diff --git a/www/scst_user_spec.txt b/www/scst_user_spec.txt deleted file mode 100644 index e1fd9071c..000000000 --- a/www/scst_user_spec.txt +++ /dev/null @@ -1,876 +0,0 @@ - SCST USER SPACE DEVICE HANDLER MODULE. - - USER SPACE INTERFACE DESCRIPTION. - - Version 1.0.0 - - - I. Description. - -SCST user space device handler module scst_user is a device handler for -SCST, which provides a way to implement in the user space complete, full -feature virtual SCSI devices in the SCST environment. - -This document assumes that the reader is familiar with the SCST -architecture and the states through which SCSI commands go during -processing in SCST. Module scst_user basically only provides hooks to -them. Their description could be found on the SCST web page on -http://scst.st.net. - - - II. User space API. - -Module scst_user provides /dev/scst_user character device with the -following system calls available: - - - open() - allows to open the device and get a file handle, which - will be used in all subsequent actions until close() is called - - - close() - closes file handle returned by open() - - - poll() - allows to wait until some pending command from SCST to - process is available. - - - ioctl() - main call, which allows commands interchange with the SCST - core. - -Device /dev/scst_user could be opened in blocking or non-blocking mode -using O_NONBLOCK flag. In the blocking mode ioctl() -SCST_USER_REPLY_GET_CMD function blocks until there is a new subcommand -to process. In the non-blocking mode if there are no pending subcommands -SCST_USER_REPLY_GET_CMD function returns immediately with EAGAIN error -code, and the user space device handler can use poll() call to get -notification about new subcommands arrival. The blocking mode is the -default. - -The module scst_user API is defined in scst_user.h file. - - - III. IOCTL() functions. - -There are following IOCTL functions available. All of them has one -argument. They all, except SCST_USER_REGISTER_DEVICE return 0 for -success or -1 in case of error, and errno is set appropriately. - - - 1. SCST_USER_REGISTER_DEVICE - -SCST_USER_REGISTER_DEVICE - registers new virtual user space device. The -argument is: - -struct scst_user_dev_desc -{ - uint8_t version; - uint8_t type; - uint8_t has_own_order_mgmt; - struct scst_user_opt opt; - uint32_t block_size; - char name[SCST_MAX_NAME]; -}, - -where: - - - version - is a protocol version, shall be DEV_USER_VERSION - - - type - SCSI type of the device - - - has_own_order_mgmt - set it in non-zero, if device implements own - ORDERED commands management, i.e. guarantees commands execution order - requirements, specified by SAM. - - - opt - device options, see SCST_USER_SET_OPTIONS/SCST_USER_GET_OPTIONS below - - - block_size - block size, shall be divisible by 512 for block devices - - - name - name of the device - -SCST_USER_REGISTER_DEVICE returns registered device's handler or -1 -in case of error, and errno is set appropriately. - -In order to unregister the device just call close() on its file descriptor. - - - 3. SCST_USER_SET_OPTIONS/SCST_USER_GET_OPTIONS - -SCST_USER_SET_OPTIONS/SCST_USER_GET_OPTIONS allows to set or get -correspondingly various options that control various aspects of SCSI -commands processing. - -The argument is: - -struct scst_user_opt -{ - uint8_t parse_type; - uint8_t on_free_cmd_type; - uint8_t memory_reuse_type; - uint8_t partial_transfers_type; - uint32_t partial_len; - - uint8_t tst; - uint8_t queue_alg; - uint8_t tas; - uint8_t swp; -}, - -where: - - - parse_type - defines how the user space handler wants to process - PARSE subcommand. Possible values are: - - * SCST_USER_PARSE_STANDARD - tells SCST use standard internal parser - for this SCSI device type. - - * SCST_USER_PARSE_CALL - tells SCST generate SCST_USER_PARSE for all - SCSI commands - - * SCST_USER_PARSE_EXCEPTION - tells SCST