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diff --git a/src/spdk/doc/nvme.md b/src/spdk/doc/nvme.md new file mode 100644 index 00000000..3565156d --- /dev/null +++ b/src/spdk/doc/nvme.md @@ -0,0 +1,262 @@ +# NVMe Driver {#nvme} + +# In this document {#nvme_toc} + +* @ref nvme_intro +* @ref nvme_examples +* @ref nvme_interface +* @ref nvme_design +* @ref nvme_fabrics_host +* @ref nvme_multi_process +* @ref nvme_hotplug + +# Introduction {#nvme_intro} + +The NVMe driver is a C library that may be linked directly into an application +that provides direct, zero-copy data transfer to and from +[NVMe SSDs](http://nvmexpress.org/). It is entirely passive, meaning that it spawns +no threads and only performs actions in response to function calls from the +application itself. The library controls NVMe devices by directly mapping the +[PCI BAR](https://en.wikipedia.org/wiki/PCI_configuration_space) into the local +process and performing [MMIO](https://en.wikipedia.org/wiki/Memory-mapped_I/O). +I/O is submitted asynchronously via queue pairs and the general flow isn't +entirely dissimilar from Linux's +[libaio](http://man7.org/linux/man-pages/man2/io_submit.2.html). + +More recently, the library has been improved to also connect to remote NVMe +devices via NVMe over Fabrics. Users may now call spdk_nvme_probe() on both +local PCI busses and on remote NVMe over Fabrics discovery services. The API is +otherwise unchanged. + +# Examples {#nvme_examples} + +## Getting Start with Hello World {#nvme_helloworld} + +There are a number of examples provided that demonstrate how to use the NVMe +library. They are all in the [examples/nvme](https://github.com/spdk/spdk/tree/master/examples/nvme) +directory in the repository. The best place to start is +[hello_world](https://github.com/spdk/spdk/blob/master/examples/nvme/hello_world/hello_world.c). + +## Running Benchmarks with Fio Plugin {#nvme_fioplugin} + +SPDK provides a plugin to the very popular [fio](https://github.com/axboe/fio) +tool for running some basic benchmarks. See the fio start up +[guide](https://github.com/spdk/spdk/blob/master/examples/nvme/fio_plugin/) +for more details. + +## Running Benchmarks with Perf Tool {#nvme_perf} + +NVMe perf utility in the [examples/nvme/perf](https://github.com/spdk/spdk/tree/master/examples/nvme/perf) +is one of the examples which also can be used for performance tests. The fio +tool is widely used because it is very flexible. However, that flexibility adds +overhead and reduces the efficiency of SPDK. Therefore, SPDK provides a perf +benchmarking tool which has minimal overhead during benchmarking. We have +measured up to 2.6 times more IOPS/core when using perf vs. fio with the +4K 100% Random Read workload. The perf benchmarking tool provides several +run time options to support the most common workload. The following examples +demonstrate how to use perf. + +Example: Using perf for 4K 100% Random Read workload to a local NVMe SSD for 300 seconds +~~~{.sh} +perf -q 128 -o 4096 -w randread -r 'trtype:PCIe traddr:0000:04:00.0' -t 300 +~~~ + +Example: Using perf for 4K 100% Random Read workload to a remote NVMe SSD exported over the network via NVMe-oF +~~~{.sh} +perf -q 128 -o 4096 -w randread -r 'trtype:RDMA adrfam:IPv4 traddr:192.168.100.8 trsvcid:4420' -t 300 +~~~ + +Example: Using perf for 4K 70/30 Random Read/Write mix workload to all local NVMe SSDs for 300 seconds +~~~{.sh} +perf -q 128 -o 4096 -w randrw -M 70 -t 300 +~~~ + +Example: Using perf for extended LBA format CRC guard test to a local NVMe SSD, +users must write to the SSD before reading the LBA from SSD +~~~{.sh} +perf -q 1 -o 4096 -w write -r 'trtype:PCIe traddr:0000:04:00.0' -t 300 -e 'PRACT=0,PRCKH=GUARD' +perf -q 1 -o 4096 -w read -r 'trtype:PCIe traddr:0000:04:00.0' -t 200 -e 'PRACT=0,PRCKH=GUARD' +~~~ + +# Public Interface {#nvme_interface} + +- spdk/nvme.h + +Key Functions | Description +------------------------------------------- | ----------- +spdk_nvme_probe() | @copybrief spdk_nvme_probe() +spdk_nvme_ctrlr_alloc_io_qpair() | @copybrief spdk_nvme_ctrlr_alloc_io_qpair() +spdk_nvme_ctrlr_get_ns() | @copybrief spdk_nvme_ctrlr_get_ns() +spdk_nvme_ns_cmd_read() | @copybrief spdk_nvme_ns_cmd_read() +spdk_nvme_ns_cmd_readv() | @copybrief spdk_nvme_ns_cmd_readv() +spdk_nvme_ns_cmd_read_with_md() | @copybrief spdk_nvme_ns_cmd_read_with_md() +spdk_nvme_ns_cmd_write() | @copybrief spdk_nvme_ns_cmd_write() +spdk_nvme_ns_cmd_writev() | @copybrief spdk_nvme_ns_cmd_writev() +spdk_nvme_ns_cmd_write_with_md() | @copybrief