108 lines
3.6 KiB
ReStructuredText
108 lines
3.6 KiB
ReStructuredText
.. _virtio-blk:
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Virtio-blk
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##########
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The virtio-blk device is a simple virtual block device. The FE driver
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(in the UOS space) places read, write, and other requests onto the
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virtqueue, so that the BE driver (in the SOS space) can process them
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accordingly. Communication between the FE and BE is based on the virtio
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kick and notify mechanism.
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The virtio device ID of the virtio-blk is ``2``, and it supports one
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virtqueue, the size of which is 64, configurable in the source code.
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.. figure:: images/virtio-blk-image01.png
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:align: center
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:width: 900px
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:name: virtio-blk-arch
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Virtio-blk architecture
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The feature bits supported by the BE device are shown as follows:
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``VIRTIO_BLK_F_SEG_MAX``
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Maximum number of segments in a request is in seg_max.
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``VIRTIO_BLK_F_BLK_SIZE``
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Block size of disk is in blk_size.
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``VIRTIO_BLK_F_TOPOLOGY``
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Device exports information on optimal I/O alignment.
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``VIRTIO_RING_F_INDIRECT_DESC``
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Support for indirect descriptors
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``VIRTIO_BLK_F_FLUSH``
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Cache flush command support.
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``VIRTIO_BLK_F_CONFIG_WCE``
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Device can toggle its cache between writeback and writethrough modes.
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Virtio-blk-BE design
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********************
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.. figure:: images/virtio-blk-image02.png
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:align: center
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:width: 900px
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:name: virtio-blk-be
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The virtio-blk BE device is implemented as a legacy virtio device. Its
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backend media could be a file or a partition. The virtio-blk device
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supports writeback and writethrough cache mode. In writeback mode,
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virtio-blk has good write and read performance. To be safer,
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writethrough is set as the default mode, as it can make sure every write
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operation queued to the virtio-blk FE driver layer is submitted to
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hardware storage.
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During initialization, virtio-blk will allocate 64 ioreq buffers in a
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shared ring used to store the I/O requests. The freeq, busyq, and pendq
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shown in :numref:`virtio-blk-be` are used to manage requests. Each
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virtio-blk device starts 8 worker threads to process request
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asynchronously.
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Usage:
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******
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The device model configuration command syntax for virtio-blk is::
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-s <slot>,virtio-blk,<filepath>[,options]
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- ``filepath`` is the path of a file or disk partition
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- ``options`` include:
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- ``writethru``: write operation is reported completed only when the
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data has been written to physical storage.
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- ``writeback``: write operation is reported completed when data is
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placed in the page cache. Needs to be flushed to the physical storage.
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- ``ro``: open file with readonly mode.
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- ``sectorsize``: configured as either
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``sectorsize=<sector size>/<physical sector size>`` or
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``sectorsize=<sector size>``.
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The default values for sector size and physical sector size are 512
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- ``range``: configured as ``range=<start lba in file>/<sub file size>``
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meaning the virtio-blk will only access part of the file, from the
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``<start lba in file>`` to ``<start lba in file> + <sub file site>``.
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A simple example for virtio-blk:
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1. Prepare a file in SOS folder::
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dd if=/dev/zero of=test.img bs=1M count=1024
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mkfs.ext4 test.img
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#. Add virtio-blk in the DM cmdline, slot number should not duplicate
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another device::
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-s 9,virtio-blk,/root/test.img
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#. Launch UOS, you can find ``/dev/vdx`` in UOS.
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The ``x`` in ``/dev/vdx`` is related to the slot number used. If
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If you start DM with two virtio-blks, and the slot numbers are 9 and 10,
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then, the device with slot 9 will be recognized as ``/dev/vda``, and
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the device with slot 10 will be ``/dev/vdb``
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#. Mount ``/dev/vdx`` to a folder in the UOS, and then you can access it.
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Successful booting of the User OS verifies the correctness of the
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device.
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