generate SCST_USER_PARSE - for unknown SCSI commands or SCSI commands that produce errors in - the standard parser. - - - on_free_cmd_type - defines how the user space handler wants to - process ON_FREE_CMD subcommand. Possible values are: - - * SCST_USER_ON_FREE_CMD_CALL - tells SCST generate - SCST_USER_ON_FREE_CMD for all SCSI commands - - * SCST_USER_ON_FREE_CMD_IGNORE - tells SCST do nothing on this event. - - - memory_reuse_type - defines how memory allocated by the user space - handler for a SCSI commands data buffers is then reused by the SCST - core as data buffer for subsequent commands. Possible values are: - - * SCST_USER_MEM_NO_REUSE - no memory reuse is possible, for each - commands the user space handler will each time allocate a dedicated - data buffer - - * SCST_USER_MEM_REUSE_READ - memory reuse by only READ-type commands - (i.e. which involve data transfer from target to initiator) is - allowed. For all WRITE-type commands (i.e. which involves data - transfer from initiator to target) the user space handler will - each time allocate a dedicated data buffer - - * SCST_USER_MEM_REUSE_WRITE - memory reuse by only WRITE-type - commands is allowed. For all WRITE-type commands the user space - handler will each time allocate a dedicated data buffer - - * SCST_USER_MEM_REUSE_ALL - unlimited memory reuse is possible. - - - partial_transfers_type - defines if the user space handler supports - partial data transfers, when a SCSI command, which required big data - transfer, is broken on several subcommands with smaller data - transfers. This allows to improve performance by simultaneous data - transfers from/to remote initiator and to/from the underlying storage - device as well as lower allocation memory requirements for each - (sub)command. All subcommands will have the same unique value in - "parent_cmd_h" field and SCST_USER_SUBCOMMAND flag in "partial" field - of struct scst_user_scsi_cmd_exec. The final subcommand will also - have in that field SCST_USER_SUBCOMMAND_FINAL flag. All the - subcommands will have the original unmodified CDB. Possible values - are: - - * SCST_USER_PARTIAL_TRANSFERS_NOT_SUPPORTED - the partial data - transfers are not supported - - * SCST_USER_PARTIAL_TRANSFERS_SUPPORTED_ORDERED - the partial data - transfers are supported, but all the subcommands must come in order - of data chunks. Could be used, e.g., for tape devices. - - * SCST_USER_PARTIAL_TRANSFERS_SUPPORTED - the partial data transfers - are supported without limitations. - - - tst, queue_alg, tas and swp - set values for TST, QUEUE ALGORITHM MODIFIER, - TAS and SWP fields from control mode page correspondingly, see SPC. - -Flags parse_type and on_free_cmd_type are designed to improve -performance by eliminating context switches to the user space handler, -when processing of the corresponding events isn't needed. - -Flag memory_reuse_type is designed to improve performance by eliminating -memory allocation, preparation and then freeing each time for each -commands, if the same memory will be allocated again and again. See -SCST_USER_ALLOC_MEM description below for more info. - -SCST_USER_SET_OPTIONS should not be used from the same and the only -thread, which also handles incoming commands, otherwise there could be a -"deadlock", when SCST_USER_SET_OPTIONS waits for active commands finish, -but nobody handles them. This "deadlock" will be resolved only when -initiator, which sent those commands, aborts them after timeout. - - - 4. SCST_USER_REPLY_AND_GET_CMD - -SCST_USER_REPLY_AND_GET_CMD allows at one call reply on the current -subcommand from SCST and get the next one. If 0 is returned by ioctl(), -SCST_USER_REPLY_AND_GET_CMD returns a SCST subcommand in the argument, -which is defined as the following: - -struct scst_user_get_cmd -{ - uint64_t preply; - uint32_t cmd_h; - uint32_t subcode; - union { - struct