71ea826507
Change-Id: Ia88ae52117068ac395dad9ad3d7ac818e41077fb Signed-off-by: Dariusz Stojaczyk <dariuszx.stojaczyk@intel.com> Reviewed-on: https://review.gerrithub.io/380956 Tested-by: SPDK Automated Test System <sys_sgsw@intel.com> Reviewed-by: Paul Luse <paul.e.luse@intel.com> Reviewed-by: Daniel Verkamp <daniel.verkamp@intel.com>
42 lines
1.8 KiB
Markdown
42 lines
1.8 KiB
Markdown
# Virtio SCSI driver {#virtio}
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# Introduction {#virtio_intro}
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Virtio SCSI driver is an initiator for SPDK @ref vhost application. The
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driver allows any SPDK app to connect to another SPDK instance exposing
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a vhost-scsi device. The driver will enumerate targets on the device (which acts
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as a SCSI controller) and create *virtual* bdevs usable by any SPDK application.
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Sending an I/O request to the Virtio SCSI bdev will put the request data into
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a Virtio queue that is processed by the host SPDK app exposing the
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controller. The host, after sending I/O to the real drive, will put the response
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back into the Virtio queue. Then, the response is received by the Virtio SCSI
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driver.
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The driver, just like the SPDK @ref vhost, is using pollers instead of standard
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interrupts to check for an I/O response. It bypasses kernel interrupt and context
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switching overhead of QEMU and guest kernel, significantly boosting the overall
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I/O performance.
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Virtio SCSI driver supports two different usage models:
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* PCI - This is the standard mode of operation when used in a guest virtual
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machine, where QEMU has presented the virtio-scsi controller as a virtual
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PCI device.
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* User vhost - Can be used to connect to a vhost-scsi socket directly on the
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same host.
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# Multiqueue {#virtio_multiqueue}
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The Virtio SCSI controller will automatically manage virtio queue distribution.
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Currently each thread doing an I/O on a single bdev will get an exclusive queue.
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Multi-threaded I/O on bdevs from a single Virtio-SCSI controller is not supported.
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# Limitations {#virtio_limitations}
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The Virtio SCSI driver is still experimental. Current implementation has many
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limitations:
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* supports only up to 8 hugepages (implies only 1GB sized pages are practical)
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* single LUN per target
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* only SPDK vhost-scsi controllers supported
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* no RPC
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* no multi-threaded I/O for single-queue virtio devices
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