Re: [Libguestfs] [nbdkit PATCH v2 13/13] RFC: plugins: Add callbacks for FUA semantics

Limitation: The kernel will (with today's default settings) typically be willing to send up to 128 requests of 128kB size to the driver in parallel. We wanted to support 128 parallel read operations on different areas of the disk without requiring 128 separate threads and connections for the driver. Right now in nbdkit that is impossible. The main loop in connection.c will pull an nbd request off the socket and block until that read request is complete before sending a response and getting the next request, blocking other requests on the socket unless running X connections/threads in parallel.

Change: We introduced an additional set of functions to the nbdkit_plugin struct that supports asynchronous handling of the requests and a few helper functions for the plugin to use to respond when it has finished the request. This is very similar to the fuse filesystem low level api (async supported) vs the high level fuse fs api (sync only). The design goal here is that a single connection/thread on nbdkit can support as many requests in parallel as the plugin allows. The nbdkit side pulls the request off the socket and if the async function pointer is non-null it will wrap the request in an op struct and use the async plugin call for read/write/etc capturing any buffer allocated and some op details into the op pointer. The plugin async_* will start the op and return to nbdkit while the plugin works on it in the background. Nbdkit will then go back to the socket and begin the next request. Our plugin uses 1 connection/nbdkit thread and 2-4 threads internally with boost asio over sockets to service the requests to cloud. We are able to achieve ~1GB/s (yes bytes) read/write performance to aws s3 from an ec2 node with 10 gigabit networking on < 100MB of memory in the driver with this approach.

Here are some of what our function prototypes look like that support an asynchronous nbdkit model #define CBDKIT_THREAD_MODEL_SERIALIZE_REQUESTS 2 #define CBDKIT_THREAD_MODEL_PARALLEL 3 #define CBDKIT_THREAD_MODEL_ASYNC 4 struct cbdkit_plugin { ... int (*pread) (void *handle, void *buf, uint32_t count, uint64_t offset); int (*pwrite) (void *handle, const void *buf, uint32_t count, uint64_t offset); int (*flush) (void *handle); int (*trim) (void *handle, uint32_t count, uint64_t offset); int (*zero) (void *handle, uint32_t count, uint64_t offset, int may_trim); int errno_is_preserved; void (*async_pread) (void *op, void *handle, void *buf, uint32_t count, uint64_t offset); void (*async_pwrite) (void *op, void *handle, const void *buf, uint32_t count, uint64_t offset, int fua); void (*async_flush) (void *op, void *handle); void (*async_trim) (void *op, void *handle, uint32_t count, uint64_t offset, int fua); void (*async_zero) (void *op, void *handle, uint32_t count, uint64_t offset, int may_trim, int fua); ... } Additionally there are a few helper functions for the plugin to use to respond back to nbdkit when the job is eventually finished. The plugin contract when using the async functions is that every async func guarantees it will call an appropriate async_reply function. /* call for completion of successful async_pwrite, async_flush, async_trim, or async_zero */ extern CBDKIT_CXX_LANG_C int cbdkit_async_reply (void *op); /* call for complete of successful async_pread */ extern CBDKIT_CXX_LANG_C int cbdkit_async_reply_read (void *op); /* call for completion of any async operation with error */ extern CBDKIT_CXX_LANG_C int cbdkit_async_reply_error (void *op, uint32_t error); If there is any interest in supporting async ops in the next api version I am able to share the entire modified nbdkit (cbdkit) source that we use that supports this async op framework, fua, as well as some buffer pooling.