Patent Publication Number: US-2022214834-A1

Title: Ssd architecture supporting low latency operation

Description:
RELATED APPLICATION 
     This application is a divisional of U.S. application Ser. No. 15/800,742 filed on Nov. 1, 2017, which claims the benefit of U.S. provisional application No. 62/542,108 filed on Aug. 7, 2017 (Expired), which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     This invention generally relates to solid state drive (SSD) architectures, and more particularly, to SSD architectures supporting low latency operation. 
     BACKGROUND OF THE INVENTION 
     Conventionally, SSD architectures and designs have primarily focused on obtaining a high average bandwidth or throughput for input and output (I/O) operations (i.e., reading and writing data). Compared to traditional magnetic storage devices such as hard disk drives (HDDs), SSDs are capable of performing I/O operations that are hundreds, if not thousands, of times greater per second as compared to HDDs. Such conventional SSDs are capable of obtaining such high average bandwidth through parallelism in its architecture. 
     An SSD typically comprises a number of non-volatile memory dies, such as NAND flash memory, that are arranged in groups coupled to channels controlled by a channel controller. For example, 128 non-volatile memory dies are arranged as sixteen groups of eight dies, each group coupled to a channel. A physical storage block from each of the non-volatile memory dies are commonly selected to create logical blocks or superblocks for one or more host devices, such as a computer or storage appliance, to write and read data to and from, respectively. Selecting a physical block from each of the non-volatile memory dies to form superblocks allows parallel access to all of the non-volatile memory dies across all channels, achieving maximum bandwidth or throughput. A die may further be organized into multiple “planes” (each die comprising two, four, or more planes), where each plane may process an I/O operation in parallel. 
     While such an SSD architecture maximizes the bandwidth or throughput of an SSD, this architecture also suffers from a number of issues that impact I/O latency (i.e., the amount of time it takes to complete an I/O operation). Due to physical limitations of the non-volatile memory dies, only a single physical block per plane per non-volatile memory die can perform an I/O operation at a time, which leads to collisions between I/O operations to different physical blocks of the same plane of the same non-volatile memory die where an I/O operation must wait until the previous operation to a different block in the same plane has completed. Relatedly, because there are multiple non-volatile memory dies per channel controller, there may also be collisions between commands for I/O operations to different superblocks at the channel controller, due to the shared nature of a channel in which only one data transfer may proceed at any time between the controller and any non-volatile memory die, leading to bottlenecks at each channel controller of the SSD. 
     In addition to I/O operations from hosts, the SSD must perform maintenance operations throughout the lifetime of the SSD, such as garbage collection to consolidate valid data and erase invalid data to create free areas for new data to be written and data reliability operations to read data stored in the non-volatile memory dies and re-write them as necessary to ensure that the data is not lost over time due to the nature of non-volatile memory dies such as NAND flash memory dies. These maintenance operations take place at typically indeterminate times throughout the lifetime of the SSD as needed and last for an indeterminate period of time, which inevitably lead to collisions with host I/O operations at both the channel controllers and the non-volatile memory dies. These collisions, either due to host I/O operations or SSD maintenance operations causes inconsistent and unpredictable SSD latency performance. 
     For example, a typical host read operation can take anywhere between 70-100 μs to complete absent any collisions. However, if there is a collision, the latency of the read operation may be greatly increased. If the non-volatile memory die is currently being written to, the read operation will collide with the write (program) operation already taking place and will have to wait until the write operation completes before the read operation can be performed. In this instance, the latency of the read operation may be more than doubled to 2500-3000 μs for TLC NAND memory. Even worse, if a maintenance operation is currently being performed, such as garbage collection which requires all three of a read, a write, and an erase operation, the latency of the read operation to a physical block of a non-volatile memory die currently undergoing maintenance may be significantly greater—in the millisecond range to complete the read operation. This problem is further exacerbated in a data center or storage center environment storing data across large numbers of SSDs that are being constantly accessed by multiple host applications (i.e., the SSDs are constantly being written to and read from), greatly increasing the likelihood of collisions between I/O operations and maintenance operations. Given the ever increasing need for storage in data centers and the growing popularity of cloud-based and web-based applications, the inconsistent and unpredictable SSD performance provided by conventional SSD architectures focused on achieving high average bandwidth is no longer sufficient to meet the current demands and requirements of such consumers. 
     What is needed, therefore, is an improved SSD architecture that supports low latency operation to provide consistent and predictable I/O operation and performance. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In one embodiment, a solid state drive (SSD) comprises a plurality of non-volatile memory dies communicatively arranged in one or more communication channels, each of the plurality of non-volatile memory dies comprising a plurality of physical blocks, one or more channel controllers communicatively coupled to the one or more communication channels, respectively, and a memory controller communicatively coupled to the plurality of non-volatile memory dies via the one or more channel controllers, wherein the memory controller is configured to assign (i) the plurality of physical blocks of a first die of the plurality of non-volatile memory dies to only a first region and (ii) the plurality of physical blocks of a second die of the plurality of non-volatile memory dies to only a second region, perform only read operations on the first region in a first operation mode, and perform write operations or maintenance operations on the second region in a second operation mode concurrently with read operations on the first region in the first operation mode. In one embodiment, at least one of the first region and the second region comprises the plurality of physical blocks of each of the non-volatile memory dies in one or more communication channels. In one embodiment, at least one of the first region and the second region comprises the plurality of physical blocks of one or more non-volatile memory dies from each of the one or more communication channels. 
     In one embodiment, the memory controller is further configured to create one or more namespaces within at least one of the first region and the second region. In one embodiment, the memory controller is further configured to maintain a maintenance counter for each of the first region and the second region, monotonically decrement the maintenance counter when the corresponding region is in the first operation mode, and monotonically increment the maintenance counter when the corresponding region is in the second operation mode. In one embodiment, each of the first region and the second region operates in the first operation mode when the corresponding maintenance counter is above a first threshold, and operates in the second operation mode when the corresponding maintenance counter is below a second threshold. In one embodiment, the memory controller is further configured to maintain a maintenance counter for each of the first region and the second region, monotonically increment the maintenance counter when the corresponding region is in the first operation mode, and monotonically decrement the maintenance counter when the corresponding region is in the second operation mode. In one embodiment, each of the first region and the second region operates in the first operation mode when the corresponding maintenance counter is below a first threshold, and operates in the second operation mode with the corresponding maintenance counter is above a second threshold. 