spdk_nvme_ns_cmd_write_with_md() +spdk_nvme_ns_cmd_write_zeroes() | @copybrief spdk_nvme_ns_cmd_write_zeroes() +spdk_nvme_ns_cmd_dataset_management() | @copybrief spdk_nvme_ns_cmd_dataset_management() +spdk_nvme_ns_cmd_flush() | @copybrief spdk_nvme_ns_cmd_flush() +spdk_nvme_qpair_process_completions() | @copybrief spdk_nvme_qpair_process_completions() +spdk_nvme_ctrlr_cmd_admin_raw() | @copybrief spdk_nvme_ctrlr_cmd_admin_raw() +spdk_nvme_ctrlr_process_admin_completions() | @copybrief spdk_nvme_ctrlr_process_admin_completions() +spdk_nvme_ctrlr_cmd_io_raw() | @copybrief spdk_nvme_ctrlr_cmd_io_raw() +spdk_nvme_ctrlr_cmd_io_raw_with_md() | @copybrief spdk_nvme_ctrlr_cmd_io_raw_with_md() + +# NVMe Driver Design {#nvme_design} + +## NVMe I/O Submission {#nvme_io_submission} + +I/O is submitted to an NVMe namespace using nvme_ns_cmd_xxx functions. The NVMe +driver submits the I/O request as an NVMe submission queue entry on the queue +pair specified in the command. The function returns immediately, prior to the +completion of the command. The application must poll for I/O completion on each +queue pair with outstanding I/O to receive completion callbacks by calling +spdk_nvme_qpair_process_completions(). + +@sa spdk_nvme_ns_cmd_read, spdk_nvme_ns_cmd_write, spdk_nvme_ns_cmd_dataset_management, +spdk_nvme_ns_cmd_flush, spdk_nvme_qpair_process_completions + +### Scaling Performance {#nvme_scaling} + +NVMe queue pairs (struct spdk_nvme_qpair) provide parallel submission paths for +I/O. I/O may be submitted on multiple queue pairs simultaneously from different +threads. Queue pairs contain no locks or atomics, however, so a given queue +pair may only be used by a single thread at a time. This requirement is not +enforced by the NVMe driver (doing so would require a lock), and violating this +requirement results in undefined behavior. + +The number of queue pairs allowed is dictated by the NVMe SSD itself. The +specification allows for thousands, but most devices support between 32 +and 128. The specification makes no guarantees about the performance available from +each queue pair, but in practice the full performance of a device is almost +always achievable using just one queue pair. For example, if a device claims to +be capable of 450,000 I/O per second at queue depth 128, in practice it does +not matter if the driver is using 4 queue pairs each with queue depth 32, or a +single queue pair with queue depth 128. + +Given the above, the easiest threading model for an application using SPDK is +to spawn a fixed number of threads in a pool and dedicate a single NVMe queue +pair to each thread. A further improvement would be to pin each thread to a +separate CPU core, and often the SPDK documentation will use "CPU core" and +"thread" interchangeably because we have this threading model in mind. + +The NVMe driver takes no locks in the I/O path, so it scales linearly in terms +of performance per thread as long as a queue pair and a CPU core are dedicated +to each new thread. In order to take full advantage of this scaling, +applications should consider organizing their internal data structures such +that data is assigned exclusively to a single thread. All operations that +require that data should be done by sending a request to the owning thread. +This results in a message passing architecture, as opposed to a locking +architecture, and will result in superior scaling across CPU cores. + +## NVMe Driver Internal Memory Usage {#nvme_memory_usage} + +The SPDK NVMe driver provides a zero-copy data transfer path, which means that +there are no data buffers for I/O commands. However, some Admin commands have +data copies depending on the API used by the user. + +Each queue pair has a number of trackers used to track commands submitted by the +caller. The number trackers for I/O queues depend on the users' input for queue +size and the value read from controller capabilities register field Maximum Queue +Entries Supported(MQES, 0 based value). Each tracker has a fixed size 4096 Bytes, +so the maximum memory used for each I/O queue is: (MQES + 1) * 4 KiB. + +I/O queue pairs can be allocated in host memory, this is used for most NVMe controllers, +some NVMe controllers which can support Controller Memory Buffer may put I/O queue +pairs at controllers' PCI BAR space, SPDK NVMe driver can put I/O submission queue +into controller memory buffer, it depends on users' input and controller capabilities. +Each submission queue entry (SQE) and completion queue entry (CQE) consumes 64 bytes +and 16 