Patent Publication Number: US-9430373-B2

Title: Apparatus including memory channel control circuit and related methods for relaying commands to logical units

Description:
PRIORITY APPLICATION INFORMATION 
     This application is a Continuation of U.S. application Ser. No. 13/149,461, filed May 31, 2011, issued as U.S. Pat. No. 8,543,758 on Sep. 24, 2013, which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates generally to apparatus, such as semiconductor memory devices, systems, and controllers, and related methods, and more particularly, to memory system controllers, for example. 
     BACKGROUND 
     Memory devices are typically provided as internal, semiconductor, integrated circuits in computers or other electronic devices. There are many different types of memory including volatile and non-volatile memory. Volatile memory can require power to maintain its information, e.g., data, and includes random-access memory (RAM), dynamic random access memory (DRAM), synchronous dynamic random access memory (SDRAM), and static random access memory (SRAM) among others. Non-volatile memory can provide persistent information by retaining stored information when not powered and can include NAND flash memory, NOR flash memory, read only memory (ROM), Electrically Erasable Programmable ROM (EEPROM), Erasable Programmable ROM (EPROM), and phase change random access memory (PCRAM), among others. 
     Memory devices can be combined together to form a solid state drive (SSD). A solid state drive can include non-volatile memory, e.g., NAND flash memory and NOR flash memory, and/or can include volatile memory, e.g., DRAM and SRAM, among various other types of non-volatile and volatile memory. Flash memory devices, including floating gate flash devices and charge trap flash (CTF) devices using semiconductor-oxide-nitride-oxide-semiconductor and metal-oxide-nitride-oxide-semiconductor capacitor structures that store information in charge traps in the nitride layer, may be utilized as non-volatile memory for a wide range of electronic applications. Flash memory devices typically use a one-transistor memory cell that allows for high memory densities, high reliability, and low power consumption. 
     An SSD can be used to replace hard disk drives as the main storage device for a computing system, as the solid state drive can have advantages over hard drives in terms of performance, size, weight, ruggedness, operating temperature range, and power consumption. For example, SSDs can have superior performance when compared to magnetic disk drives due to their lack of moving parts, which may avoid seek time, latency, and other electro-mechanical delays associated with magnetic disk drives. SSD manufacturers can use non-volatile flash memory to create flash SSDs that may not use an internal battery supply, thus allowing the drive to be more versatile and compact. 
     An SSD can include a number of memory devices, e.g., a number of memory chips (as used herein, “a number of” something can refer to one or more of such things, e.g., a number of memory devices Can refer to one or more memory devices). As one of ordinary skill in the art will appreciate, a memory chip can include a number of dies and/or logical units (LUNs), e.g., where a LUN can be one or more die. Each die can include a number of memory arrays and peripheral circuitry thereon. The memory arrays can include a number of memory cells organized into a number of physical pages, and the physical pages can be organized into a number of blocks. An array of flash memory cells can be programmed a page at a time and erased a block at a time. SSD controllers may use an embedded processor to perform memory management and allocation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a functional block diagram of a computing system including at least one memory system in accordance with a number of embodiments of the present disclosure. 
         FIG. 2  is a functional block diagram of a memory system in accordance with a number of embodiments of the present disclosure. 
         FIG. 3A  is a functional block diagram of a transport layer interface between a host bus adapter (HBA) and a serial attachment (SA) programming compliant device in accordance with a number of embodiments of the present disclosure. 
         FIG. 3B  is a functional block diagram of an HBA and an SA programming compliant device in accordance with a number of embodiments of the present disclosure. 
         FIG. 4  illustrates a functional block diagram of a block management device in accordance with a number of embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure includes memory system controllers. Memory system controllers can include a switch and non-volatile memory control circuitry coupled to the switch. The non-volatile memory control circuitry can include a channel control circuit coupled to logical units. The channel control circuitry can be configured to relay an erase command to a first one of the logical units and relay a particular command from the switch to a second one of the logical units while the erase command is being executed on the first one of the plurality of logical units. 
     In the following detailed description of the present disclosure, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration how a number of embodiments of the disclosure may be practiced. These embodiments are described in sufficient detail to enable those of ordinary skill in the art to practice the embodiments of this disclosure, and it is to be understood that other embodiments may be utilized and that process, electrical, and/or structural changes may be made without departing from the scope of the present disclosure. As used herein, the designator “N,” particularly with respect to reference numerals in the drawings, indicates that a number of the particular feature so designated can be included with a number of embodiments of the present disclosure. 
     The figures herein follow a numbering convention in which the first digit or digits correspond to the drawing figure number and the remaining digits identify an element or component in the drawing. Similar elements or components between different figures may be identified by the use of similar digits. For example,  108  may reference element “ 08 ” in  FIG. 1 , and a similar element may be referenced as  208  in  FIG. 2 . As will be appreciated, elements shown in the various embodiments herein can be added, exchanged, and/or eliminated so as to provide a number of additional embodiments of the present disclosure. In addition, as will be appreciated, the proportion and the relative scale of the elements provided in the figures are intended to illustrate the embodiments of the present invention, and should not be taken in a limiting sense. 
       FIG. 1  is a functional block diagram of a computing system  100  including at least one memory system  104  in accordance with a number of embodiments of the present disclosure. In the embodiment illustrated in  FIG. 1 , the memory system  104 , e.g., a solid state drive (SSD), can include a physical host interface  106 , a memory system controller  108 , e.g., an SSD controller, and a number of solid state memory devices  110 - 1 , . . . ,  110 -N. The solid state memory devices  110 - 1 , . . . ,  110 -N can provide a storage volume for the memory system. In a number of embodiments, the solid state memory system controller  108  can be an application specific integrated circuit (ASIC), where the controller  108 , e.g., in the form of an ASIC, is coupled to a printed circuit board including the physical interface  106  and solid state memory devices  110 - 1 , . . . ,  110 -N. 
     As illustrated in  FIG. 1 , the memory system controller  108 , e.g., a solid state memory system controller, can be coupled to the physical host interface  106  and to the solid state memory devices  110 - 1 , . . . ,  110 -N. The physical host interface  106  can be used to communicate information between the memory system  104  and another device such as a host  102 . Host  102  can include a memory access device, e.g., a processor. One of ordinary skill in the art will appreciate that “a processor” can intend a number of processors, such as a parallel processing system, a number of coprocessors, etc. Example hosts include laptop computers, personal computers, digital cameras, digital recording and playback devices, mobile telephones, PDAs, memory card readers, interface hubs, and the like. For a number of embodiments, the physical host interface  106  can be in the form of a standardized physical interface. For example, when the memory system  104  is used for information storage in a computing system  100 , the physical host interface  106  can be a serial advanced technology attachment (SATA) physical interface, a peripheral component interconnect express (PCIe) physical interface, a small computer system interface (SCSI) interface, a serial attachment SCSI (SAS) interface, or a universal serial bus (USB) physical interface, among other interfaces. In general, however, the physical host interface  106  can provide an interface for passing control, address, information, e.g., data, and other signals between the memory system  104  and a host  102  having compatible receptors for the physical host interface  106 . 