     In one embodiment, a method of operating an SSD comprises assigning a first die of a plurality of non-volatile memory die communicatively arranged in one or more communication channels to only a first region, each of the plurality of non-volatile memory dies comprising a plurality of physical blocks, assigning a second die of the plurality of non-volatile memory dies to only a second region, performing only read operations on the first region in a first operation mode, and performing write operations or maintenance operations on the second region in a second operation mode concurrently with read operations on the first region in the first operation mode. In one embodiment, at least one of the first region and the second region comprises the plurality of physical blocks of each of the non-volatile memory dies in one or more communication channels. In one embodiment, at least one of the first region and the second region comprises the plurality of physical blocks of one or more non-volatile memory dies from each of the one or more communication channels. In one embodiment, the method further comprises creating one or more namespaces within at least one of the first region and the second region. In one embodiment, the method further comprises maintaining a maintenance counter for each of the first region and the second region, monotonically decrementing the maintenance counter when the corresponding region is in the first operation mode; and monotonically incrementing the maintenance counter when the corresponding region is in the second operation mode. In one embodiment, the method further comprises operating the first region and the second region in the first operation mode when the corresponding maintenance counter is above a first threshold; and operating the first region and the second region in the second operation mode when the corresponding maintenance counter is below a second threshold. In one embodiment, the method further comprises maintaining a maintenance counter for each of the first region and the second region, monotonically incrementing the maintenance counter when the corresponding region is in the first operation mode, and monotonically decrementing the maintenance counter when the corresponding region is in the second operation mode. In one embodiment, the method further comprises operating the first region and the second region in the first operation mode when the corresponding maintenance counter is below a first threshold, and operating the first region and the second region in the second operation mode when the corresponding maintenance counter is above a second threshold. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a block diagram of the structure of an SSD, according to one embodiment of the invention. 
         FIG. 2 a    is a block diagram of an SSD architecture supporting low latency operation, according to one embodiment of the invention. 
         FIG. 2 b    is a block diagram of two isolation region configurations, according to various embodiments of the present invention. 
         FIG. 3  is a timing diagram of an operation of an SSD architecture supporting low latency operation, according to one embodiment of the invention. 
         FIG. 4  is a plot of read latencies corresponding to two SSD architectures supporting low latency operation according to various embodiments of the present invention. 
         FIGS. 5 a  and 5 b    are block diagrams of allocations of isolation regions within an SSD, according to various embodiments of the invention. 
         FIGS. 5 c -5 e    are block diagrams of various configurations of isolation regions within an SSD, according to various embodiments of the invention. 
         FIG. 6  is a block diagram of various configurations of namespaces within isolation regions, according to one embodiment of the invention. 
         FIG. 7  is a block diagram of the flow of data within an SSD architecture supporting low latency operation, according to one embodiment of the invention. 
         FIG. 8  is a flowchart of method steps for configuring isolation regions and namespaces within an SSD, according to one embodiment of the invention. 
         FIG. 9  is a timing diagram of an operation of an array of SSDs each having architectures supporting low latency operation, according to one embodiment of the invention. 
         FIG. 10  is a flow chart of method steps for low latency operation of an SSD, according to one embodiment of the invention. 
         FIG. 11  is a flow chart of method steps for read data error handling by an SSD, according to one embodiment of the invention. 
         FIG. 12  is a flow chart of method steps for read data error handling by a host device connected to one or more SSDs, according to one embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Part 1: I/O Determinism and the SSD Architecture Supporting I/O Determinism for Low Latency Operation 
       FIG. 1  is a block diagram of the structure of an SSD  100 , according to one embodiment of the invention. As shown in  FIG. 1 , an SSD memory controller  120  is in communication with one or more host devices or host applications (not shown) through a host interface  110 . The host device may comprise any suitable device, such as a computer or storage appliance. The SSD  100  includes both a volatile memory  130 , and an array of non-volatile memory dies  140 . The volatile memory device  130  and the array of non-volatile memory dies  140  are in communication  124  and  122 , respectively, with the SSD memory controller  120 . 
     The array of non-volatile memory dies  140  comprises non-volatile memory dies  142   a - d ,  144   a - d ,  146   a - d , and  148   a - d  that may be arranged in one or more channels in communication  122  with the SSD memory controller  120 . While 16 non-volatile memory dies  142   a - d ,  144   a - d ,  146   a - d , and  148   a - d  are shown in  FIG. 1 , the array of non-volatile memory dies  140  of the SSD  100  may comprise any suitable number of non-volatile memory dies  140  that are arranged in one or more channels in communication  122  with the SSD memory controller  120 . In one embodiment, the volatile memory device  130  comprises a volatile memory DRAM buffer. The volatile memory device  130  need not be a single device of a unitary type, and may comprise multiple devices of different types capable of providing a volatile memory buffer for the SSD  100 . In one embodiment, the non-volatile memory dies  142   a - d ,  144   a - d ,  146   a - d , and  148   a - d  comprise NAND flash memory. 
       FIG. 2 a    is a block diagram of an SSD architecture supporting low latency operation, according to one embodiment of the invention. As shown in  FIG. 2   a,  32 non-volatile memory dies are arranged across 8 channels (CH 0 -CH 7 ), with 4 non-volatile memory dies coupled to each channel. Rather than superblocks formed by selecting a physical block from each of the non-volatile memory dies across all 8 channels as is done in the prior art, the SSD architecture of  FIG. 2 a    includes superblocks formed by selecting all of the physical blocks from each of the non-volatile memory dies on the same channel and an adjacent channel (denoted by similar shading). Each of these superblocks forms an isolation region within the SSD that is relatively unaffected by activities in other regions of the SSD. Thus, as shown in  FIG. 2 a   , the physical blocks of the non-volatile memory dies on the first two channels (starting from left to right) comprise an isolation region  202 , the physical blocks of the non-volatile memory dies in the next two channels comprise an isolation region  204 , and so on and so forth for isolation regions  206  and  208 . 
