration. The region of the SSD being mirrored might itself contain a RAID, or any virtualized configuration of data. 
     The invention is not limited to embodiments including a single SSD. For example, two SSDs might mirror each other, with a third copy being provided by a mirroring set of RMDs. This configuration would allow one SSD to go offline without losing high-speed reads and without losing redundancy. In other embodiments, a single data object, dataset, or storage extent could be spread across portions of multiple SSDs; this object, dataset, or extent could be mirrored to, and striped across, a plurality of RMDs or RMD portions. 
     In some embodiments, there can be multiple mirror copies of data from an SSD maintained on RMDs. An SSD mirrored to two sets of RMDs provides more redundancy than mirroring to just one set, without appreciably sacrificing either read or write speed. 
     In some embodiments of the invention, a virtual or RAID configuration of data is maintained on one or more SSDs for IO operations. One or more mirrors of this virtual or RAID configuration are maintained on respective sets of rotational media drive. 
     In still more general embodiments of the invention, data, which might include a virtual or a RAID configuration of data, is maintained on one or more fast-write storage devices. One or more mirrors of this data are maintained on respective sets of slow-write storage devices, where the fast-write storage devices have higher IO capacities for writing than the slow-write storage devices, in some embodiments by a factor of 10, 100, or more. Striping of data across sufficiently many RMDs can result in 25%, 50%, or more of the write IO capacity of an SSD. 
     The storage system in some embodiments of the invention includes a controller. The controller receives requests for IO operations that involve the portion of the SSD being mirrored. The controller contains a management module and a storage module. The storage module interacts with the PDisks to read and write data thereon. The management module directs the storage module in determining the physical device, and locations thereon, to use for particular IO operations. The management module contains logic that determines whether the fast-write storage device, the slow-write storage devices, or both will be used to respond to a request. If the fast-write storage device is faster for reading than the slow-write storage device, as will typically be the case, the management module will direct the storage module to use the fast-write storage device to service read requests under normal operating conditions. Upon an event that results in the fast-write storage device becoming unavailable, the controller will direct read requests to the mirror copy on the slow-write storage devices, unless another mirror copy is being maintained on a faster device. 
     The controller, the management module, and the storage module are defined functionally, and not by the enclosure housing them. The controller might be contained in a single device, or distributed over several devices. The controller might reside in any storage system level. For example, it might be in a host or hosts; or in one or more physical storage devices; or in a separate network device. It might be spread over two or more of these levels. For example, the management module might be housed in a network device, and each physical storage device might contain a portion of an aggregate storage module. 
     The storage system may be connected to a digital communication system. The digital communication system may be a wide area network (WAN), a local area network (LAN), a personal area network (PAN), or some combination thereof. The network may be wired, wireless, or implemented in whole or in part by a hardware bus. In some embodiments, the digital communication system will incorporate Fibre Channel technology. The digital communication system provides access by one or more hosts to the storage system. The hosts may run application software, which might access databases or other data objects represented in storage that is included in the mirroring operation. Requests to the controller for IO operations commonly originate from such hosts. 
     The controller may provide an interface whereby contents of the storage system can be accessed as a VDisk. This VDisk might be so simple as to just represent the data being mirrored between the fast-write storage device and the slow-write storage devices. Also within the scope of the invention, however, the mirror might be a single element in a complex virtualization scheme, represented by the VDisk, that involves various RAIDs or RAID-like storage management techniques. 
     When the copy of data mirroring a portion of an SSD is being striped across a plurality of RMDs, some of the RMDs may be experiencing load from other processes using portions of the drive not involved in the mirroring relationship. Prioritization of the load on a particular drive based on various factors may cause the stripes in the mirror copy to be written in some time order different from the sequence in which the data is arranged on the fast-write storage device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating a storage system according to an embodiment of the present invention, involving an SSD mirrored by a plurality of RMDs over which data is striped. 
         FIG. 2  is a block diagram illustrating mirroring according to an embodiment of the invention, involving an SSD mirrored to two differently configured RAID copies on RMDs, over which data is striped. 