     The solid state memory system controller  108  can communicate with the solid state memory devices  110 - 1 , . . . ,  110 -N to read, write, and erase information, among other operations. The solid state memory system controller  108  can have firmware and/or circuitry that may be a number of integrated circuits and/or discrete components. For a number of embodiments, the circuitry in solid state memory system controller  108  may include control circuitry for controlling access across the solid state memory devices  110 - 1 , . . . ,  110 -N and circuitry for providing a translation layer between a host  102  and the memory system  104 . Thus, a memory controller could selectively couple an I/O connection (not shown in  FIG. 1 ) of a solid state memory device  110 - 1 , . . . ,  110 -N to receive the appropriate signal at the appropriate I/O connection at the appropriate time. Similarly, the communication protocol between a host  102  and the memory system  104  may be different than what is required for access of a solid state memory device  110 - 1 , . . . ,  110 -N. Solid state memory system controller  108  could then translate the commands received from the host  102  into the appropriate commands to achieve the desired access to a solid state memory device  110 - 1 , . . . ,  110 -N. 
     A solid state memory device  110 - 1 , . . . ,  110 -N can include a number of arrays of memory cells, e.g., non-volatile memory cells. The arrays can be flash arrays with a NAND architecture, for example. In a NAND architecture, the control gates of memory cells of a “row” can be coupled with an access, e.g., word, line, while the memory cells can be coupled in series source to drain in a “string” between a select gate source transistor and a select gate drain transistor. The string can be connected to a data, e.g., bit, line by the select gate drain transistor. The use of the terms “row” and “string” implies neither a linear nor an orthogonal arrangement of memory cells. As will be appreciated by those of ordinary skill in the art, the manner of connection of the memory cells to the bit lines and source lines depends on whether the array is a NAND architecture, a NOR architecture, or some other memory array architecture. 
     The solid state memory devices  110 - 1 , . . . ,  110 -N can include a number of memory cells that can be grouped. As used herein, a group can include a number of memory cells, such as a page, block, plane, die, an entire array, or other groups of memory cells. For example, some memory arrays can include a number of pages of memory cells that make up a block of memory cells. A number of blocks can be included in a plane of memory cells. A number of planes of memory cells can be included one a die. As an example, a 128 GB memory device can include 4320 bytes of information per page, 128 pages per block, 2048 blocks per plane, and 16 planes per device. 
     The memory system  104  can implement wear leveling, e.g., garbage collection and/or reclamation, to control the wear rate on the solid state memory devices  110 - 1 , . . . ,  110 -N. A solid state memory array can experience errors, e.g., failures, after a number of program and/or erase cycles. Wear leveling can reduce the number of program and/or erase cycles performed on a particular group by spreading the cycles more evenly over the entire array. Wear leveling can include dynamic wear leveling to minimize the amount of valid blocks moved to reclaim a block. Dynamic wear leveling can include a technique called garbage collection. Garbage collection can include reclaiming, e.g., erasing and making available for writing, blocks that have the most invalid pages, e.g., according to a “greedy algorithm.” Alternatively, garbage collection can include reclaiming blocks with more than a threshold amount of invalid pages. If sufficient free blocks exist for a writing operation, then a garbage collection operation may not occur. An invalid page, for example, can be a page of information that has been updated to a different page. Static wear leveling includes writing static information to blocks that have high erase counts to prolong the life of the block. 
     Write amplification is a process that may occur when writing information to solid state memory devices  110 - 1 , . . . ,  110 -N. When randomly writing information in a memory array, the solid state memory system controller  108  scans for available space in the array. Available space in a memory array can be individual cells, pages, and/or blocks of memory cells that are not storing information and/or have been erased. If there is enough available space to write the information in a selected location, then the information is written to the selected location of the memory array. If there is not enough available space in the selected location, the information in the memory array is rearranged by reading, copying, moving, or otherwise rewriting and erasing the information that is already present in the selected location to a new location, leaving available space for the new information that is to be written in the selected location. The rearranging of old information in the memory array is called write amplification because the amount of writing performed by the memory device is amplified over an amount of writing that would occur if there were sufficient available space in the selected location. 
     The computing system  100  illustrated in  FIG. 1  can include additional circuitry beyond what is illustrated. The detail of the computing system  100  illustrated in  FIG. 1  has been reduced so as not to obscure embodiments of the present disclosure. For example, the memory system  104  can include address circuitry to latch address signals provided over I/O connections through I/O circuitry. Address signals can be received and decoded by a row decoder and a column decoder to access the solid state memory devices  110 - 1 , . . . ,  110 -N. It will be appreciated by those skilled in the art that the number of address input connections can depend on the density and architecture of the solid state memory devices  110 - 1 , . . . ,  110 -N. 
       FIG. 2  is a functional block diagram of a memory system  204  in accordance with a number of embodiments of the present disclosure. The memory system  204  can include solid state memory system controller  208 . The solid state memory system controller  208  can be coupled to a number of solid state memory devices, e.g., solid state memory devices  110 - 1 , . . . ,  110 -N illustrated in  FIG. 1 . In the example shown in  FIG. 2 , the number of memory devices include non-volatile memory including a number of logical units (LUNs)  250  and the controller  208  includes volatile memory  212 . A LUN  250  can be a portion of non-volatile memory that can be independently controllable. Memory system  204  and solid state memory system controller  208  can be analogous to memory system  104  and solid state memory system controller  108  respectively, illustrated in  FIG. 1 . 
     The solid state memory system controller  208  can include host interface (I/F) circuitry  214  for interfacing with a host, e.g., host  102  illustrated in  FIG. 1 , via physical host interface  206 . The solid state memory system controller  208  can include host-memory translation circuitry  216 , memory management circuitry  218 , a switch  220 , non-volatile memory control circuitry  222 , and/or volatile memory control circuitry  224 . As described herein, the solid state memory system controller  208  can be provided in the form of an ASIC, however, embodiments are not so limited. 