     By configuring the superblocks using the physical blocks from the same non-volatile memory dies in the same channel(s) to form isolation regions, such as shown in  FIG. 2 a   , I/O operation collisions at the non-volatile memory dies and the channel controllers can be minimized or avoided altogether, allowing for lower latency, greater predictability, and more consistent I/O performance of the SSD. Of course, configuring the superblocks in the manner shown in  FIG. 2 a    decreases the maximum possible throughput of the SSD as compared to the prior art configuration because the isolation regions  202 ,  204 ,  206 , and  208  of  FIG. 2 a    are not leveraging all 8 available channels to achieve maximum parallel access to all of the non-volatile memory dies. However, this tradeoff is offset in the architecture shown in  FIG. 2 a    by selecting physical blocks from each of the non-volatile memory dies across two channels per isolation region  202 ,  204 ,  206  and  208 , thereby leveraging two channel controllers per isolation region to handle host I/O operations. 
     The selection and arrangement of physical blocks to form isolation regions, as well as the aggregation of isolation regions and the flow of data within the SSD using this architecture is explained in greater detail below. 
       FIG. 2 b    is a block diagram of two isolation region configurations  220  and  230 , according to various embodiments of the present invention. As shown in  FIG. 2   b,  16 non-volatile memory dies  210   a - 210   d ,  212   a - 212   d ,  214   a - 214   d , and  216   a - 216   d  are arranged across four channels CH 0  to CH 3 , with four non-volatile memory dies per channel. Similar to the configuration shown in  FIG. 2 a   , isolation region  220  comprises each of the physical blocks of the non-volatile memory dies  210   a - 210   d  of channel CH 0  and the non-volatile memory dies  212   a - 212   d  of channel CH 1 . Again, as previously explained with respect to  FIG. 2 a   , the isolation region  220  minimizes I/O operation collisions at both the non-volatile memory dies  210   a - d  and  212   a - d  and the channel controllers of channels CH 0  and CH 1  to enable lower latency and more predictable operation at the expense of maximum I/O throughput. 
     Alternatively, rather than selecting each of the physical blocks of all of the non-volatile memory dies of a channel or multiple channels, such as isolation region  220  shown in  FIG. 2 b    or the isolation regions  202 ,  204 ,  206  and  208  shown in  FIG. 2 a   , an isolation region  230  comprises each of the physical blocks of non-volatile memory dies  210   a  and  210   b  of channel CH 0 , each of the physical blocks of non-volatile memory dies  212   a  and  212   b  of channel CH 1 , each of the physical blocks of non-volatile memory dies  214   a  and  214   b  of channel CH 2 , and each of the physical blocks of non-volatile memory dies  216   a  and  216   b.    
     As compared to isolation region  220 , isolation region  230  will have improved I/O throughput as it provides parallel access to physical blocks from the non-volatile memory dies across all four channels CH 0  to CH 3 , rather than being limited to two channels CH 0  and CH 1  in the case of isolation region  220 . However, with the physical block configuration of isolation region  230 , there may be some I/O operation collisions at the channel controllers of channels CH 0 -CH 3  assuming other regions of the SSD are similarly configured as isolation region  230  (i.e., comprising each of the physical blocks from non-volatile memory dies across channels CH 0 -CH 3 ), such as an isolation region comprising the physical blocks of non-volatile memory dies  210   c  and  210   d  of channel CH 0 , the physical blocks of non-volatile memory dies  212   c  and  212   d  of channel CH 1 , the physical blocks of non-volatile memory dies  214   c  and  214   d  of channel CH 2 , and the physical blocks of non-volatile memory dies  216   c  and  216   d  of channel CH 3 . I/O operations to non-volatile memory dies in the isolation region  230  may collide with I/O operations to other non-volatile memory dies of other similarly-configured isolation regions that are on the same channel (i.e., a channel collision), potentially causing some unpredictable I/O performance and higher than expected latencies. 
     As will be explained in greater detail below, collisions of I/O operations at both the non-volatile memory dies and the channel controllers can be minimized or avoided for SSDs architectures utilizing either the configuration of isolation region  220  or the configuration of isolation region  230  through management of the operation of the isolation regions and the SSD. Moreover, it is not necessary for the SSD architecture to only comprise a single isolation region configuration (i.e., only isolation regions like isolation region  220  or only isolation regions like isolation region  230 ), and may comprise isolation regions comprising each physical block of the non-volatile memory dies of one or more channels (similar to that of isolation region  220 ) and isolation regions comprising each physical block of the non-volatile memory dies across multiple channels (similar to that of isolation region  230 ). 
       FIG. 3  is a timing diagram  300  of an operation of an SSD architecture supporting low latency operation, according to one embodiment of the invention. As shown in  FIG. 3 , an isolation region according to the various configurations shown and described in  FIGS. 2 a  and 2 b    operates in two distinct modes over a period of time—a deterministic mode  302  and a non-deterministic or maintenance mode  304 . In the deterministic mode  302 , the isolation region performs only read operations, which as previously explained, are completed relatively quickly with minimal latency (e.g., between 60 μs to 100 μs typically for TLC NAND flash memory). In one embodiment, when an isolation region is in the deterministic mode  302  write commands for the isolation region are queued in the memory controller of the SSD until the isolation region enters the non-deterministic mode  304 . In another embodiment, the host device is aware that the isolation region is in the deterministic mode  302  and will not issue write commands to the isolation region until the isolation region enters the non-deterministic mode  304 . 
     Because the isolation region comprises all of the physical blocks of a given non-volatile memory die, the read operations during the deterministic mode  302  will not be affected by other I/O operations and maintenance operations in other isolation regions within the SSD at the non-volatile memory die level. Moreover, if the isolation region is configured in the same manner as isolation region  220  shown in  FIG. 2 b    or isolation regions  202 - 208  as shown in  FIG. 2 a   , then channel controller collisions are also avoided as the isolation region will have dedicated channel controller(s) for the non-volatile memory dies. Thus, when operating in the deterministic mode  302 , the I/O performance of the isolation region will be deterministic, with read operations completing with minimal latency in this mode. 
     As read operations are performed in the isolation region during the deterministic mode  302 , the need to perform maintenance operations on the isolation region will correspondingly increase. Additionally, there may be an accumulation of write operations pending at the host application for the isolation region (if only read operations are performed during the deterministic mode  302 ). As such, after a period in the deterministic mode  302 , the isolation region will enter the non-deterministic mode  304  once a threshold level of maintenance tasks become necessary. In the non-deterministic mode  304 , the isolation region will perform maintenance tasks and write operations. While it is possible, it is not necessary for all of the maintenance tasks and write operations to be performed before the isolation region re-enters the deterministic mode  302 . In one embodiment, the isolation region may enter the deterministic mode  302  from the non-deterministic mode  304  once a threshold level of maintenance tasks have been completed. 
