2017-10-02 19:31:06 +02:00
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# 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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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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2018-01-30 13:53:45 +01:00
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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. If used inside a VM, it bypasses interrupt
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and context switching overhead of QEMU and guest kernel, significantly boosting
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the overall I/O performance.
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2017-10-02 19:31:06 +02:00
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# Limitations {#virtio_limitations}
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2018-01-30 13:53:45 +01:00
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Current Virtio-SCSI implementation has a couple of limitations:
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2017-10-02 19:31:06 +02:00
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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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