scst_user_sess sess; - struct scst_user_scsi_cmd_parse parse_cmd; - struct scst_user_scsi_cmd_alloc_mem alloc_cmd; - struct scst_user_scsi_cmd_exec exec_cmd; - struct scst_user_scsi_on_free_cmd on_free_cmd; - struct scst_user_on_cached_mem_free on_cached_mem_free; - struct scst_user_tm tm_cmd; - }; -}, - -where: - - - preply - pointer to the reply data or, if 0, there is no reply. See - SCST_USER_REPLY_CMD for description of struct scst_user_reply_cmd - fields - - - cmd_h - command handle used to identify the command in the reply. - - - subcode - subcommand code, see 4.1 below - -Union contains command's specific payload. - -For all received subcommands the user space device handler shall call -SCST_USER_REPLY_AND_GET_CMD or SCST_USER_REPLY_CMD function to tell SCST -that the subcommand's processing is finished, although some subcommands -don't return a value. - - - 4.1. Possible subcommands: - - - 4.1.1. SCST_USER_ATTACH_SESS - -SCST_USER_ATTACH_SESS notifies the user space handler that a new -initiator's session is about to be attached to the device. Payload -contains struct scst_user_sess, which is defined as the following: - -struct scst_user_sess -{ - uint64_t sess_h; - uint64_t lun; - uint16_t threads_num; - uint8_t rd_only; - char initiator_name[SCST_MAX_NAME]; -}, - -where: - - - sess_h - session's handle, may not be 0 - - - lun - assigned LUN for this device in this session - - - threads_num - specifies amount of additional threads, requested by - the corresponding target driver - - - rd_only - if true, this device is read only in this session - - - initiator_name - name of the remote initiator, which initiated this - session - -When SCST_USER_ATTACH_SESS is returned, it is guaranteed that there are -no other commands are being executed or pending. - -After SCST_USER_ATTACH_SESS function completed, the user space device -handler shall reply using "result" field of the corresponding reply -command. - - - 4.1.2. SCST_USER_DETACH_SESS - -SCST_USER_DETACH_SESS notifies the user space handler that the -corresponding initiator is about to be detached from the particular -device. Payload contains struct scst_user_sess, where only handle field -is valid. - -When SCST_USER_DETACH_SESS is returned, it is guaranteed that there are -no other commands are being executed or pending. - -This command doesn't reply any return value, although -SCST_USER_REPLY_AND_GET_CMD or SCST_USER_REPLY_CMD function must be -called. - - - 4.1.3. SCST_USER_PARSE - -SCST_USER_PARSE returns SCSI command on PARSE state of the SCST -processing. The PARSE state is intended to check validity of the -command, determine data transfer type and the necessary data buffer -size. This subcommand is returned only if SCST_USER_SET_OPTIONS -parse_type isn't set to SCST_USER_PARSE_STANDARD. In this case the -standard SCST internal parser for this SCSI device type will do all the -job. - -Payload contains struct scst_user_scsi_cmd_parse, which is defined as -the following: - -struct scst_user_scsi_cmd_parse -{ - uint64_t sess_h; - - uint8_t cdb[SCST_MAX_CDB_SIZE]; - int32_t cdb_len; - - uint32_t timeout; - int32_t bufflen; - - uint8_t queue_type; - uint8_t data_direction; - - uint8_t expected_values_set; - uint8_t expected_data_direction; - int32_t expected_transfer_len; - - uint32_t sn; -}, - -where: - - - sess_h - corresponding session handler - - - cdb - SCSI CDB - - - cdb_len - SCSI CDB length - - - timeout - CDB execution timeout - - - bufflen - command's buffer length - - - queue_type - SCSI task attribute (queue type) - - - data_direction - command's data flow direction, one of SCST_DATA_* - constants - - - expected_values_set - true if expected_data_direction and - expected_transfer_len contain valid values - - - expected_data_direction - remote initiator supplied command's data - flow direction - - - expected_transfer_len - remote initiator