     In this manner, the isolation region will toggle back-and-forth between the deterministic mode  302  and the non-deterministic mode  304 . The deterministic and non-deterministic operations of the isolation region, as well as coordination between multiple isolation regions within and external to an SSD will be explained in greater detail below. 
       FIG. 4  is a plot of read latencies corresponding to two SSD architectures supporting low latency operation according to various embodiments of the present invention. As shown in  FIG. 4 , three read latency plots  402 ,  404 , and  406  are depicted. Read latency plot  402  corresponds to the SSD architecture of the prior art, where a physical block from each non-volatile memory die across all channels is picked to form a logical block (a superblock). Read latency plot  404  corresponds to the SSD architecture shown in  FIG. 2 b    where the isolation regions of the SSD comprise each physical block of non-volatile memory dies across all channels, similar to the configuration of isolation region  230 . Finally, read latency plot  406  corresponds to the SSD architecture shown in  FIGS. 2 a  and 2 b   , whereby the isolation regions of the SSD comprise each physical block of all the non-volatile memory dies from one or more channels, similar to the configuration of isolation regions  202 - 208  shown in  FIG. 2 a    or isolation region  220  shown in  FIG. 2   b.    
     The data represented by all three read latency plots  402 ,  404 , and  406  were measured using the same test system and parameters (i.e., same processor, motherboard, operating system, and workload). For the read latency plots  404  and  406 , the SSD architecture comprises four isolation regions, with one isolation region in deterministic mode (i.e., read operations only) while the remaining three isolation regions are all performing write operations only. The read latency plots  404  and  406  are generated by measuring the read operation latencies of the isolation region in deterministic mode. 
     As shown in  FIG. 4 , for read latency plot  402 , the read latency of 90 percent of read operations are spread across a wide range of latencies, from about 100 μs to 5 ms. The latency for the 99 th  percentile of read operations further increases to about 10 ms, and gets much worse for the 99.9 th  percentile and 99.99 th  percentile of read operations reaching nearly 20 ms to complete. While a 10 ms to 30 ms read operation may appear to have a relatively low chance of occurring (i.e., one out of every thousand to ten thousand read operations), a typical SSD will have tens or hundreds of thousands of I/O operations occurring per second which means that during every second of operation there is a strong possibility that multiple read operations will take 10 ms to 30 ms. The dramatically increasing latencies of the read latency plot  402  are attributable to read operations colliding with other I/O operations and maintenance operations at both the non-volatile memory die level and at the channel controllers due to the SSD architecture of the prior art, as previously explained in the background. Further, in datacenters where operations are at “hyperscale” (i.e., an operation such as serving a web page is spread out as thousands of sub-operations among thousands of servers, with the results of the thousands of sub-operations aggregated), a one-in-thousands chance of a latency outlier means that every operation will be delayed. 
     The read latency plot  404  shows a marked improvement over the read latency plot  402 , with the read latency of 90 percent of read operations being less than about 400 μs, and the read latency of 99.99 percent of read operations being less than about 1100 μs. The read latency plot  404  shows about a fifteen times (15×) improvement over the read latency plot  402 , with a much smaller range of variation. The read latency plot  404  shows that read operations are almost guaranteed to be completed within about 1100 μs, with the majority of read operations (99 percent) completing within about 700 μs. Thus, the SSD architecture corresponding to the read latency plot  404 , in which the isolation regions of the SSD comprise each physical block of non-volatile memory dies across all channels, similar to the configuration of isolation region  230  of  FIG. 2 b   , provides for much lower read latency operation and more predictable, deterministic, operation as compared to the SSD architecture of the prior art. 
     However, as shown in the read latency plot  404 , there still is an increase in read latency at higher percentiles of operation. The increase in read latency is attributable to collisions at the channel controller due to the configuration of the isolation regions of this SSD architecture (i.e., the isolation regions of the SSD comprise each physical block of non-volatile memory dies across all channels, similar to the configuration of isolation region  230  of  FIG. 2 b   ). As previously explained, with this SSD architecture read operations to the isolation region in the deterministic mode may be queued up behind write operations to other isolation regions at the channel controller level as multiple isolation regions share the same channel controller, and so the read operations to the isolation region in the deterministic mode must wait for those operations to complete before being executed. 
     Read latency plot  406  continues the trend of improving read latencies with the read latency of 90 percent of read operations between about 100 μs and 180 μs. The 99 th  percentile, 99.9 th  percentile, and 99.99 th  percentile read latencies reach a maximum at about 200 μs, 220 μs, and 280 μs, respectively, tapering off at about a little more than 300 μs in the 99.9999 th  percentile of read operations. The read latency plot  406  shows a more than fifty times (50×) improvement over the read latency plot  402  and an even smaller range of variation as compared to the read latency plot  404 , with 99.99999 percent of read operations completing in between about 100 μs and 300 μs. The SSD architecture corresponding to the read latency plot  406  has improved low latency operation and deterministic behavior compared to the SSD architecture corresponding to the read latency plot  404 , which is already greatly improved over the SSD architecture of the prior art corresponding to the read latency plot  402  as explained above, because the SSD architecture corresponding to the read latency plot  406  does not spread multiple isolation regions across a given channel, thus avoiding collisions at the channel controller level as well as the non-volatile memory die level. 
     As such, similar to the SSD architecture corresponding to the read latency plot  404 , the SSD architecture corresponding to the read latency plot  406  also provides for low latency operation and predictable, deterministic, operation as compared to the SSD architecture of the prior art. 