     The host I/F circuitry  214  can be coupled to host-memory translation circuitry  216 . The host I/F circuitry  214  can be coupled to and/or incorporated with a physical interface to a host, such as physical interface  206 . The host I/F circuitry  214  can interface with a host, e.g., host  102  in  FIG. 1 . The host I/F circuitry  214  can include a peripheral component interconnect express (PCIe) circuit  230  providing a physical layer, link layer, and transport or transaction layer interface, e.g., where the host is configured to transmit information according to a PCIe standard. A transport layer according to a serial advanced technology attachment (SATA) standard and/or a serial attachment SCSI (SAS) standard can be analogous to a transaction layer according to a PCIe standard. The PCIe circuit  230  can be coupled to a host bus adapter (HBA)  234 , e.g., an advanced host controller interface (AHCI) compatible HBA. The HBA  234  can be coupled to an SA compliant device  236 , which can be coupled to the host-memory translation circuitry  216 . 
     In general, the host I/F circuitry  214  is responsible for converting command packets received from the host, e.g., from a PCIe bus, into command instructions for the host-memory translation circuitry  216  and for converting host-memory translation responses into host commands for transmission to the requesting host. For example, the host I/F circuitry  214  can construct SA command packets from PCIe based transaction layer packets. The HBA  234  and SA compliant device  236  are described in more detail with respect to  FIGS. 3A-3B  below. 
     The host-memory translation circuitry  216  can be coupled to the host I/F circuitry  214 , to the memory management circuitry  218 , and/or to the switch  220 . The host-memory translation circuitry  216  can be configured to translate host addresses to memory addresses, e.g., addresses associated with a received command such as a read and/or write command. For example, such translation may be performed by SATA to memory circuitry  238 . Although identified as “SATA to memory” circuitry  238 , the circuitry can be configured for other serial attachments such as SAS, as described herein. The host-memory translation circuitry  216  might, for example, convert host sector read and write operations to commands directed to specific LUNs  250 . The host-memory translation circuitry  216  can include error detection/correction circuitry, such as RAID exclusive or (XOR) circuitry  226 . The RAID XOR circuitry  226  can calculate parity information based on information received from the host I/F circuitry  214 . 
     The memory management circuitry  218  can be coupled to the host-memory translation circuitry  216  and to the switch  220 . The memory management circuitry  218  can control a number of memory operations including but not limited to initialization, wear leveling, e.g., garbage collection and/or reclamation, and/or error detection/correction. While the memory management circuitry  218  can include a processor  228 , a number of embodiments of the present disclosure provide for control of memory operations in circuitry, e.g., hardware, without relying on the execution of instructions, e.g., software and/or firmware, by the processor  228 . Such embodiments can provide for faster memory operations relative to some previous approaches that rely more heavily on a processor to control memory operations. Memory management circuitry  218  can include block management circuitry  240 , which is described in more detail with respect to  FIG. 4 . 
     The switch  220  can be coupled to the host-memory translation circuitry  216 , the memory management circuitry  218 , the non-volatile control circuitry  222 , and/or the volatile memory control circuitry  224 . The switch  220  can be a crossbar switch and can include and/or be coupled to a number of buffers. For example, the switch  220  can include internal static random access memory (SRAM) buffers (ISBs)  225 . The switch can be coupled to a plurality of dynamic random access memory (DRAM) buffers  227  included in the volatile memory  212 . The switch can include a buffer allocation management (BAM) circuit  221 , which can include a buffer tag pool  223 . The switch  220  can provide an interface between various components of the solid state memory system controller  208 . The switch  220  can account for variations in defined signaling protocols that may be associated with different components of the solid state memory system controller  208  in order to provide consistent access and implementation between components. In a number of embodiments, the switch  220  can be a direct memory access (DMA) module. 
     The plurality of tags in the buffer tag pool  223  in the BAM circuit  221  can each identify a respective one of the plurality of DRAM buffers  227  or the plurality of ISBs  225 . When a particular tag points to a DRAM buffer  227 , a programmable “BAR” address can be appended to the buffer tag to fill out the address for the volatile memory  212 . The BAM circuit  221  can be configured to allocate a tag to one of a number of hardware masters in response to an allocation request from the one of the number of hardware masters. Hardware masters can include those hardware components that can access memory. 
     The BAM circuit  221  can be configured to prioritize allocation of a tag identifying one of the plurality of ISBs  225  over a tag identifying one of the plurality of DRAM buffers  227 . The ISBs  225  can be faster and lower powered than the DRAM buffers  227 , and therefore preferable thereto. Thus, the BAM circuit  221  can be configured to prioritize allocation of tags such that a tag identifying one of the plurality of DRAM buffers  227  is allocated only after all tags identifying ISBs  225  have been allocated. In a number of embodiments, the solid state memory system controller  208  can include 768 DRAM buffers  227  and  256  ISBs  225  for a total of 1024 buffers. In such embodiments, the buffer tags can be 10-bit tags that identify one of the 1024 buffers. The buffer tag may have no particular meaning to the hardware master, but is passed along by the hardware master during the processing of commands, e.g., read and/or write commands, to allow the correct buffer to be referenced. The number of hardware masters can be configured to request allocation of a tag generically without requesting allocation of a tag specific to one of the plurality of ISBs  225  or one of the plurality of DRAM buffers  227 . In a number of embodiments, each of the DRAM buffers  227  and the ISBs  225  can be of a size equal to a size of the largest supported memory page, e.g., 4 KB. 
     The BAM circuit  221  can be configured to remove tags from the buffer pool  223  that are associated with non-functioning buffers. The processor  228  can be configured to access the BAM circuit  221  and remove tags from the buffer pool  223  that are associated with non-functioning buffers. Such embodiments can help the solid state memory system controller  208  to function properly even if one or more ISBs  225  fail. In some embodiments, the ability to remove, e.g., “map out,” “bad” buffers can be used in lieu of testing ISBs  225  during manufacturing. The switch  220  may be “over-provisioned” with ISBs  225  such that losing a number of the ISBs  225  does not does not noticeably affect operation of the solid state memory system controller  208 . 
     The number of hardware masters can be configured to request allocation of a tag from the BAM circuit  221  in conjunction with a read command or a write command and to request deallocation of an allocated tag from the BAM circuit  221  in conjunction with completion of the read command or the write command. Hardware masters do not use the same tags for multiple commands and therefore request a new tag for each command. The hardware masters can be configured to prioritize deallocation requests over allocation requests, e.g., to help ensure that sufficient buffers, particularly ISBs  225 , are available for multiple commands from multiple hardware masters. 
     The use of buffer tags can facilitate read operations. The hardware masters can be configured to send a read command to the non-volatile memory control circuitry  222  along with an allocated tag. The non-volatile memory control circuitry  222  can be configured to store information corresponding to the read command in a buffer identified by the allocated tag and to notify the particular hardware master that sent the read command that the information is ready. 