         FIG. 3  is a block diagram illustrating mirroring according to an embodiment of the invention, in which an extent spanning two SSDs is mirrored to a plurality of RMDs over which data is striped. 
         FIG. 4  is a block diagram illustrating mirroring according to an embodiment of the invention. 
         FIG. 5  is a flowchart illustrating the processing of read and write requests in an embodiment of the invention. 
         FIG. 6  is a flowchart illustrating how processes, other than the mirroring relationship to which the invention pertains, that access an RMD may affect the order in which stripes are written. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  illustrates a storage system  160  according to an embodiment of the present invention. The storage system  160  includes at least one digital fast-write storage device  101  and at least one digital slow-write storage device  111 . The slow-write storage devices  111  mirror data from at least one of the fast-write storage devices  101 . “Fast-write” and “slow-write” refer to the respective IO capacities for writing, or the write speed, of the devices. In some embodiments, the fast-write storage devices  101  also have significantly higher read speeds than the slow-write storage devices  111 , but in other embodiments, their read speeds might be comparable. It is even possible, within the scope of the invention, that the slow-write storage devices  111  might be faster for reading, possibly substantially so, than the fast-write storage devices  101 . 
     The data being mirrored is striped across the slow-write storage devices  111 . In some embodiments of the invention, the fast-write storage devices  101  are solid state drives (SSDs)  100 . In some embodiments, the slow-write storage devices  111  are rotational media disks (RMDs)  110 . 
     The storage system  160  includes a controller  120  that carries out requests for operations that, for example, read data from, or write data to, the storage system  160 . The controller  120  includes a management module  121  and a storage module  122 . The storage module  122  accesses the storage devices  102  for IO operations. The management module  121  receives requests through a host communication system  133  to which the controller  120  is connected. Such requests typically originate from application hosts  140 . The management module  121  controls how the storage module  122  fulfills the requests. The management module  121  may also monitor system operations, collect performance and operations statistics, and transmit diagnostics in response to various conditions or events, such as device failures. In some embodiments, the management module  121  also automatically reconfigures the storage system  160  as necessary under certain circumstances. For example, if a device containing mirrored data fails, the management module  121  may direct the storage module  122  to execute all IO operations using devices holding another copy. If the storage system  160  is virtualized, the management module  121  will respond to requests from hosts  140  for IO as if it were a single hard drive, while causing the storage system  160  to implement those operations necessary to implement the virtualization configuration, which might include operations such as mirroring and striping. 
     The management module  121  and the storage module  122  implement logic that executes instructions. The instructions may be stored in digital storage of any kind. Within the scope of the invention, however, the logic, and any instructions that embody the logic, may be implemented in hardware or software, or any combination thereof. 
     Communications among the controller  120  and the storage devices  102  are transmitted across a storage communication system  132 . In general, a digital communication system  130  must be capable of transferring information in digital form. It can be wired or wireless. It can be a network  131 , such as a wide area network (WAN), a local area network (LAN), a personal area network (PAN); or a hardware bus. It can be some combination of communications means or networks  131 . 
     The storage system  160  will be used by one or more application hosts  140  for storage and retrieval of data. Preferably, configuration of data on the physical storage devices  102  will be handled by the controller  120 , which will provide access to the hosts  140  through a virtualized configuration, which might include one or more levels of RAID. Virtualization presents a facade that hides from the hosts complexities such as allocation of data to physical drives, mirroring, and striping. Communications between the hosts  140  and the storage system  160  utilize a host communication system  133 , which is another digital communication system  130  and, in some embodiments, a network  131 . In some embodiments, actual transfer of data between a host  140  and a physical storage device may incorporate a Fibre Channel system. The host communication system  133  and the storage communication system  132  may be the same communication system  130  in some embodiments. The connections  150  of the various devices and systems to the communication systems  130  are all shown with a single reference number, but they can be of any types. 