     Part 2: Isolation Regions, Namespaces, and the Flow of Data within the SSD Architecture 
       FIG. 5 a    is a block diagram of a plurality of superblocks in a non-volatile memory array of an SSD, according to one embodiment of the invention. The non-volatile memory array of  FIG. 5 a    includes eight channels (CH 0 -CH 7 ) with four non-volatile memory dies (D 0 -D 3 ) coupled to each channel by a channel bus (not shown). Each non-volatile memory die includes a plurality of physical blocks (not shown). During the initial boot up of the SSD, the firmware mapped the physical blocks of the non-volatile memory array into four superblocks  502 ,  504 ,  506 , and  508 . Each of superblocks  502 ,  504 ,  506 , and  508  is a logical block that has been assigned a plurality of physical blocks in the non-volatile memory array. Superblock  502  includes all physical blocks on dies D 0 -D 3  of channels CH 0  and CH 1 . Superblock  504  includes all physical blocks on dies D 0 -D 3  of channels CH 2  and CH 3 . Superblock  506  includes all physical blocks on dies D 0 -D 3  of channels CH 4  and CH 5 . Superblock  508  includes all physical blocks on dies D 0 -D 3  of channels CH 6  and CH 7 . Each of superblocks  502 ,  504 ,  506 , and  508  has an identical number of physical blocks, which in the  FIG. 5 a    embodiment provides 512 GB (about 0.5 TB) of storage capacity. Although each superblock in the  FIG. 5 a    embodiment includes all non-volatile memory dies on two channels, a superblock including all of the non-volatile memory dies on any number of channels is within the scope of the invention. 
     During the initial boot up of the SSD, the firmware also assigned each of superblocks  502 ,  504 ,  506 ,  508  to an isolation region. An isolation region can contain one or more superblocks and each superblock can belong to only one isolation region. For example, in one embodiment the firmware assigns superblock  502  to an isolation region  510  and assigns superblock  504  and superblock  506  to an isolation region  520 . In this embodiment, isolation region  510  has a storage capacity of 512 GB and the firmware controls maintenance operations such as garbage collection and reliability operations for the physical blocks assigned to isolation region  510  independently from maintenance operations for all other isolation regions in the non-volatile memory array. For example, during garbage collection a page of valid data in a physical block that is to be erased is moved to another physical block within isolation region  510 . In this embodiment, isolation region  520  has a storage capacity of 1 TB (the 512 GB storage capacity of superblock  504  plus the 512 GB storage capacity of superblock  506 ). The firmware controls maintenance operations for the physical blocks assigned to isolation region  520  independently from maintenance operations for all other isolation regions in the non-volatile memory array. As each of superblocks  502 ,  504 ,  506 , and  508  has two dedicated channels, I/O operations and maintenance operations on one of superblocks  502 ,  504 ,  506 ,  508  do not introduce any delay in I/O operations on any other of superblocks  502 ,  504 ,  506 ,  508 . 
       FIG. 5 b    is a block diagram of a plurality of superblocks in a non-volatile memory array of an SSD, according to one embodiment of the invention. The non-volatile memory array of  FIG. 5 b    includes eight channels (CH 0 -CH 7 ) with two non-volatile memory dies (D 0 -D 1 ) coupled to each channel by a channel bus (not shown). Each non-volatile memory die includes a plurality of physical blocks (not shown). During the initial boot up of the SSD, the firmware mapped the physical blocks of the non-volatile memory array into four superblocks  512 ,  514 ,  516 , and  518 . Each of superblocks  512 ,  514 ,  516 , and  518  is a logical block that has been assigned a plurality of physical blocks in the non-volatile memory array. Superblock  512  includes all physical blocks on non-volatile memory dies D 0 -D 1  of channels CH 0  and CH 1 . Superblock  514  includes all physical blocks on non-volatile memory dies D 0 -D 1  of channels CH 2  and CH 3 . Superblock  516  includes all physical blocks on non-volatile memory dies D 0 -D 1  of channels CH 4  and CH 5 . Superblock  518  includes all physical blocks on non-volatile memory dies D 0 -D 1  of channels CH 6  and CH 7 . Each of superblocks  512 ,  514 ,  516 , and  518  has an identical number of physical blocks, which in the  FIG. 5 b    embodiment provides 256 GB (about 0.25 TB) of storage capacity. 
     During the initial boot up of the SSD, the firmware also assigned each of superblocks  512 ,  514 ,  516 ,  518  to an isolation region. An isolation region can contain one or more superblocks and each superblock can belong to only one isolation region. For example, in one embodiment the firmware assigns superblock  512  to an isolation region  530  and assigns superblock  514 , superblock  516 , and superblock  518  to an isolation region  540 . In this embodiment, isolation region  530  has a storage capacity of 256 GB and isolation region  540  has a storage capacity of 768 GB (the 256 GB storage capacity of each of superblock  514 , superblock  516 , and superblock  518 ). 
     As described above in conjunction with  FIGS. 5 a  and 5 b   , the firmware of the SSD creates a plurality of superblocks of uniform size and then allocates the superblocks to create isolation regions of a variety of sizes. In this fashion the total storage capacity of an SSD can be organized into one or more isolation regions in various configurations to satisfy different users&#39; needs.  FIGS. 5 c -5 e    are block diagrams of various configurations of superblocks and isolation regions within an SSD, according to various embodiments of the invention. The non-volatile memory array of the SSD of  FIG. 5 c    includes four isolation regions  560 ,  562 ,  564 , and  566  where each isolation region is comprised of one superblock that includes all physical blocks on all non-volatile memory dies coupled to two channels. The non-volatile memory array of the SSD of  FIG. 5 d    includes three isolation regions  570 ,  572 , and  574 . Each of isolation regions  570  and  572  is comprised of one superblock that includes all physical blocks on all non-volatile memory dies coupled to two channels: isolation region  570  includes all physical blocks on non-volatile memory dies D 0 -D 3  of channels CH 0  and CH 1  and isolation region  572  includes all physical blocks on non-volatile memory dies D 0 -D 3  of channels CH 2  and CH 3 . Isolation region  574  is comprised of two superblocks, a first superblock that includes all physical blocks on non-volatile memory dies D 0 -D 3  of channels CH 4  and CH 5  and a second superblock that includes all physical blocks on non-volatile memory dies D 0 -D 3  of channels CH 6  and CH 7 . The non-volatile memory array of the SSD of  FIG. 5 e    includes one isolation region  580  that includes four superblocks. 