supplied transfer length - - - sn - command's SN, which might be used for task management - -In the PARSE state of SCSI commands processing the user space device -handler shall check and provide SCST values for command data buffer -length, data flow direction and timeout, which it shall reply using the -corresponding reply command. - -In case of any error the error reporting should be deferred until -SCST_USER_EXEC subcommand, where the appropriate SAM status and sense -shall be set. - - - 4.4.4. SCST_USER_ALLOC_MEM - -SCST_USER_ALLOC_MEM returns SCSI command on memory allocation state of -the SCST processing. On this state the user space device handler shall -allocate the command's data buffer with bufflen length and then return -it to SCST using the corresponding reply command. Then SCST internally -will convert it in SG vector in order to use it itself and by target -drivers. - -If the memory reuse type is disabled (i.e. set to SCST_USER_MEM_NO_REUSE) -there are no special requirements for buffer memory or its alignment, it -could be just what malloc() returned. If the memory reuse type is enabled, -the buffer shall be page size aligned, for example using memalign() -function. - -Payload contains struct scst_user_scsi_cmd_alloc_mem, which is defined -as the following: - -struct scst_user_scsi_cmd_alloc_mem -{ - uint64_t sess_h; - - uint8_t cdb[SCST_MAX_CDB_SIZE]; - int32_t cdb_len; - - int32_t alloc_len; - - uint8_t queue_type; - uint8_t data_direction; - - uint32_t sn; -}, - -where: - - - sess_h - corresponding session handler - - - cdb - SCSI CDB - - - cdb_len - SCSI CDB length - - - alloc_len - command's buffer length - - - queue_type - SCSI task attribute (queue type ) - - - data_direction - command's data flow direction, one of SCST_DATA_* - constants - - - sn - command's SN, which might be used for task management - -Memory allocation, preparation and freeing are ones of the most -complicated and expensive operations during SCSI commands processing. -Module scst_user provides a way to almost completely eliminate those -operations by reusing once allocated memory for subsequent SCSI -commands. It is controlled by memory_reuse_type option, which could be -set by SCST_USER_SET_OPTIONS function. If any type memory reusage is -enabled, then SCST will use its internal SGV cache in order to cache -allocated and fully built SG vectors for subsequent commands of this -type, so for them SCST_USER_ALLOC_MEM subfunction will not be called and -in SCST_USER_EXEC pbuf pointer will point to that reused buffer. - -SGV cache is a backend cache made on top of Linux kernel SLAB cache. It -caches SG vectors with power of 2 data sizes: 4K, 8K, ..., 4M. So, if -there is a 5K SCSI command coming, then the user space handler will be -asked for 8K allocation, from which only 5K will be used for this -particular SCSI command. Then that SG vector will be cached and -subsequently reused for all SCSI commands between 4K and 8K. For a 1M -SCSI command the user space handler will be asked for another buffer -(while the previous 5K one will be kept), which will not be then reused -for 5K SCSI commands, since 1M and 5K vectors belong to different cache -data sizes, it will be reused only for commands between 512K and 1M. -Then, after some time of inactivity or when the system is under memory -pressure, the cache entries will be freed and the user space handler -will be notified using SCST_USER_ON_CACHED_MEM_FREE. - -Since SGV cache caches SG vectors with power of 2 data sizes, alloc_len -field could be up to 2 times more, than actually required by the SCSI -command. - -The memory reuse could be used in both SCSI tagged and untagged queuing -environments. In the SCSI tagged queuing environment the SGV cache will -take care that several commands don't use the same buffer simultaneously -by asking the user space handler to allocate