     The use of buffer tags can facilitate write operations. The hardware masters can be configured to send a write command to the non-volatile memory control circuitry  222  along with an allocated tag and to store information corresponding to the write command in a buffer identified by the allocated tag. The non-volatile memory control circuitry  222  can be configured to retrieve the information from the buffer identified by the allocated tag, write the information to non-volatile memory, e.g., to the appropriate LUN  250 , and notify the hardware master that sent the write command that the information has been written. 
     The processor  228  in the memory management circuitry  218  can be a hardware master. The processor  228  can be configured to enable direct memory access (DMA) operations between the non-volatile memory control circuitry  222  and the volatile memory control circuitry  224  with access to the entire volatile memory  212  space without regard to the plurality of tags. Other hardware masters may not enable the DMA feature and therefore rely on tags for access to buffers rather than directly accessing the memory space. The processor  228  can enable DMA operations via information in a field used in conjunction with commands sent through the command pipeline that bypasses the need for a buffer to access memory space. For example, the information “DMA_En BARSEL” can be included in a field used with a command, where “DMA_En” indicates to the receiver that a DMA operation has been enabled, and where “BARSEL” indicates that an address should be included for the DRAM in lieu of a buffer tag, as described herein. Thus, the processor  228  can move information between the volatile memory  212  and the LUNs  250  using the same logic as buffer-based access for other hardware masters. The “DMA_En” can be echoed to the switch  220  by the channel control circuitry  248  when accessing a LUN  250 . From the perspective of the channel control circuit  248 , the DMA access is virtually identical to the buffer-based access. If “DMA_En” is asserted, the switch  220  can force the request to the volatile memory  212 . The “BARSEL” is used by the switch  220  to swap a unique “BAR” analogous to the “BAR” address appended to the buffer tag when it points to a DRAM buffer  227 . For example, the “BAR” address can be a two-bit address pointing to one of four locations, however embodiments are not so limited. Use of the “BAR” address can allow different simultaneous DMAs. 
     The non-volatile memory control circuitry  222  can be coupled to the switch  220 . The non-volatile memory control circuitry  222  can be coupled to non-volatile memory devices.  FIG. 2  shows the non-volatile memory devices including a number of LUNs  250 . The number of LUNs  250  can be coupled to the non-volatile memory control circuitry  222  by a number of channels. In some embodiments, the number of channels can be controlled collectively by the non-volatile memory control circuitry  222 . In a number of embodiments, each memory channel is coupled to a discrete channel control circuit  248 , as illustrated in  FIG. 2 . A particular channel control circuit  248  can control and be coupled to more than one LUN  250  by a single channel. In a number of embodiments, the channel control circuits  248  can be coupled to a plurality of LUNs  250  by an Open NAND Flash Interface (ONFI) compliant bus. 
     The non-volatile memory control circuitry  222  includes at least a portion of a command pipeline where commands reside while waiting to be executed by a channel control circuit  248 . All hardware masters that initiate requests to the LUNs  250  can share the same command pipeline through the switch  220 . For read commands from a host (host reads), there is no control over which LUN  250  is accessed because the host needs particular information, which is stored in one or more LUNs  250 . Likewise, there is no control over the volume of host traffic. The command pipeline described herein can provide flexibility in hiding program time, read time, and erase time conflicts between LUNs  250 . For example, erase time may be longer than program time or read time. Each channel control circuit  248  may be coupled by a plurality of channels to a plurality of LUNs  250 . Therefore, while an erase command is being executed by a particular channel control circuit  248  for a particular LUN, the particular channel control circuit  248  may execute another command for a different LUN  250  on the same channel. 
     The non-volatile memory control circuitry  222  can include a channel request queue (CRQ)  242  coupled to each of the channel control circuits  248 . Each channel control circuit  248  can include a LUN request queue (LRQ)  244  coupled to a plurality of LUN command queues (LCQs)  246 . The LRQ  244  can be an L-number-deep queue circuit, where L is equal to a number of LUNs  250  per channel control circuit  248 . The CRQ  242  can be a C-number-deep queue circuit, where C is equal to, for example, x*(the plurality of channel control circuits  248 ), where x is a whole number, e.g., 4. For example, the non-volatile memory control circuitry  222  can include 32 channels with 16 LUNs per channel, one 128-deep CRQ  242  configured to store up to 128 entries for command storage shared between channels, one 16-deep LRQ  244  per channel configured to store up to 16 commands between the LUNs  250  on a particular channel, and a 2-deep LCQ  246  per channel, where the LCQ  246  is a first-in-first-out (FIFO) circuit. For example, the 2-deep FIFO can be configured to queue a current command and a next command to be executed subsequent to the current command. Such embodiments provide (512 LUNs*2-deep LCQ) for 1024 entries, plus (one 16-deep LRQ per channel*32 channels) for 512 entries, plus (one 128-deep CRQ) for a total of (1024+512+128)=1664 command storage entries. As described herein, such a command pipeline structure can provide the same performance as 524,288 command storage entries (32 LUNs per channel*16 channels*1024-deep FIFO per LUN=524,288) at a fraction of the size. 
     The CRQ  242  can be configured to receive a command from the switch  220  and relay the command to one of the LRQs  244 , e.g., the LRQ  244  associated with the channel that is associated with the particular LUN  250  for which the command is targeted. The LRQ  244  can be configured to relay a first plurality of commands for a particular LUN  250  to the LCQ  246  associated with the particular LUN  250  in an order that the first plurality of commands were received by the LRQ  244 . The command pipeline is structured such that commands to a same LUN  250  move in order, e.g., in the order that they were received by the LRQ  244 . The LRQ  244  can be configured to queue a command for a particular LUN  250  in response to the LCQ  246  associated with the particular LUN  250  being full and the CRQ  242  can be configured to queue a command for a particular LRQ  244  in response to the particular LRQ  244  being full. 
     The LRQ  244  can be configured to relay a second plurality of commands for different LUNs  250  to the LCQs  246  associated with the different LUNs  250  in an order according to a status of the different LUNs  250 . For example, the status of the different LUNs  250  can be a ready/busy status. The command pipeline is structured such that the commands between different LUNs  250  can move out of order, e.g., in an order different from the order in which they were received by the LRQ  244  according to what is efficient for overall memory operation at the time. For example, the LRQ  244  can be configured to relay a first one of the second plurality of commands to a first LCQ  246  before relaying the second one of the second plurality of commands to a second LCQ  246  in response to the status of the different LUN  250  associated with the second LCQ  246  being busy, where the first one of the second plurality of commands is received later in time than the second one of the second plurality of commands. The LRQ  244  can be configured to relay the second one of the second plurality of commands to the second LCQ  246  in response to the status of the LUN  250  associated with the second LCQ  246  being ready, e.g., subsequent to relaying the first one of the second plurality of commands. 