     The basic concept of the invention is to mirror fast storage to slower storage, striping the mirrored data over a plurality of the slow-write storage devices  111 . The idea is to use sufficiently many of the slow-write storage devices  111  that a write operation to the mirror copy on the slow-write storage devices  111  will complete in a time not significantly longer than a simultaneous write to the fast-write storage devices  101 , thus compensating for the write speed differential. The number of slow-write storage devices  111  can be chosen to satisfy any specified degree of compensation. The data on the fast-write storage devices  101  can have any structure, and will ordinarily not be separated into noncontiguous stripes. However, there is no reason why, in theory, the data on the fast storage could not exist in noncontiguous stripes; embodiments of the invention encompass that situation as well as the normal one. In most embodiments, the mirroring is carried out by the controller  120 . Under normal operations, whichever storage device  102  has the faster read speed will is used for read access. Generally, those commercial devices that are faster for write operations are also faster for read operations, but not necessarily. A write operation is not completed until all stripes have been written to the slow-write storage devices  111 . Striping over a sufficiently many slow-write storage devices  111  can compensate to any desired degree for their reduced write speed. In the case of a failure of a fast-write storage device  101 , the slower storage can be used for both read and write access to the data. Ordinarily, this arrangement will be temporary until fast-write storage device  101  can be repaired or replaced. Input and output operations will ordinarily be under control of the storage module  122 . Determination of which device to use for a particular operation will ordinarily be done by the management module  121 , which will instruct the storage module  122  to direct or redirect accesses appropriately. 
       FIGS. 2 through 4  illustrate mirroring schemes in exemplary embodiments of the invention. Many configurations other than those shown in the figure are included within the concept of the invention, as already described herein. 
     In the exemplary embodiment of  FIG. 2 , data on a fast-write storage device  101 , namely the SSD  100  labeled  210 , is being mirrored to three slow-write storage devices  111 , namely the RMDs  110  labeled  221 ,  222 , and  223 . In this example, as in others, the data involved in the mirror may occupy all the storage space on a given storage device  102 . More generally, however, the data being mirrored may only take up a portion of any given storage device  102 . This is particularly important with respect to write operations on slow-write storage devices  111 , which may have load from other processes that can contend with the mirroring relationship. 
     The relevant data on the SSD  210  is logically divided into six subextents  200  that correspond to stripes  201 . Of course, actual data might be divided into any number of stripes  201 . Individual stripes  201  and corresponding subextents  200  will be labeled cDs, where c is the copy index number and s is the stripe  201  index number. (This convention is also used in  FIGS. 3 and 4 . For example, 2D4 refers to the fourth stripe of the second copy  220  of the data. The six stripes  201  in the second copy  220  are labeled 2D1 through 2D6. In the illustration, these are spread across the three RMDs  110 , with stripes 2D1 and 2D4 on RMD  221 , 2D2 and 2D5 on  222 , and 2D3 and 2D6 on  223 . The subextents  200  corresponding to the stripes  201  on the RMDs  110  in the first copy  215  of the data, located on the SSD  100 , are labeled 1D1 through 1D6. The mirroring relationship is designated by an arrow  240 . As described herein, under normal operations all reads will be done against the SSD  100  because it is faster for read operations than the RMDs  110 . Writing the two copies of the data will only complete when the slowest device finishes writing its portion of the data. But the use of three RMDs  110  will substantially reduce the time to about one third the time in which a solitary RMD  110  can write the data. Obviously, more RMDs  110  would reduce the time further. Based upon typical current prices and relative IO capacities, a second copy striped to 50 RMDs  110  would take about twice as long to create as would writing a second copy to a second SSD  100 . The cost of the RMD  110  hardware itself would be half that of a new SSD  100 , but many companies have legacy RMDs  110  available that could be used for the task. 
     Note that the devices, whether fast-write storage devices  101  or slow-write storage devices  111 , can be accessed in any order for reading or writing the stripes. That is, subextents that are logically consecutive on the fast-write storage device  101 , which correspond to stripes, do not need to be written consecutively in time. Load on a device from other processes may dictate when logic on the controller  120  causes a particular stripe  201  to be written to that device. 