bytes respectively. Therefore, the maximum memory used for each I/O queue +pair is (MQES + 1) * (64 + 16) Bytes. + +# NVMe over Fabrics Host Support {#nvme_fabrics_host} + +The NVMe driver supports connecting to remote NVMe-oF targets and +interacting with them in the same manner as local NVMe SSDs. + +## Specifying Remote NVMe over Fabrics Targets {#nvme_fabrics_trid} + +The method for connecting to a remote NVMe-oF target is very similar +to the normal enumeration process for local PCIe-attached NVMe devices. +To connect to a remote NVMe over Fabrics subsystem, the user may call +spdk_nvme_probe() with the `trid` parameter specifying the address of +the NVMe-oF target. + +The caller may fill out the spdk_nvme_transport_id structure manually +or use the spdk_nvme_transport_id_parse() function to convert a +human-readable string representation into the required structure. + +The spdk_nvme_transport_id may contain the address of a discovery service +or a single NVM subsystem. If a discovery service address is specified, +the NVMe library will call the spdk_nvme_probe() `probe_cb` for each +discovered NVM subsystem, which allows the user to select the desired +subsystems to be attached. Alternatively, if the address specifies a +single NVM subsystem directly, the NVMe library will call `probe_cb` +for just that subsystem; this allows the user to skip the discovery step +and connect directly to a subsystem with a known address. + +# NVMe Multi Process {#nvme_multi_process} + +This capability enables the SPDK NVMe driver to support multiple processes accessing the +same NVMe device. The NVMe driver allocates critical structures from shared memory, so +that each process can map that memory and create its own queue pairs or share the admin +queue. There is a limited number of I/O queue pairs per NVMe controller. + +The primary motivation for this feature is to support management tools that can attach +to long running applications, perform some maintenance work or gather information, and +then detach. + +## Configuration {#nvme_multi_process_configuration} + +DPDK EAL allows different types of processes to be spawned, each with different permissions +on the hugepage memory used by the applications. + +There are two types of processes: +1. a primary process which initializes the shared memory and has full privileges and +2. a secondary process which can attach to the primary process by mapping its shared memory +regions and perform NVMe operations including creating queue pairs. + +This feature is enabled by default and is controlled by selecting a value for the shared +memory group ID. This ID is a positive integer and two applications with the same shared +memory group ID will share memory. The first application with a given shared memory group +ID will be considered the primary and all others secondary. + +Example: identical shm_id and non-overlapping core masks +~~~{.sh} +./perf options [AIO device(s)]... + [-c core mask for I/O submission/completion] + [-i shared memory group ID] + +./perf -q 1 -o 4096 -w randread -c 0x1 -t 60 -i 1 +./perf -q 8 -o 131072 -w write -c 0x10 -t 60 -i 1 +~~~ + +## Limitations {#nvme_multi_process_limitations} + +1. Two processes sharing memory may not share any cores in their core mask. +2. If a primary process exits while secondary processes are still running, those processes +will continue to run. However, a new primary process cannot be created. +3. Applications are responsible for coordinating access to logical blocks. + +@sa spdk_nvme_probe, spdk_nvme_ctrlr_process_admin_completions + + +# NVMe Hotplug {#nvme_hotplug} + +At the NVMe driver level, we provide the following support for Hotplug: + +1. Hotplug events detection: +The user of the NVMe library can call spdk_nvme_probe() periodically to detect +hotplug events. The probe_cb, followed by the attach_cb, will be called for each +new device detected. The user may optionally also provide a remove_cb that will be +called if a previously attached NVMe device is no longer present on the system. +All subsequent I/O to the removed device will return an error. + +2. Hot remove NVMe with IO loads: +When a device is hot removed while I/O is occurring, all access to the PCI BAR will +result in a SIGBUS error. The NVMe driver automatically handles this case by installing +a SIGBUS handler and remapping the PCI BAR to a new, placeholder memory location. +This means I/O in flight during a hot remove will complete with an appropriate error +code and will not crash the application. + +@sa spdk_nvme_probe |