     A number of embodiments including discrete non-volatile memory channel control circuits for each channel can include discrete error detection/correction circuitry  232 , e.g., error correction code (ECC) circuitry, coupled to each channel control circuit  248  and/or a number of error detection/correction circuits  232  that can be used with more than one channel. The error detection/correction circuitry  232  can be configured to apply error correction such as BCH error correction, as will be understood by one of ordinary skill in the art, to detect and/or correct errors associated with information stored in the LUNs  250 . For example, the error detection/correction circuitry can provide 29 bits of error correction over a 1080-bit code word. The error detection/correction circuitry  232  can be configured to provide differing error correction schemes for single and/or multi level cell (SLC/MLC) operation. 
     The volatile memory control circuitry  224  can be coupled to the switch  220  and to the volatile memory  212 , e.g., a number of volatile memory devices. Among other information, the number of volatile memory devices can store an LBA table and/or a block table as described in more detail with respect to  FIG. 4 . 
       FIG. 3A  is a functional block diagram of a transport layer interface between a host bus adapter (HBA)  334  and a serial attachment (SA) programming compliant device  336  in accordance with a number of embodiments of the present disclosure. The HBA  334  can be analogous to HBA  234  illustrated in  FIG. 2 . The SA programming compliant device  336  can be analogous to the SA programming compliant device  236  illustrated in  FIG. 2 . In a number of embodiments, the SA programming compliant device  336  can be a serial advanced technology (SATA) programming compliant device. In a number of embodiments, the SA programming compliant device  336  can be a serial attachment SCSI (SAS) programming compliant device. 
     SA protocols are conceptually defined using layers. In order from low to high, for SATA, these include the physical layer, the link layer, the transport layer, and the command layer. In order from low to high, for SAS, these include the physical layer, the PHY layer, the link layer, the port layer, the transport layer, and the application layer. Command layer and/or application layer information can be communicated on the transport layer by breaking commands up into frame information structures (FISes). According to some previous approaches, FISes were communicated on the link layer using primitives. Primitives were communicated on the physical layer using codewords, which were transmitted on a SATA or SAS cable using 8b10b encoding. Communication on the SATA or SAS cable consisted of two differential pairs of wires referenced from the host&#39;s perspective as transmit (Tx) and receive (Rx). These wires could transmit either commands or information, and were arbitrated for control. One limitation of this scheme is that once a communication is arbitrated on the cable, it is the only communication that occurs. That is, SATA or SAS is only capable of transmitting a single FIS at a given time. 
     If the HBA  334 -SA programming compliant device  336  interface is entirely within the controller, e.g., as in the case of the solid state memory system controller  208  illustrated in  FIG. 2 , information flow is free from the constraints of a cable-connected SA physical layer. A number of embodiments of the present disclosure abandon use of the physical and link layers while preserving usage of the FISes and commands supported by the command layer  352  and the transport layer  354 . Concurrency can be achieved by defining physical communication as four sets input/outputs, e.g., command (Cmd), response (Rsp), write data (WrData), and read data (RdData). Thus, by employing a function-specific interconnect, conflicts no longer exist between commands, responses, and information delivery. They can be pipelined and operate concurrently. That is, the function-specific interconnect can be configured to simultaneously transfer a command, a response, and information between the HBA  334  and the SA programming compliant device  336 . The function-specific interconnect can include: a Cmd output on the HBA  334  coupled to a Cmd input on the SA programming compliant device  336 , a Rsp output on the SA programming compliant device  336  coupled to a Rsp input on the HBA  334 , a WrData output on the HBA  334  coupled to a WrData input on the SA programming compliant device  336 , and a RdData output on the SA programming compliant device  336  coupled to a RdData input on the HBA  334 . In a number of embodiments, each of the Cmd, Rsp, WrData, and RdData input/output pairs can be unidirectional and employ neither 8b10b encoding nor primitives. 
     The function-specific interconnect can be configured to operate a command interface (the Cmd output on the HBA  334  and the Cmd input on the SA programming compliant device  336 ), a response interface (the Rsp input on the HBA  334  and the Rsp output on the SA programming compliant device  336 ), and an information interface (the WrData output on the HBA  334  and the WrData input on the SA programming compliant device  336  and/or the RdData input on the HBA  334  and the RdData output on the SA programming compliant device  336 ) concurrently in response to a first command protocol, e.g., native command queuing (NCQ) and/or tagged command queuing (TCQ), among others. The function-specific interconnect can be configured to operate the command interface, response interface, and the information interface atomically in response to a second command protocol, e.g., NonData, programmed input/output (PIO), and/or direct memory access (DMA), among others. 
     The command interface can be used to deliver host-device (HD) FISes from the HBA  334  to the SA programming compliant device  336 . The WrData output on the HBA  334  can be used to deliver information to the WrData input on the SA programming compliant device  336 . The RdData output on the SA programming compliant device  336  can be used to deliver information to the RdData input on the HBA  334 . The response interface can be used to qualify information delivery, e.g., using PIOSetups, DMAActivates, DMASetups, etc., and communicate completions, e.g., device-host (DH) and/or set device bits (SDB), etc. Qualifying information delivery can include defining an order in which information is delivered. The use of buffering can allow the movement of information to begin prior to the corresponding response FIS. 
       FIG. 3B  is a functional block diagram of an HBA  334  and an SA programming compliant device  336  in accordance with a number of embodiments of the present disclosure. The HBA  334  can be analogous to HBA  234  illustrated in  FIG. 3A , with more detail shown. The SA programming compliant device  336  can be analogous to the SA programming compliant device  236  illustrated in  FIG. 3A , with more detail shown. 
     The HBA  334  can include a command fetcher  356  providing the Cmd output to the SA programming compliant device  336 . The HBA  334  can include a response receiver  358  receiving the Rsp input from the SA programming compliant device  336 . The HBA  334  can include a downstream DMA device  360  providing the WrData output to the SA programming compliant device  336 . The HBA  334  can include an upstream DMA device  362  receiving the RdData input from the SA programming compliant device  336 . Each of the Cmd fetcher  356 , the Rsp receiver  358 , the downstream DMA  360 , and the upstream DMA  362  can be hardware components that can operate independently of each other, but, in a number of embodiments, can synchronize operation with the HBA FSM  372 , e.g., an advanced host controller interface (AHCI) state machine so that AHCI protocol can be enforced. 