a new data buffer, when all -cached ones are busy. - -Some important notes: - -1. If the user space handler needs to call fork(), it must call -madvise() with MADV_DONTFORK flag for all allocated data buffers, -otherwise parent or child process could loose the connection with them, -which could lead to data corruption. See http://lwn.net/Articles/171941/ -for details. - -2. The interface assumes that all allocated memory by the user space -handler is DMA'able by the target hardware. This is almost always true -for most modern systems, except if the target hardware isn't capable of -using 64-bit address space and the system has >4GB of memory or the -memory addresses are in address space, which is unavailable with 32-bit -addresses. - -In case of any error the error reporting should be deferred until -SCST_USER_EXEC subcommand, where the appropriate SAM status and sense -should be set. - - - 4.4.5. SCST_USER_EXEC - -SCST_USER_EXEC returns SCSI command on execution state of the SCST -processing. The user space handler should execute the SCSI command and -reply using the corresponding reply command. - -In some cases for performance reasons for READ-type SCSI commands -SCST_USER_ALLOC_MEM subcommand isn't returned before SCST_USER_EXEC. -Thus, if pbuf pointer is 0 and the SCSI command needs data transfer, -the user space handler should be prepared to allocate the data buffer -with size alloc_len, which could be up to 2 times more, than actually -required by the SCSI command. But field bufflen will contain the correct -value. All the memory reusage rules, described for SCST_USER_ALLOC_MEM, -apply to SCST_USER_EXEC as well. - -Payload contains struct scst_user_scsi_cmd_exec, which is defined as the -following: - -struct scst_user_scsi_cmd_exec -{ - uint64_t sess_h; - - uint8_t cdb[SCST_MAX_CDB_SIZE]; - int32_t cdb_len; - - int32_t data_len; - int32_t bufflen; - int32_t alloc_len; - uint64_t pbuf; - uint8_t queue_type; - uint8_t data_direction; - uint8_t partial; - uint32_t timeout; - - uint32_t sn; - - uint32_t parent_cmd_h; - int32_t parent_cmd_data_len; - uint32_t partial_offset; -}, - -where: - - - sess_h - corresponding session handler - - - cdb - SCSI CDB - - - cdb_len - SCSI CDB length - - - data_len - command's data length. Could be different from bufflen for - commands like VERIFY, which transfer different amount of data, than - process, or even none of them - - - bufflen - command's buffer length - - - alloc_len - command's buffer length, which should be allocated, if pbuf - is 0 and the command requires data transfer - - - pbuf - pointer to command's data buffer or 0 for SCSI commands - without data transfer. - - - queue_type - SCSI task attribute (queue type) - - - data_direction - command's data flow direction, one of SCST_DATA_* - constants - - - partial - specifies, if the command is a partial subcommand, could - have the following OR'ed flags: - - * SCST_USER_SUBCOMMAND - set if the command is a partial subcommand - - * SCST_USER_SUBCOMMAND_FINAL - set if the subcommand is a final one - - - timeout - CDB execution timeout - - - sn - command's SN, which might be used for task management - - - parent_cmd_h - has the same unique value for all partial data - transfers subcommands of one original (parent) command - - - parent_cmd_data_len - for partial data transfers subcommand has the - size of the overall data transfer of the original (parent) command - - - partial_offset - has offset of the subcommand in the original - (parent) command - -It is guaranteed that only commands of the same queue_type per session -can be returned simultaneously. - -In case of any error it should be reported via appropriate SAM status -and sense. If it happens for a subcommand of a partial data transfers -command, all other subcommands of this command, which already passed the -the user space handler or will be passed in the