     Striping a copy of the data, especially across a large number of devices, introduces vulnerability because the loss of a single device renders the entire copy unreadable. Consequently, it often makes sense to have one source mirror copy and two or more target mirror copies.  FIG. 3  illustrates this principle in another embodiment of the invention. In this case, there are three copies of the data, one (labeled  315 ) on an SSD  100  labeled  310 , and two (labeled  320  and  330 ) on respective sets of RMDs  110 . The mirroring relationships are indicated by arrows labeled  340  and  341 . Note that the two sets of RMDs  110  have different RAID configurations. The first set  320  includes three drives (labeled  431 ,  432 , and  433 ), each with two stripes  201 . The second set  330  includes two drives (labeled  434  and  435 ), each with three stripes  201 . This illustrates that, in general, the striping schemes across the respective sets of slow-write storage devices  111  may be different. The drives in set  320  might perhaps perform writes more slowly than those in set  330 , motivating striping across more drives to compensate. In the examples of  FIG. 2  and  FIG. 3 , one or more additional copies (not shown) could be made to other fast-write storage devices  101 , such as SSDs  100 . 
       FIG. 4  illustrates an embodiment of the invention in which data spread across a plurality of fast-write storage devices  101  is mirrored to a plurality of slow-write storage devices  111 . In the exemplary case, there are two copies of the data, a first copy  410  and a second copy  430 . The two copies are mirrors of each other, as indicated by an arrow  420 . The first copy  410  is maintained on two SSDs  100 , labeled  411  and  412 . The second copy  430  includes the data in striped form spread across five RMDs  110 , labeled consecutively from  431  to  435 . Each RMD  110  contains two stripes  201  that reflect subextents  200  of the data on the SSDs  100 . The illustrated mirroring scheme is an example of a class of embodiments in which there are at least two fast-write storage devices  101  mirrored to at least two slow-write storage devices  111 . The number of stripes into which the data is divided must be at least two, but could be any larger number. 
       FIG. 5  is a flowchart illustrating how read and write requests, which are typically received from hosts  140  connected to a network  131 , are handled in an embodiment of the invention under normal conditions. This embodiment assumes that the fast-write storage device  101  is also faster than the slow-write storage devices  111  for reading. The subextents  200  of data on one or more fast-write storage devices  101  are associated  510  in a mirroring relationship with stripes  201  spread across a plurality of slow-write storage devices  111 . In the exemplary embodiment, the fast-write storage devices  101  are SSDs  100 , and the slow-write storage devices  111  are RMDs  110 . FIGS.  2  through  4  show some typical kinds of mirroring relationships that might be established by step  510 . In some embodiments, a controller  120  maintains this association. At some point in the process, a check is made  515  to determine whether the process should end. For convenience, this step is shown at this point in the flowchart, but in other embodiments it could be elsewhere. An IO request is received  520  from a host communication system  133 , typically transmitted by an application host  140 , accessing portions of data storage involved in the mirroring relationship. Typically, this request will be received by the controller  120 . The management module  121  will direct the storage module  122 , which will actually perform any read or write access to the physical media, including the SSDs  100  and the RMDs  110 . If  530  a read operation is to be performed, the data will be read  540  from the SSD  100 . If  560  a write operation is to be performed, the data will be written  570  to both the SSD  100  and to the RMDs  110 . Enough RMDs  110  will be used in the striping so that a write operation will complete on the set of RMDs  110  within some factor (typically, but not necessarily, greater or equal to 1) of the time it takes the SSD  100  to complete the write. If some other request is received, then it will be handled  580  appropriately. Different orderings of these steps are possible within the scope of the invention. For example, the test for a write operation might precede the test for a read operation. Some steps might be omitted (e.g., step  580 ), again within the scope of the invention. 
       FIG. 6  is a flowchart of an exemplary embodiment of the invention, showing a reason why writing of stripes to the RMDs  110  might not occur in time order corresponding to their original sequence as stored on the SSD. As an introduction to this flowchart, consider the striping configuration of  FIG. 2 . If the three RMDs  110  in that figure had the same speed and were dedicated to the mirroring operation (i.e., no processes were using those disks for other purposes), then one might expect stripes 2D1, 2D2, and 2D3 to be written nearly simultaneously in parallel. Subsequent to completion of those operations, stripes 2D4, 2D5, and 2D6 would be written approximately in parallel. 