     The downstream DMA device  360  and the Cmd fetcher  356  can be coupled to a bus interface (I/F)  364  for downstream writes and/or upstream reads. The bus I/F  364  can be configured to receive requests from the Cmd fetcher  356 , pass along the requests to the PCIe I/F, e.g., an interface with PCIe circuit  230  illustrated in  FIG. 2 , and return replies to the Cmd fetcher  356  when appropriate. The bus I/F  364  can also be configured to receive information from the PCIe I/F and send requests, e.g., write information fetches, thereto. The downstream DMA device  360  can be configured to pipeline multiple PCIe read requests via the Bus I/F  364  when adequate buffering exists to receive the write information. Each request sent via the Bus I/F  364  to the PCIe I/F can cause assignment of a rotating buffer tag that directs information from the PCIe I/F to the respective write buffer. Information can then be removed from the write buffers in a rotating fashion to be sent to the SA programming compliant device  336 . The Rsp receiver  358  and the upstream DMA device  362  can be coupled to a bus I/F  366  for downstream reads and/or upstream writes. The bus I/F  366  can be configured to transmit information and/or requests via the PCIe I/F. 
     The HBA  334  architecture can present a single AHCI port to host device driver software, but can extend the number of command slots within the port, e.g., to  256 . Proprietary host device drivers can access the slots by control registers  370 , e.g., eight 32-bit command-issue registers. The control registers  370  may be connected to the PCIe I/F via control interfaces  368  for input of write control information and output of read control information. In a number of embodiments, the Cmd fetcher  356  can retrieve and forward commands to the SA programming compliant device  336  in an order that the commands are issued. A number of commands, e.g., 32 commands, can be received simultaneously within the control registers  370  and enqueued for the Cmd fetcher  356 . 
     The number of command slots can be grouped into command slot groups and have a message signaled interrupt (MSI), e.g., MSI-X, vector assigned thereto. Such embodiments can be beneficial versus some previous approaches that include 32b AHCI registers for commands for multiple ports. A number of embodiments of the present disclosure include one port with 256b registers for commands, which can make command grouping with MSI vectors advantageous, such as to improve a likelihood that interrupt performance can be optimized in the host, e.g., host  102  illustrated in  FIG. 1 . 
     The HBA  334  can use context arrays for NCQ reads to be returned in an interleaved fashion. The information within a particular read command processes in order, however portions of multiple outstanding read commands may be interleaved with each other. When a read context is initiated by the SA programming compliant device  336 , the HBA  334  can retrieve a record of the current status of the read command from the context arrays and continue from the point it was last discontinued. When the read context is completed, the HBA  334  can store the updated command progress values in the context arrays. A context array location can exist for each possible outstanding read command, e.g., 256 outstanding read commands. An analogous context array may be used for writes. 
     The SA programming compliant device  336  can include a command finite state machine (FSM)  374  receiving the Cmd input from the HBA  334 , e.g., the command fetcher  356  of the HBA  334 . The SA programming compliant device  336  can include a device-host arbiter (DH ARB)  376  providing the Rsp output to the HBA  334 , e.g., the response receiver  358  of the HBA  334 . The SA programming compliant device  336  can include a write FSM  378  receiving the WrData input from the HBA  334 , e.g., the downstream DMA  360  of the HBA  334 . The SA programming compliant device  336  can include a read FSM  380  providing the RdData output to the HBA  334 , e.g., the upstream DMA  362  of the HBA  334 . The Wr FSM  378  can provide an output to memory, and the Rd FSM  380  can provide an input from memory, e.g., via the host-memory translation circuitry  216  illustrated in  FIG. 2 . 
     The DH ARB  376  can have an input from the Cmd FSM  374 , e.g., for communication of DHs, SDBs, etc. The DH ARB  376  can include an input from the Wr FSM  378 , e.g., for write DMASetups. The DH ARB  376  can include an input from the Rd FSM  380 , e.g., for read DMASetups. The DH ARB  376  can be configured to qualify an order of information delivery and communicate completions to the HBA  334  based on the input from the Cmd FSM  374 , the Wr FSM  378 , and the Rd FSM  380 . The Wr FSM  378  can include a write buffer configured to buffer write information before the DH ARB  376  qualifies the order of write information delivery. The Rd FSM  380  can include a read buffer configured to buffer read information before the DH ARB  376  qualifies the order of read information delivery. 
     In a number of embodiments, the Cmd FSM  374  can be configured to operate concurrently by default. The Cmd FSM  374  can be configured to operate atomically, e.g., in “CurrencyDisabled” mode, in response to detection of a single-context command protocol such as PIO or DMA. When operating atomically, the Cmd FSM  374  can be configured to assert one of two “PassControl” signals, one to the Wr FSM  378  and one to the Rd FSM  380 . Subsequently, the Cmd FSM  374  can enter a “WaitForCtl” state until a reset command is received or a “RetrunControl” signal is asserted from either the Wr FSM  378  or the Rd FSM  380 . If the detected protocol is NonData, then neither PassControl signal is asserted. The Cmd FSM  374  can service the command itself and return to idle upon completion. If logic indicates that command tags are ready to be retired, the Cmd FSM can generate an appropriate SDB FIS and return to idle. 
     In a number of embodiments, the Wr FSM  378  and the Rd FSM  380  can be configured to operate concurrently by default. The Wr FSM  378  and the Rd FSM  380  can be configured to operate atomically on receipt of the PassControl signal from the Cmd FSM  374  and/or detection of a single-context command protocol such as PIO or DMA. The Wr FSM  378  and/or the Rd FSM  380  can operate atomically following the SA protocol, e.g., SATA protocol or SAS protocol, until either a command count is completed or an error condition is met, in which case the respective FSM can assert its RtnCtl signal, which can release the Cmd FSM  374  from its WaitForCtl state. Conversely, concurrent operation allows each FSM to move information independently. 
       FIG. 4  illustrates a functional block diagram of a block management device  440  in accordance with a number of embodiments of the present disclosure. The block management device  440  can be analogous to the block management device  240  illustrated in  FIG. 2  and can be included in memory management circuitry. The block management device  440  can communicate with volatile memory  412 , e.g., DRAM, which can be analogous to the volatile memory  212  illustrated in  FIG. 2 . Thus, for example, the block management device  440  can communicate with the volatile memory  412  via a switch and volatile memory control circuitry. The volatile memory  412  can store a logical block address (LBA) table  482 , a block table  484 , and/or a transaction log  486 , among other information. 