       FIG. 6  is a block diagram of various configurations of namespaces within isolation regions, according to one embodiment of the invention. In one embodiment, each isolation region in an SSD is configured as a namespace. For example, a 4 TB SSD can be configured as four 1 TB isolation regions where each 1 TB isolation region is defined as a single 1 TB namespace. But in other embodiments namespaces are defined as a subset of the physical blocks allocated to an isolation region, as shown in  FIG. 6 . An isolation region  610  of an SSD includes a namespace  612 , a namespace  614 , a name space  616 , and an unallocated portion  618  of physical blocks. An isolation region  620  of the SSD includes a namespace  622 , a namespace  624 , and an unallocated portion  628  of physical blocks. An isolation region  630  of the SSD includes a namespace  632  and an unallocated portion  638  of physical blocks. To maintain the advantages of isolation regions, each of namespaces  612 ,  614 ,  616 ,  622 ,  624 , and  632  is contained within a single isolation region. For example, maintenance operations such as garbage collection occurring in namespace  612  of isolation region  610  will not affect read operations in namespace  632  of isolation region  630 . Each of unallocated portions  618 ,  628 , and  638  can be allocated to one or more namespaces within its corresponding isolation region  610 ,  620 , and  630 , respectively. 
       FIG. 7  is a block diagram of the flow of data within an SSD architecture supporting low latency operation, according to one embodiment of the invention. The SSD architecture includes, but is not limited to, a host interface layer  710 , a data cache  715 , a flash manager layer  725 , a flash translation layer  720 , an address map table  730 , a flash interface layer  735 , a flash interface layer  740 , four dual-channel controllers  745 ,  750 ,  755 , and  760 , and a non-volatile memory array  770 . Non-volatile memory array  770  includes an isolation region  772 , an isolation region  774 , an isolation region  776 , and an isolation region  778 . Each of isolation regions  772 ,  774 ,  776 , and  778  comprises a superblock that includes all of the physical blocks on all of the non-volatile memory dies coupled to two channels. Address map table  730  stores the assignment of addresses of physical blocks in non-volatile memory array  770  to the logical superblocks and isolation regions. Dual channel controller  745  manages communications over the two channels of isolation region  772 , dual channel controller  750  manages communications over the two channels of isolation region  774 , dual channel controller  755  manages communications over the two channels of isolation region  776 , and dual channel controller  760  manages communications over the two channels of isolation region  778 . In the  FIG. 7  embodiment, each of isolation regions  772 ,  774 ,  776 , and  778  corresponds to a namespace. In other embodiments the non-volatile memory array can include other numbers of channels such as four or sixteen, and the SSD architecture can include single channel controllers, one for each channel of the non-volatile memory array. 
     The four isolation regions  772 ,  774 ,  776 , and  778  of non-volatile memory array  770  allow the SSD to manage the exchange of data between one or more host devices or applications (not shown) and non-volatile memory array  770  as four independent “data pipes” such that data exchanges between a host application and one isolation region do not interfere with data exchanges between another host application and a different isolation region. Data cache  715 , which is a set of memory locations in a volatile memory such as a DRAM of the SSD, caches commands and data for each isolation region independently. Flash translation layer  720  manages data  702  written to or read from isolation region  772 , data  704  written to or read from isolation region  774 , data  706  written to or read from isolation region  776 , and data  708  written to or read from isolation region  778  independently from each other. For example, data  702  to be written to isolation region  772  is routed by flash translation layer  720  to flash interface layer  735  and dual channel controller  745 . Similarly, data  706  read from isolation layer  776  is routed through dual channel controller  755  and flash interface layer  740 , and flash translation layer  720  causes data  706  to be stored in the appropriate area of data cache  715 . 
       FIG. 8  is a flowchart of method steps for creating superblocks and isolation regions in a non-volatile memory array of an SSD, according to one embodiment of the invention. In a step  802 , firmware of the SSD detects that the SSD is being booted or started up for the first time (i.e., the initial boot up). In a step  804 , the firmware reads a configuration file that was previously programmed into the firmware to identify the capacity of each superblock and each isolation region to be created in the non-volatile memory array. The capacity of each superblock is defined by the SSD manufacturer and varies based on the total storage capacity of the SSD, e.g., 1 TB, 2 TB, or 4 TB. The capacity of each isolation region defined in the configuration file can be customized to satisfy a user&#39;s requirements. For example, a user may want to create a plurality of isolation regions of equal size, or may want to create a plurality of isolation regions where each isolation region is of a different size. In a step  806 , the firmware selects the appropriate number of channels for each superblock. The appropriate number of channels may depend on the hardware architecture of the SSD. In one embodiment, the firmware selects two channels for each superblock. In a step  808 , the firmware assigns physical blocks of the non-volatile memory array to each superblock to satisfy the superblock requirements set forth in the configuration file and creates a separate list of physical blocks for each logical superblock. In one embodiment the firmware selects two adjacent channels and assigns each physical block of each non-volatile memory die on those two channels to a first superblock. The firmware then selects the next two adjacent channels and assigns each physical block of each non-volatile memory die on those two channels to a second superblock. The firmware continues in this fashion until all of the physical blocks of the non-volatile memory array have been assigned to a superblock. The firmware creates a list of the addresses of the physical blocks assigned to each superblock and stores these separate lists in an address mapping table. 
     In a step  810 , the firmware assigns superblocks to isolation regions and stores the assignments in a table. The firmware assigns superblocks to isolation regions so as to satisfy the isolation region capacities set forth in the configuration file. Generally, each isolation region includes one or more superblocks. For example, in one embodiment the firmware assigns one superblock to each of a plurality of isolation regions such that the number of isolation regions equals the number of superblocks; such an embodiment is shown in  FIG. 5 c   . In another embodiment, the firmware assigns one superblock to a first isolation region, one superblock to a second isolation region, and two superblocks to a third isolation region; such an embodiment is shown in  FIG. 5 d   . In an optional step  812 , the firmware assigns isolation regions to namespace identifiers. In one embodiment, the firmware assigns each isolation region to a separate namespace identifier, such that the number of namespaces in the SSD matches the number of isolation regions. In another embodiment, namespaces are defined at some time after the initial boot up of the SSD. As described above in conjunction with  FIG. 6 , multiple namespaces can be defined within a single isolation region. 
     Part 3: Low Latency Operation of SSDs 
       FIG. 9  is a timing diagram of an operation of an array of SSDs  910 ,  920 , and  930  each having architectures supporting low latency operation, according to one embodiment of the invention. For simplicity of explanation, the timing diagram of  FIG. 9  shows the operation of a single isolation region within each of the SSDs  910 ,  920 , and  930 . However, the operation shown and described in connection with the timing diagram of  FIG. 9  is not so limited, and is equally applicable to three isolation regions within an SSD or spread across one or more SSDs. 