     But if one or more of the RMDs  110  in  FIG. 2  were being used concurrently for other processes, a different ordering of writes might well occur.  FIG. 6  is a flowchart that shows how load can be considered in choosing when to write particular stripes  201  to a slow-write storage device  111 . Load can influence the timing of writes of individual stripes to the slow-write storage device  111 . Because storage on an SSD  100  (and presumably on any fast-write storage device  101 ) is relatively expensive, a given mirroring relationship is likely to only involve a small fraction of the SSD  100 . It follows that only a small fraction of any given RMD  110 , which might have a large storage capacity, will typically be involved in the mirroring relationship. Other processes may be accessing the RMD  110  for other purposes. In the flowchart, a target RMD  110  upon which to write data in stripes  201  is selected  610 . The stripes  201  correspond to subextents  200  of the SSD  100  that are assigned to that RMD  110  by a mirroring relationship, such as one of those illustrated by  FIGS. 2 through 4 . This selection will ordinarily be done by the controller  120 . Typically, all the data will be assigned to stripes  201 , to be divided among the RMDs  110  using striping schemes well-known to practitioners in the art. The flowchart pertains to any one of the RMDs  110 . Data is selected  620  to be written as a particular stripe  201  to that target RMD  110 . The data may be written to the RMD  110  directly, or it might be first written to an SSD  100  participating in the mirroring operation and then copied to the RMD  110 . Load factors on the target RMD  110  are then considered  630  to determine whether to write the stripe  201  at this time. Factors relevant to determining load include: whether an IO operation is in process on the device; queued requests for use of the device; head thrashing; and priorities associated with, or assigned to, particular operations. Based on consideration of the load factors, a determination is made  640  whether to perform the write now. If so, then the stripe  201  is written  650  to the RMD  110 . Otherwise, the process waits until the load factor check indicates that the write should be performed. If there is more data to write  660  to this drive, the process continues; otherwise it ends.  FIG. 6  is exemplary of embodiments of this aspect of the present invention. Other embodiments may perform the steps in a different order. For example, the check of whether there is more data to stripe might be done at the start rather than at the end of the process. 
     Because the load balancing process illustrated by  FIG. 6  may be done independently for plurality of RMDs  110  upon which stripes  201  are to be written, possibly for each stripe, the order in which stripes  201  are written to the RMDs  110  might not follow the same sequence in which the data appears upon the SSD  100 . Two different RMDs  110  might have very different loads due to other processes competing for their resources. Of course, in the case of the embodiment described in  FIG. 6 , as throughout this description, one or more fast-write storage devices  101  of any type might replace the SSD  100 , and slow-write storage devices  111  of any type might replace the RMDs  110 . The resulting embodiment remains within scope of the invention. 
     It is worth repeating that the invention can be applied to any types of devices such that the write IO capacity of the fast-write storage device  101  is substantially greater than that of the slow-write storage devices  111 . In some embodiments, the fast-write storage device  101  also has a faster read speed than the slow-write storage device  111 . This is true for example where the fast-write storage device  101  is an SSD  100  and the slow-write storage devices  111  are conventional RMDs  110 . In this case, read requests will ordinarily all be directed to the fast-write storage device  101 . In other embodiments, the fast-write storage device  101  and the slow-write storage devices  111  might have comparable read speeds. For example, the fast-write storage device  101  might be a DRAM SSD  100  and the slow-write storage devices  111  might be Flash-based SSDs  100 , or Flash SSD  100 . A DRAM SSD  100  reads only about 20% faster than a Flash SSD  100 , but might write 10 to 20 times as fast. Currently, the cost per gigabyte of DRAM SSD  100  is roughly 16 times that of Flash SSD  100 . Given the price differential, it makes sense to use the approach of the invention in this case, striping a mirror copy over a plurality of Flash SSDs  100 . Ordinarily, in this case, read requests would still go to the DRAM SSD  100 . 
     Embodiments of the present invention in this description are illustrative, and do not limit the scope of the invention. It will be apparent other embodiments may have various changes and modifications without departing from the scope and concept of the invention. The invention is intended to encompass the following claims and their equivalents.