future, will be aborted -by scst_user, the user space handler should ignore them. - - - 4.4.6. SCST_USER_ON_FREE_CMD - -SCST_USER_ON_FREE_CMD returns SCSI command when the command is about to -be freed. At this stage, the user space device handler could do any -necessary cleanups, for instance, free allocated for data buffer memory. - -NOTE! If the memory reusage is enabled, then the data buffer must not be -freed, it will be reused by subsequent SCSI commands. The buffer must be -freed only on SCST_USER_ON_CACHED_MEM_FREE event. - -Payload contains struct scst_user_scsi_on_free_cmd, which is defined -as the following: - -struct scst_user_scsi_on_free_cmd -{ - uint64_t pbuf; - int32_t resp_data_len; - uint8_t buffer_cached; - uint8_t aborted; - uint8_t status; - uint8_t delivery_status; -}, - -where: - - - pbuf - pointer to command's data buffer or 0 for SCSI commands - without data transfer. - - - resp_data_len - length of the response data - - - buffer_cached - true, if memory reusage is enabled for this command - - - aborted - true, if command was aborted - - - status - SAM status of the commands execution - - - delivery_status - status of cmd's status/data delivery to remote - initiator. Can be: - - * SCST_CMD_DELIVERY_SUCCESS - delivery succeeded - - * SCST_CMD_DELIVERY_FAILED - delivery failed - -The user space handler should reply using the corresponding reply -command. No error code is needed. - - - 4.4.7. SCST_USER_ON_CACHED_MEM_FREE - -SCST_USER_ON_CACHED_MEM_FREE subcommand is returned, when SGV cache -decided that this buffer isn't needed anymore. This happens after some -time of inactivity or when the system is under memory pressure. - -Payload contains struct scst_user_on_cached_mem_free, which is defined -as the following: - -struct scst_user_scsi_cmd_alloc_mem -{ - uint64_t pbuf; -}, - -where: - - - pbuf - pointer to buffer, which should be freed - - - 4.4.8. SCST_USER_TASK_MGMT - -SCST_USER_TASK_MGMT subcommand returns a task management functions. -Payload contains struct scst_user_tm, which is defined as the following: - -struct scst_user_tm -{ - uint64_t sess_h; - uint32_t fn; - uint32_t cmd_h_to_abort; - uint32_t cmd_sn; - uint8_t cmd_sn_set; -}, - -where: - - - sess_h - corresponding session handler - - - fn - task management function, see below - - - cmd_h_to_abort - handle of command to abort. Valid only if fn is - SCST_ABORT_TASK - - - cmd_sn - if cmd_sn_set is set, contains maximum commands SN, which - this task management function affects. See iSCSI RFC 3720 10.5.1 for - more details. - - - cmd_sn_set - specifies if cmd_sn is valid - -After TM function is completed, the user space device handler shall -reply using "result" field of the corresponding reply command. It isn't -necessary to wait for aborted command(s) finished, the result of TM -function shall be returned immediately. SCST core will take care that -the reply to the TM function isn't sent before all affection SCSI -commands finished. - -Possible values of "fn" field: - - - SCST_ABORT_TASK - cmd_h_to_abort shall be aborted - - - SCST_ABORT_TASK_SET - task set on the device shall be aborted - - - SCST_CLEAR_ACA - ACA status shall be cleared - - - SCST_CLEAR_TASK_SET - task set on the device shall be cleared - - - SCST_LUN_RESET, SCST_TARGET_RESET - reset of the device shall be done - - - SCST_NEXUS_LOSS_SESS - notifies about nexus loss event for the session - - - SCST_ABORT_ALL_TASKS_SESS - all tasks in the session shall be aborted - - - SCST_NEXUS_LOSS - notifies about global nexus loss event - - - SCST_ABORT_ALL_TASKS - all tasks shall be aborted - -Possible return values are: - - - SCST_MGMT_STATUS_SUCCESS - success - - - SCST_MGMT_STATUS_TASK_NOT_EXIST - task does not exist - - - SCST_MGMT_STATUS_LUN_NOT_EXIST - LUN does not exist - - - SCST_MGMT_STATUS_FN_NOT_SUPPORTED - task management function not - supported - - - SCST_MGMT_STATUS_REJECTED - task management function was rejected - - - SCST_MGMT_STATUS_FAILED - task management function failed - - - - 5. SCST_USER_REPLY_CMD - -SCST_USER_REPLY_CMD IOCTL function allows the user space handler to -return the result of a command's execution. Its argument is defined as: - -struct scst_user_reply_cmd -{ - uint32_t cmd_h; - uint32_t subcode; - union { - int32_t result; - struct scst_user_scsi_cmd_reply_parse parse_reply; - struct scst_user_scsi_cmd_reply_alloc_mem alloc_reply; - struct scst_user_scsi_cmd_reply_exec exec_reply; - }; -}, - -where: - - - cmd_h - command handle used to identify the command in the reply. - - - subcode - subcommand code, see 4.1 - -Union contains the subcommand's specific payload: - - - result - subcommand's result code - - -struct scst_user_scsi_cmd_reply_parse -{ - uint8_t queue_type; - uint8_t data_direction; - int32_t data_len; - int32_t bufflen; -}, - -where: - - - queue_type - SCSI task attribute (queue type ) - - - data_direction - command's data flow direction, one of SCST_DATA_* constants - - - data_len - command's data length - - - bufflen - command's buffer length - - -struct scst_user_scsi_cmd_reply_alloc_mem -{ - uint64_t pbuf; -}, - -where: - - - pbuf - pointer to command's data buffer - - -struct scst_user_scsi_cmd_reply_exec -{ - int32_t resp_data_len; - uint64_t pbuf; - - uint8_t reply_type; - - uint8_t status; - uint8_t sense_len; - aligned_u64 psense_buffer; -}, - -where: - - - resp_data_len - length of the response data - - - pbuf - pointer to command's data buffer. Used only when in the - original SCST_USER_EXEC subcommand pbuf field is 0 - - - reply_type - could be one of the following constants: - - * SCST_EXEC_REPLY_BACKGROUND - tells SCST send to the remote - initiator GOOD status, but the command not yet completed by the - user space handler, it is being executed in the background. When - it will be completed, the user space handler will call - SCST_USER_REPLY_CMD again with reply_type - SCST_EXEC_REPLY_COMPLETED. - - * SCST_EXEC_REPLY_COMPLETED - the user space handler completed the command - - - status - SAM status of the commands execution - - - sense_len - length of sense data in psense_buffer, if any - - - psense_buffer - pointed to sense buffer - - - IV. Commands processing flow example. - -As the example consider a simple synchronous VTL, which serves one -virtual SCSI tape device and can process only one command at time from -any initiator. - -At the beginning the VTL opens using open() call /dev/scst_user in the -default blocking mode. - -Then it using SCST_USER_REGISTER_DEVICE ioctl() function registers the -tape device. Since only one command at time is supported, the allocated -command's data memory could be reused for both READ-type (i.e. which -involve data transfer from target to initiator) and WRITE-type (i.e. -which involve data transfer from initiator to target) commands. So the -device is configured with parse_type SCST_USER_PARSE_STANDARD, -on_free_cmd_type SCST_USER_ON_FREE_CMD_IGNORE, memory_reuse_type -SCST_USER_MEM_REUSE_ALL and partial_transfers_type -SCST_USER_PARTIAL_TRANSFERS_NOT_SUPPORTED. - -Then it prepares struct scst_user_get_cmd with reply set to 0, calls -SCST_USER_REPLY_AND_GET_CMD ioctl() and waits until some initiator -connects to its tape device. On that event the VTL receives -SCST_USER_ATTACH_SESS subcommand. Since the VTL doesn't use any -initiator specific data, it can do nothing on that subcommand, so it -prepares scst_user_reply_cmd structure, where: - - - cmd_h set to returned by SCST_USER_REPLY_AND_GET_CMD ioctl() cmd_h - - - subcode set to SCST_USER_ATTACH_SESS - - - result set to 0 - -Then it prepares struct scst_user_get_cmd with reply set to the prepared -scst_user_reply_cmd structure, calls SCST_USER_REPLY_AND_GET_CMD ioctl() -and waits for some SCSI command arrives from the