     The LBA table  482  can store the physical page address of pages in the LUNs, e.g., LUNs  250  illustrated in  FIG. 2 , and include corresponding logical addresses. That is, the LBA table  482  can store logical to physical and/or physical to logical address translations. Thus, the LBA table  482  can be used to look-up physical page addresses that correspond to logical block addresses where corresponding information can be stored. The LBA table  482  can be indexed by the LBA that is contained in an associated SA command. The block table  484  can store information for erasable blocks in the number of LUNs. Information stored in the block table  484  can include valid page information, erase count, and other health and/or status information. Information accessed from the block table  484  can be indexed by physical block address. The transaction log  486  can be used to record information about writes that occur in the LUNs. In a number of embodiments, the transaction log  486  can be updated contemporaneously with the writes to the LUNs. The transaction log  486  can include information about writes to the LUNs that have occurred since the last time that the LBA table  482  was saved in the non-volatile memory, e.g., in order to facilitate recreation of portions of the LBA table  482  that may be lost due to sudden power loss or other errors between updates of the LBA table  482  to the non-volatile memory. 
     Some of the objects depicted within the block management device  440  are indicative of the functionality provided by the block management device  440 . The LBA table lookup function  490  can reference the LBA table  482  in the volatile memory  412  to perform logical to physical address translation. The LBA table lookup function  490  can update the LBA table  482  with a new physical address corresponding to a logical address when information associated with the logical address is updated. The block table lookup function  491  can reference the block table  484  in the volatile memory  412 , e.g., to determine candidates for wear leveling such as reclamation and/or garbage collection. Reclamation can involve moving all valid pages from a block to be erased to new locations before the block is erased. The block reclamation function  493  can reference the transaction log  486  in the volatile memory  412 . 
     The block array  492  stored in local memory of the memory management circuitry, e.g., block management device  440 , can track reclamation page candidates  494 , erase block candidates  495 , reclamation write block candidates  496 , and/or host write block candidates  497 , e.g., as referenced to the block array  492  by the LBA table lookup function  490  and/or the block table lookup function  491 . Such candidates can be selected for each LUN in the system using dedicated hardware to analyze the health and/or status information of each block as it is read or written to the volatile memory  412 , e.g., DRAM, at volatile memory speed. The current candidates for each LUN can be stored in the block array  492 . Each time the block table  484  is accessed, a pipelined structure can retrieve the current best candidate from the block array  492  and compare it to the new block table  484  access. If the new block table  484  access, e.g., as a result of a write, erase, or error event, reveals a better candidate than the current candidate stored in the block array  492 , then the new block can replace that candidate in the block array  492 . Candidate selection can occur without stalling access to the volatile memory  412 , which allows the process to proceed at volatile memory  412  speed using the pipelined structure. 
     The block management device  440  can be configured to store health and status information for each of a plurality of blocks in a block table  484  in the volatile memory  412 . The block management device  440  can be configured to store a candidate block table, e.g., block array  492  including reclamation page candidates  494 , erase block candidates  495 , reclamation write block candidates  496 , and/or host write block candidates  497 , in the local memory. The candidate block table can identify a candidate block for a particular operation, e.g., a host write, a reclamation read, a reclamation write, and/or an erase, based on a number of criteria for the particular operation. The block management device  440  can be configured to update the health and status information for a particular block in the block table  484 , e.g., in response to a write, an erase, or an error event for the particular block. The block management device  440  can be configured to compare the updated health and status information for the particular block with the candidate block according to the number of criteria. The block management device  440  can be configured to update the candidate block table to identify the particular block at least partially in response to the comparison indicating that the particular block better satisfies the number of criteria, e.g., on a same clock cycle during which the health and status information for the particular block is updated in the volatile memory  412 . 
     The candidate selection process described above can be supplemented with a table walking process that does not rely on a new block table  484  access as a result of a write, erase, or error event, for example. The table walking process can recover the history of block information that is not stored locally in the block array  492  in the block management device  440 . Table walking can be a slower background process relative to block table  484  accesses as a result of a write, erase, or error event. Once the entire block table  484  has been walked, the table walking process may be ceased, e.g., to reduce volatile memory  412  power consumption. In some instances, a new block table  484  access can reinitiate the table walking process. Thus, the block management device  440  can be configured to compare the updated health and status information for each of the plurality of blocks in the block table  484  in the volatile memory  412  with the candidate block according to the number of criteria independent of a write, an erase, or an error event for any of the plurality of blocks. The comparison of the blocks can be ceased after each of the plurality of blocks has been compared. 
     In a number of embodiments of the present disclosure, host operations are not stalled during a reclamation process according to the use of a coherency point to update the LBA table  482 . During a reclamation process, a reclamation page candidate, e.g., from reclamation page candidates  494 , is read from a first location and written to a second location. During this read and write, the host may have written new information to the LBA currently being processed for reclamation and updated the LBA table  482  with a new physical address. When the reclamation has finished the page read and write, the LBA table  482  can be updated with the new physical address only if the LBA table  482  entry for the LBA under reclamation has the same physical address as the information read from the first location, e.g., if the host has not updated the entry in the LBA table  482 . The page corresponding to the reclamation read can be marked as invalid, e.g., storing stale information, in the block table  484 . If the physical address is different than the address corresponding to the reclamation read, that indicates that the host has made an update, and the LBA table will not be updated with the new physical address per the reclamation write. The reclamation write can be invalidated in the block table  484  to indicate that the physical location corresponding to the reclamation write stores invalid information. 
     The memory management circuitry, e.g., the block management device  440 , can be configured to retrieve a first physical address corresponding to a logical address for a particular block from the block table  484  prior to information being read from the particular block during a reclamation operation on the particular block. The block management device  440  can be configured to retrieve a second physical address corresponding to the logical address from the LBA table  482  after the information is written to a different block during the reclamation operation. The block management device  440  can be configured to update the LBA table  482  with a third physical address corresponding to the different block at least partially in response to the second physical address being equal to the first physical address. The block management device  440  can be configured to invalidate the reclamation operation at least partially in response to the second physical address being different than the first physical address. 
     The block management device  440  can be coupled between a host command queue  498  and a memory command queue  499 . The host command queue  498  can be associated with a host, such as host  102  illustrated in  FIG. 1 , SA to memory circuitry  238  as illustrated in  FIG. 2 , and/or a number of components of the host interface  214  illustrated in  FIG. 2 . The memory command queue  499  can be analogous to one or more of the CRQ  242 , LRQ  244 , and LCQs  246  illustrated in  FIG. 2 , and/or other components. 
     Memory management circuitry, e.g., the block management device  440 , can be coupled to non-volatile memory control circuitry, e.g., non-volatile memory control circuitry  222  illustrated in  FIG. 2  via a switch, e.g., switch  220  illustrated in  FIG. 2 . The memory management circuitry can be configured to allocate a write block cluster for host writes based on an information width of a host bus and a protocol of the host bus. A write block cluster can include a number of blocks from a number of LUNs  250 . The host bus can be part of a host, e.g., host  102  illustrated in  FIG. 1 . For example, the host bus can be a PCIe bus with bus widths of x1, x2, x4, x8, x16, x32, etc., and protocols of PCIe generations 1-3, among others. The write block cluster can include one block from fewer than all of the LUNs that are coupled to a plurality of channel control circuits in the non-volatile memory control circuitry. In a number of embodiments, the memory management circuitry can be configured to limit the size of the write block cluster to a minimum number of the LUNs used to support a maximum host bus bandwidth according to the information width of the host bus and the protocol of the host bus. 