     The isolation regions of the SSDs  910 ,  920 , and  930  may be configured in the manner as described above in connection with  FIGS. 2 a , 2 b , 5 a - e   , with either the physical blocks from each of the non-volatile memory dies of one or more channels, similar to the isolation regions  202 - 208  as shown in  FIG. 2 a    and isolation region  220  as shown in  FIG. 2 b   , or the physical blocks from non-volatile memory dies across multiple channels, similar to the isolation region  230  as shown in  FIG. 2 b   . Similar to the operation described previously with respect to  FIG. 3 , each of the isolation regions of the SSDs  910 ,  920 , and  930  alternate between periods of operating in a deterministic mode  902  and a non-deterministic (i.e., maintenance) mode  904 . As previously discussed, in the deterministic mode  902  the isolation region performs only read operations, and in the non-deterministic mode  904  the isolation region performs maintenance tasks such as garbage collection and performs write operations that have accumulated for the isolation region while in the deterministic mode  902 . 
     As shown in  FIG. 9 , at any given point in time at least one of the isolation regions of the SSDs  910 ,  920 , and  930 , is in the deterministic mode  902  to provide deterministic read operations. Where each of the isolation regions of the SSDs  910 ,  920 , and  930  have copies of the same data, which typically occurs in data center and storage center environments to ensure sufficient redundancy and back-up of critical data, the operation shown in  FIG. 9  of the SSDs  910 ,  920 , and  930  guarantees that at least one copy of the data can be deterministically read at any time. In other embodiments, where, to provide redundancy erasure coding is employed and the data is written in N coded fragments such that any M fragments (where M&lt;N) are required to be able to reconstruct the data, the operation of the SSDs guarantees that M of N SSDs can be read deterministically at any time. In addition, necessary maintenance and writes can be performed for the isolation regions without interfering with deterministic read operations of the data. The operation of the SSDs  910 ,  920 , and  930  is particularly well-suited for large-scale data center or storage center environments where data is constantly being read from and written to the SSDs by multiple devices concurrently. 
     While three isolation regions/SSDs  910 ,  920 , and  930  are shown in  FIG. 9 , again the operation shown and described in connection with the timing diagram of  FIG. 9  is not so limited and can be implemented in two or more isolation regions in one or more SSDs according to various embodiments of the invention. For example, where two isolation regions within a single SSD are used to store copies of the data then one isolation region should be in the deterministic mode to perform read operations of the data while the other isolation region undergoes maintenance and/or performs write operations in the non-deterministic mode. Similarly, where four isolation regions across two SSDs are used to store copies of the data, for example, up to three of the isolation regions can be in the non-deterministic mode at any point in time, with the remaining one or more isolation regions are in the deterministic mode to provide for read operations. However, it may be preferable to operate the SSDs such that two or more isolation regions are operating in the deterministic mode at all times to provide flexibility for read data error handling as will be described in greater detail below. 
       FIG. 10  is a flow chart of method steps  1000  for low latency operation of an SSD, according to one embodiment of the invention. The method steps  1000  are described with respect to a single isolation region within the SSD for simplicity of explanation, and can similarly be applied to other isolation regions of the SSD or an array of SSDs consistent with the various embodiments of the present invention. As shown in  FIG. 10 , in a step  1002 , the isolation region enters the deterministic mode. At step  1004 , data is read from the isolation region. In a step  1006 , one or more maintenance counters for the isolation region is decreased. In one embodiment, the maintenance counter tracks a number of read operations in the isolation region and the starting value of the maintenance counter is a typical number of read operations that can be performed before maintenance operations are required. In this or other embodiments, the maintenance counter tracks increments of time, a number of write operations queued for the isolation region, or a combination of a number of read operations, a number of write operations, and/or increments of time. In a step  1008 , a check of the maintenance counter is made to determine whether the maintenance counter is below a minimum threshold indicating that maintenance needs to be performed on the isolation region. If the maintenance counter is still above the minimum threshold, then maintenance does not yet need to be performed and the isolation region can continue processing read commands and reads data again at step  1004 . 
     However, if the maintenance counter is below the minimum threshold indicating that maintenance needs to be performed on the isolation region, then in a step  1010 , the isolation region enters the non-deterministic mode. In a step  1012 , the isolation region performs maintenance operations and accumulated write operations. In a step  1014 , as maintenance operations are performed the maintenance counter is correspondingly increased. In a step  1016 , the maintenance counter is checked to determine whether sufficient maintenance operations have been performed and the isolation region can return to operating in the deterministic mode. If so, then the isolation region re-enters the deterministic mode at step  1002  and the method steps  1000  repeats. If not, then additional maintenance operations and write operations are performed again at step  1012 . 
     The maintenance counter can be implemented in any suitable manner, for example, as a status register in one embodiment. In one embodiment, the status register monotonically decreases as data is read from the isolation region at steps  1004  and  1006 , and monotonically increases as maintenance and write operations are performed at steps  1012  and  1014 . Additionally, it is not critical that the maintenance counter decreases at step  1006  as read operations are performed at step  1004  and increases at step  1014  as maintenance operations are performed at step  1012 . In another embodiment, the maintenance counter increases as read operations are performed at step  1004  and decreases as maintenance operations are performed  1012 . In this embodiment, the check at step  1008  will determine whether the maintenance counter is above a maximum threshold indicating that maintenance needs to be performed on the isolation region, and the check at step  1016  will determine whether the maintenance counter is below an acceptable threshold indicating that sufficient maintenance operations have been performed and the isolation region can return to operating in the deterministic mode. 
     In one embodiment, the isolation region does not automatically transition from the deterministic mode of operation to the non-deterministic mode of operation at step  1010  when the maintenance counter is determined below the minimum threshold at step  1008  and transition from the non-deterministic mode of operation to the deterministic mode of operation at step  1002  when the maintenance counter is determined to be above an acceptable threshold at step  1016 . Rather, the host device can be configured to set the operation mode (i.e., deterministic or non-deterministic) of the isolation region. In this embodiment, the SSD provides an estimate to the host device as to when the isolation region will require maintenance. 
     For example, when the maintenance counter is determined to be below the minimum threshold, the SSD may send a notification to the host device indicating to the host that maintenance will need to be performed on the isolation region within a certain period of time or within a certain number of read operations. The host device can then coordinate the timing of when the isolation region enters the non-deterministic mode to perform maintenance operations with other isolation regions storing copies of the same data to ensure that a copy of the data can be deterministically accessed from another isolation region while maintenance is being performed on the current isolation region in the manner as previously explained in connection with  FIG. 9 . 