initiator. - -If the received SCSI command is READ-type one, SCST does the necessary -preparations, then the VTL receives SCST_USER_EXEC subcommand, where -bufflen and data_len are set correctly, but memory for buffer isn't -allocated, so pbuf field is 0. The VTL then allocates the data buffer -with size alloc_len, e.g. using malloc(). Then the VTL reads the data -from disk in it, e.g. using O_DIRECT read() function, then prepares -scst_user_reply_cmd structure, where: - - - cmd_h set to returned by SCST_USER_REPLY_AND_GET_CMD ioctl() cmd_h - - - subcode set to SCST_USER_EXEC - - - exec_reply.resp_data_len set to length of the read data - - - exec_reply.pbuf set to the data buffer, where the data were read - - - exec_reply.reply_type set to SCST_EXEC_REPLY_COMPLETED - - - exec_reply.status set to the SAM defined status of the - operation - - - exec_reply.sense_len set and exec_reply.psense_buffer filled with - sense data, if necessary - -Then it prepares struct scst_user_get_cmd with reply set to the prepared -scst_user_reply_cmd structure, calls SCST_USER_REPLY_AND_GET_CMD ioctl() -and waits for the next SCSI command arrives from the initiator. That's -all for this SCSI command. For the next one the used data buffer will be -reused. - -For WRITE-type SCSI commands the processing is the same, but -SCST_USER_ALLOC_MEM will be returned before SCST_USER_EXEC, since the -data transfer from the initiator precedes the commands execution. - -In case, if the first command requires 4K data buffer, but the second -one - 1M, for it the VTL also will be asked to allocate the buffer. -Then, if no more 4K commands come for some time, for it -SCST_USER_ON_CACHED_MEM_FREE subcommand will be returned to the VTL in -order to ask it to free that buffer. - -Vladislav Bolkhovitin diff --git a/www/scstvsstgt.html b/www/scstvsstgt.html index 9a4c87e09..e27989b2c 100644 --- a/www/scstvsstgt.html +++ b/www/scstvsstgt.html @@ -53,10 +53,19 @@

Also you shouldn't be deceived by the fact that some (small) part of STGT was accepted in the kernel.
It doesn't mean that STGT has the "kernel quality". In fact, STGT as a whole similarly to any other - out-of-tree project lives on its own, hence has its own quality level, which isn't necessary better, than the - quality level of SCST. Actually, from such important aspect of quality as simplicity, it - might be quite contrary: e.g. SCST isn't required to support HIGHMEM (nowadays it - isn't necessary, but required for all in-kernel components), which allowed to simplify memory management a lot.

+ out-of-tree project lives on its own, hence has its own quality level, which isn't necessary better, than the + quality level of SCST. Actually, from such important aspect of quality as simplicity, it + might be quite contrary: e.g. SCST isn't required to support HIGHMEM (nowadays it + isn't necessary, but required for all in-kernel components), which allowed to simplify memory management a lot.

+ +

Interesting, on 2008 Linux Storage & Filesystem Workshop, namely in + "Storage Track" and + "Linux/iSCSI and a Generic Target Mode Storage + Engine for Linux v2.6" documents, a special emphasis was put on the fact that SCST has "older" design, + while STGT has the "current" design. Well, this seems to be a good sign, because SCST opponents have come to so + little to say against it, so they started to use psychological arguments, exploiting the fact that most people + think that "newer" is always better.

+ diff --git a/www/targets.html b/www/targets.html index db1762b6c..ce46d45de 100644 --- a/www/targets.html +++ b/www/targets.html @@ -55,7 +55,7 @@
  • Fibre Channel QLogic 22xx/23xx series
  • Fibre Channel QLogic ISP Chipsets, including 24xx
  • Infiniband SCSI RDMA Protocol (SRP)
    • -
  • LSI/MPT SCSI
    • +
  • LSI/MPT cards (parallel SCSI, including Wide Ultra320, SAS, Fibre Channel)
  • ...