     The write bandwidth of the memory system controller can be a function of the maximum bandwidth supported by the host bus, the number of memory channels in the system, and the number of LUNs per channel. Allocating more LUNs than the host bus can support may be a waste of resources that could otherwise be used for reclamation operations. Thus, according to a number of embodiments of the present disclosure, the host write block cluster size can be limited to something less than the total number of LUNs in the memory system. 
     A remainder of blocks from the LUNs, e.g., those not allocated to the write block cluster, can be allocated for reclamation operations. Memory management circuitry can be configured to temporarily deallocate a number of the allocated LUNs from reclamation operations. The write block cluster can be allocated for host writes such that blocks of the write block cluster are written in a particular sequence of LUNs. Reclamation bandwidth may be limited to improve the write bandwidth of the memory system controller by suspending reclamation operations to LUNs that may be used for writes in the near future. At least one of the allocated LUNs can be deallocated from reclamation operations at least partially in response to the particular sequence of LUNs indicating that the LUN is within a threshold number of LUNs of being next in the particular sequence of LUNs for a host write. The threshold number can be different for each of a reclamation read operation, a reclamation write operation, and a reclamation erase operation. The threshold number can represent an “exclusion zone” that is a number of LUNs where no reclamation operations may occur prior to a write operation. 
     Using multiple LUNs per channel can lead to situations where more commands are issued than a particular channel can handle, leading to the commands being queued with respect to the memory channel, e.g., as described with respect to the CRQ  242 , LRQ  244 , and LCQs  246  in  FIG. 2 . The memory management circuitry can be configured to limit a number of commands issued for reclamation operations to a particular LUN at least partially in response to the number of queues associated with the particular LUN being within a threshold of being full. The maximum number of commands per channel may also be limited to stay within a desired power envelope. The number of commands issued for reclamation operations can be limited at least partially in response to an amount of power caused to be used by the memory system controller exceeding a threshold amount of power. The maximum number of commands per channel may also be limited, for example in order to help ensure that back-to-back commands for a same channel and/or LUN are spaced apart sufficiently for efficient operation. The memory management circuitry can be configured to enforce at least a minimum time, e.g., number of clocks, between issuance of a first command and a second command for reclamation operations. The minimum time can be based on a total number of erased blocks in the LUNs, e.g., because reclamation bandwidth can increase as the number of erased blocks decreases. 
     Maximum host write bandwidth can be maintained in the short-term if there is an adequate supply of erased blocks available. Once the supply of erased blocks is nearly consumed, write bandwidth can decrease due to increased reclamation operations. The memory management circuitry can be configured to track a number of erased blocks for reach of the LUNs, e.g., with reference to the block table  484 . The memory management circuitry can be configured to engage reclamation operations for a particular LUN at least partially in response to the number of erased blocks in the particular LUN exceeding a reclamation threshold number, e.g., falling below the threshold number. The memory management circuitry can be configured to suspend reclamation operations for a particular LUN at least partially in response to the number of erased blocks in the particular LUN exceeding a reclamation threshold number, e.g., going over the threshold number. Host writes can be suspended for the particular LUN at least partially in response to the number of erased blocks in the particular LUN falling below a host write threshold number. The memory management circuitry can be configured to suspend writes to a particular LUN at least partially in response to the number of erased blocks in the particular LUN exceeding a write threshold number, e.g., falling below the threshold number. The memory management circuitry can be configured to re-engage writes to a particular LUN at least partially in response to the number of erased blocks in the particular LUN exceeding a reclamation threshold number, e.g., going over the threshold number. Such embodiments can help provide an equilibrium between reclamation operations and write operations, e.g., dynamic load balancing. 
     Some memory operations have long durations. The channel control circuits, e.g., channel control circuits  248  illustrated in  FIG. 2  can include polling logic configured to issue a status read to the LUNs, e.g., to determine when a particular memory operation is complete. In some instances, such polling can interfere with other commands using the same channel as a LUN being polled. According to a number of embodiments of the present disclosure, the polling logic can be idled for a particular time according to a type of operation indicated by a command issued to a LUN. The idle time can be set for specific operations, e.g., according to a time that the particular operation is expected to take, such as a read, a write, and/or an erase operation, among others. 
     CONCLUSION 
     The present disclosure includes examples of various apparatus, including memory system controllers. One such memory system controller can include a switch and non-volatile memory control circuitry coupled to the switch. The non-volatile memory control circuitry can include a channel control circuit coupled to logical units. The channel control circuitry can be configured to relay an erase command to a first one of the logical units and relay a particular command from the switch to a second one of the logical units while the erase command is being executed on the first one of the plurality of logical units. 
     It will be understood that when an element is referred to as being “on,” “connected to” or “coupled with” another element, it can be directly on, connected, or coupled with the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly on,” “directly connected to” or “directly coupled with” another element, there are no intervening elements or layers present. As used herein, the term “and/or” includes any and all combinations of a number of the associated listed items. 
     As used herein, the term “and/or” includes any and all combinations of a number of the associated listed items. As used herein the term “or,” unless otherwise noted, means logically inclusive or. That is, “A or B” can include (only A), (only B), or (both A and B). In other words, “A or B” can mean “A and/or B” or “one or more of A and B.” 
     It will be understood that, although the terms first, second, third, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another element. Thus, a first element could be termed a second element without departing from the teachings of the present disclosure. 
     Although specific embodiments have been illustrated and described herein, those of ordinary skill in the art will appreciate that an arrangement calculated to achieve the same results can be substituted for the specific embodiments shown. This disclosure is intended to cover adaptations or variations of a number of embodiments of the present disclosure. It is to be understood that the above description has been made in an illustrative fashion, and not a restrictive one. Combination of the above embodiments, and other embodiments not specifically described herein will be apparent to those of skill in the art upon reviewing the above description. The scope of the number of embodiments of the present disclosure includes other applications in which the above structures and methods are used. Therefore, the scope of a number of embodiments of the present disclosure should be determined with reference to the appended claims, along with the full range of equivalents to which such claims are entitled. 
     In the foregoing Detailed Description, some features are grouped together in a single embodiment for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that the disclosed embodiments of the present disclosure have to use more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive subject matter lies in less than all features of a single disclosed embodiment. Thus, the following claims are hereby incorporated into the Detailed Description, with each claim standing on its own as a separate embodiment.