     Further, once sufficient maintenance and write operations have been performed and the maintenance counter is above an acceptable threshold, the SSD may send another notification to the host device indicating to the host that the isolation region is ready to resume operation in the deterministic mode and the host device can coordinate the timing of when the isolation region or namespace re-enters the deterministic mode from the non-deterministic mode with other isolation regions that have a copy of the same data and that require maintenance. In one embodiment, the host device need not wait for an indication from the SSD to change the mode of operation of the isolation region from the deterministic mode to the non-deterministic mode, or vice-versa, and can do so depending on the maintenance needs of the various isolation regions storing copies of the same data. In one embodiment, rather than the SSD sending indications to the host, the host device periodically checks the maintenance counter of the SSD to determine whether maintenance is needed by the isolation region. In another embodiment, the host enters/leaves deterministic mode according to the workload balance of read and write operations at any given time. For example, deterministic mode may be selected when read operations predominate or non-deterministic mode selected when write operations predominate. In another embodiment, the host estimates durations for the deterministic mode and non-deterministic mode based on the maintenance counter thresholds and schedules switching an isolation region between the modes solely based on time but also uses the current maintenance counter values to create exceptions to this schedule. 
     Part 4: Read Data Error Handling 
     In one embodiment, an SSD such as SSD  100  of  FIG. 1  is configured to implement multiple levels of error correction to assist in correctly reading data from the non-volatile memory array. The levels of error correction differ in complexity and amount of processing resources required. Controller  120  of SSD  100  is configured to apply error correction to data read from a page of non-volatile memory starting with the lowest level of error correction complexity and if the data is not successfully recovered, to then apply the next higher level of error correction complexity until the data is successfully recovered or the available error correction is exhausted and the data is deemed unrecoverable. In one embodiment, the levels of error correction techniques, in order of increasing complexity, include hard-decision low-density parity check (LDPC), one or more levels of soft-decision LDPC, and quadruple swing-by code (QSBC). Other types of error correction coding such as BCH (Bose-Chaudhuri-Hocquenghem) block codes are within the scope of the invention. Applying multiple levels of error correction improves the SSD&#39;s ability to successfully read data but may introduce significant latency. 
     As described above in conjunction with  FIG. 9 , datacenters often store multiple copies of data in different isolation regions or namespaces within an SSD and/or in different SSDs in the same or different geographical locations. In such an environment, a host application has the ability to read the data from any of the different locations that are available. If a read command to one SSD, isolation region, or namespace is not successful, that is, the read command returned an error, the host application can attempt to read the same data from another location. For example, if SSD  910  returns an error for a read command, then the data may be read from SSD  920  or SSD  930 . 
     In one embodiment, the firmware of controller  120  is configured to enable a host application to select a maximum read recovery level, corresponding to a level of error correction, with a typical read latency. The host application can select the maximum read recovery level and its related read latency for all read commands to a particular SSD, isolation region, or namespace or make a selection for each read command individually. If a host application selects a maximum read recovery level that is lower than the highest level of error correction that the controller of the SSD is capable of, the SSD will return a read error sooner than if all levels of error correction are applied. The host application can then attempt to read the data from another SSD, isolation region, or namespace. If a host application has not “opted-in” by selecting a maximum read recovery level for a read command, the SSD will apply the error correction techniques in increasing levels of complexity until the data has been read successfully or the error correction techniques are exhausted. One embodiment of a set of read recovery levels and associated read latencies is set forth in Table 1. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Read Recovery Level 
                 ECC 
                 Average Time 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 “0” 
                 Hard-Decision LDPC 
                 100 
                 μs 
               
               
                 “1” 
                 Soft-Decision LDPC 
                 200 
                 μs 
               
               
                 “2” 
                 QSBC 
                 1 
                 ms 
               
               
                   
               
            
           
         
       
     
       FIG. 11  is a flow chart of method steps  1100  for read data error handling by an SSD, according to one embodiment of the invention. In a step  1102 , the firmware of the SSD identifies the maximum read recovery level for a read command received from a host application. In one embodiment, the SSD, isolation region, or namespace has been previously configured to use a maximum read recovery level for all read commands. In another embodiment, the host application makes a selection of a maximum read recovery level for each read command individually. In a step  1104 , the SSD executes the read command including the applicable error correction levels. For example, if the host application selected read recovery level “1” in Table 1, the SSD will first apply a hard-decision LDPC technique to the data and if the data not successfully recovered will then apply a soft-decision LDPC technique. In a step  1106 , the firmware of the SSD determines whether the data has been successfully read with the maximum read recovery level within its associated read latency. If the data was recovered successfully, then in step  1108  the SSD writes a read completion to the completion queue of the host application. If the data was not recovered successfully, then in a step  1110  the SSD reports a read error to the completion queue of the host application. 
       FIG. 12  is a flow chart of method steps  1200  for read data error handling by a host application connected to one or more SSDs, according to one embodiment of the invention. In a step  1202 , the host application sets a maximum read recovery level for a read command. In another embodiment, the host application sets a maximum read recovery level globally for all read commands issued to a particular namespace, isolation region, or SSD. In a step  1204 , the host application issues the read command to a namespace in an SSD. In a step  1206 , the host application checks its completion queue to determine whether the SSD returned an uncorrectable error for the requested data or some other error response. If the SDD returned an error, then in a step  1208  the host application determines whether another copy of the requested data is stored in another namespace, isolation region, or SSD. If so, then the method returns to step  1202  where the host application attempts to read the data from another namespace. If in step  1208  the host application determines that another copy of the data is not available in another namespace, for example if no other copy of the data exists or the other data location is experiencing a power failure, then in a step  1210  the host application determines whether a higher read recovery level is available. If no higher read recovery level is available for that namespace, then the method ends (read command returned as an error). If a higher read recovery level is available, in a step  1212  the host application selects the higher read recovery level, which may be the highest level of error correction available. The method then returns to step  1204 . 
     Other objects, advantages and embodiments of the various aspects of the present invention will be apparent to those who are skilled in the field of the invention and are within the scope of the description and the accompanying Figures. For example, but without limitation, structural or functional elements might be rearranged, or method steps reordered, consistent with the present invention. Similarly, principles according to the present invention could be applied to other examples, which, even if not specifically described here in detail, would nevertheless be within the scope of the present invention.