mirror of
https://github.com/xen-project/xen.git
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Kamala Narasimhan
73b16b07ca
Signed-off-by: Kamala Narasimhan <kamala.narasimhan@citrix.com> Acked-by: Ian Jackson <ian.jackson@eu.citrix.com> Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>
155 lines
5.1 KiB
Plaintext
155 lines
5.1 KiB
Plaintext
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----------------------------
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xl Disk Configuration Option
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----------------------------
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This document provides a brief description of xl disk configuration
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option, different attributes that can be passed through it and the
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format in which they need to be specified.
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At a higher level, xl disk configuration option takes the following
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format:
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disk = [ '[format:][path],vdev[:type],attrib',
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'[format:][path],vdev[:type],attrib', ... ]
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Not all attributes are required (the attributes enclosed within square
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brackets above are optional) and some are deprecated. Following is a
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brief description of each of the attribute along with information on
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whether or not they are mandatory.
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------------------
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Attribute Details
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------------------
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format:
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------
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Description: Specifies the format of image file.
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Supported values: raw, qcow, qcow2, vhd
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Deprecated values: None
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Mandatory: No. When not specified raw format is assumed.
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For a physical block device the format must be
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raw and need not be explicitly specified. For
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an image file the format could be one of the
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supported values and when not specified assumed
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to be raw.
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path:
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----
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Description: Block device or image file path. For a
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physical block device a /dev will be prepended
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when not specified and when the path doesn't
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start with a '/'.
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Supported values: N/A
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Deprecated values: N/A
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Mandatory: No. While a path is provided in most cases
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there is an exception. For a cdrom device, lack
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of this attribute would imply an empty cdrom
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drive.
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vdev:
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----
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Description: Virtual device as seen by the guest (also
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referred to as guest drive designation in some
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specifications). See docs/misc/vbd-interface.txt.
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Supported values: hd[x], xvd[x], sd[x] etc. Please refer to the
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above specification for further details.
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Deprecated values: None
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Mandatory: Yes
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type:
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----
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Description: Qualifies virtual device type.
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Supported values: cdrom
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Deprecated values: None
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Mandatory: No
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attrib:
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------
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Description: Specified access control information. Whether
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or not the block device is provided to the
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guest in read-only or read-write mode depends
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on this attribute.
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Supported values: 'r', 'w'
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Deprecated values: None
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Mandatory: Yes
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--------------
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Example usages
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--------------
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disk = [ 'vhd:/path/to/dev,hda,w', '/path/to/iso,hdc:cdrom,r' ]
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disk = [ 'path/to/phy/dev,hda,w', '/dev/cdrom,hdc:cdrom,r' ]
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disk = [ 'qcow:/path/to/file,hda,w' ]
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disk = [ 'qcow2:/path/to/file,hda,w', 'raw:/path/to/dev,hdc:cdrom,r' ]
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-------------------------
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Miscellaneous Information
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-------------------------
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---------------------
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Deprecated Attributes
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---------------------
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Deprecated values are acceptable and will work the way they did
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earlier except with a warning message printed to the xl log or screen.
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However it's usage is supported purely for backward compatibility
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purpose and not recommended. Also, support for these deprecated
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attributes are likely to be dropped in future versions of xl.
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The block-dev-type and access-type deprecated attributes described
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below are prepended to the format attribute. Example - tap:aio:qcow
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Further details on the deprecated attributes are below:
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block-dev-type:
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--------------
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Description: Specifies the block device type.
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Supported values: phy,file, tap, tap2
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access-type:
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-----------
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Description: Backend implementation option to choose from
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while accessing block device.
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Example: tap:aio:vhd:/path/to/file
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Supported values: 'aio', 'tapdisk', 'ioemu'
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---------------------
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Impementation Details
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---------------------
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Backend Details:
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---------------
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For 'phy' block device type, blkback is always used as the backend.
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When the running dom0 instance does not support blkback, block device
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access will fail. For block device type 'file' with format raw or
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when no format specfied, tapdisk2 is used when present otherwise qemu
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fallback option is used. For 'file', 'tap' or 'tap2' block device
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type with format 'vhd', only tapdisk2 is used as qemu does not support
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vhd format. Absence of tapdisk2 support in this case will result in
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failure. When it comes to image format and how that affects the
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backend choice, for qcow/qcow2 qemu backend is used as tapdisk2 does
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not work with these formats. For raw format image file, tapdisk2 is
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used and when not available qemu backend is used as fallback. For vhd
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format, as mentioned earlier tapdisk2 is used and tapdisk2
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unavailability will result in failure as qemu fallback option does not
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support vhd file format.
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