---
docs/misc/xl-disk-configuration.txt | 249 +++++++++++++++++++++--------------
1 files changed, 150 insertions(+), 99 deletions(-)
diff --git a/docs/misc/xl-disk-configuration.txt
b/docs/misc/xl-disk-configuration.txt
index 58332a9..de0a75e 100644
--- a/docs/misc/xl-disk-configuration.txt
+++ b/docs/misc/xl-disk-configuration.txt
@@ -1,57 +1,98 @@
+ ---------------------
+ XL DISK CONFIGURATION
+ ---------------------
+This document specifies the xl config file format disk configuration
+option. It has the following form:
- ----------------------------
- xl Disk Configuration Option
- ----------------------------
+ disk = [ '<diskspec>', '<diskspec>', ... ]
-This document provides a brief description of xl disk configuration
-option, different attributes that can be passed through it and the
-format in which they need to be specified.
+where each diskspec is in this form:
+
+ [<key>=<value>|<flag>,]*,
+ [<target>, [<format>, [<vdev>, [<access>]]]],
+ [<key>=<value>|<flag>,]*
+ [target=<target>]
-At a higher level, xl disk configuration option takes the following
-format:
- disk = [ '[format:][path],vdev[:type],attrib',
- '[format:][path],vdev[:type],attrib', ... ]
+For example, these strings are equivalent:
-Not all attributes are required (the attributes enclosed within square
-brackets above are optional) and some are deprecated. Following is a
-brief description of each of the attribute along with information on
-whether or not they are mandatory.
+ /dev/vg/guest-volume,,hda
+ /dev/vg/guest-volume,raw,hda,rw
+ format=raw, vdev=hda, access=rw, target=/dev/vg/guest-volume
+ raw:/dev/vg/guest-volume,hda,w (deprecated, see below)
+As are these:
-------------------
-Attribute Details
-------------------
+ /root/image.iso,,hdc,cdrom
+ /root/image.iso,,hdc,,cdrom
+ /root/image.iso,raw,hdc,devtype=cdrom
+ format=raw, vdev=hdc, access=ro, devtype=cdrom, target=/root/image.iso
+ raw:/root/image.iso,hdc:cdrom,ro (deprecated, see below)
+These might be specified in the domain config file like this:
-format:
-------
+ disk = [ '/dev/vg/guest-volume,,hda', '/root/image.iso,,hdc,cdrom' ]
-Description: Specifies the format of image file.
-Supported values: raw, qcow, qcow2, vhd
-Deprecated values: None
-Mandatory: No. When not specified raw format is assumed.
- For a physical block device the format must be
- raw and need not be explicitly specified. For
- an image file the format could be one of the
- supported values and when not specified assumed
- to be raw.
-path:
-----
-Description: Block device or image file path. For a
- physical block device a /dev will be prepended
- when not specified and when the path doesn't
- start with a '/'.
+More formally, the string is a series of comma-separated keyword/value
+pairs, flags and positional parameters. Parameters which are not bare
+keywords and which do not contain "=" symbols are assigned to the
+so-far-unspecified positional parameters, in the order below. The
+positional parameters may also be specified explicitly by name.
+
+Each parameter may be specified at most once, either as a positional
+parameter or a named parameter. Default values apply if the parameter
+is not specified, or if it is specified with an empty value (whether
+positionally or explicitly).
+
+Whitespace may appear before each parameter and will be ignored.
+
+
+=====================
+POSITIONAL PARAMETERS
+=====================
+
+target
+------
+
+Description: Block device or image file path. When this is
+ used as a path, /dev will be prepended
+ if the path doesn't start with a '/'.
Supported values: N/A
Deprecated values: N/A
-Mandatory: No. While a path is provided in most cases
- there is an exception. For a cdrom device, lack
+Default value: None. While a path is provided in most cases
+ there is an exception: for a cdrom device, lack
of this attribute would imply an empty cdrom
drive.
-vdev:
+Special syntax:
+
+ When this parameter is specified by name, ie with the "target="
+ syntax in the configuration file, it consumes the whole rest of the
+ <diskspec>. Therefore in that case it must come last. This is
+ permissible even if an empty value for the target was already
+ specifed as a positional parameter. This is the only way to
+ specify a target string containing metacharacters such as commas
+ and (in some cases) colons, which would otherwise be
+ misinterpreted.
+
+ Future parameter and flag names will start with an ascii letter and
+ contain only ascii alphanumerics, hyphens and underscores, and will
+ not be legal as vdevs. Targets which might match that syntax
+ should not be specified as positional parameters.
+
+
+format
+------
+
+Description: Specifies the format of image file.
+Supported values: raw, qcow, qcow2, vhd
+Deprecated values: None
+Default value: raw
+
+
+vdev
----
Description: Virtual device as seen by the guest (also
@@ -60,95 +101,105 @@ Description: Virtual device as seen by the
guest (also
Supported values: hd[x], xvd[x], sd[x] etc. Please refer to the
above specification for further details.
Deprecated values: None
-Mandatory: Yes
+Default Value: None, this parameter is mandatory.
-type:
-----
-
-Description: Qualifies virtual device type.
-Supported values: cdrom
-Deprecated values: None
-Mandatory: No
-attrib:
-------
+access
+-------
Description: Specified access control information. Whether
or not the block device is provided to the
guest in read-only or read-write mode depends
on this attribute.
-Supported values: 'r', 'w'
+Supported values: ro, r (specifies read-only)
+ rw, w (specifies read/write)
Deprecated values: None
-Mandatory: Yes
+Default value: rw
+ unless devtype=cdrom, in which case r
---------------
-Example usages
---------------
-disk = [ 'vhd:/path/to/dev,hda,w', '/path/to/iso,hdc:cdrom,r' ]
-disk = [ 'path/to/phy/dev,hda,w', '/dev/cdrom,hdc:cdrom,r' ]
-disk = [ 'qcow:/path/to/file,hda,w' ]
-disk = [ 'qcow2:/path/to/file,hda,w', 'raw:/path/to/dev,hdc:cdrom,r' ]
+==========================
+OTHER PARAMETERS AND FLAGS
+==========================
--------------------------
-Miscellaneous Information
--------------------------
+devtype=<devtype>
+-----------------
----------------------
-Deprecated Attributes
----------------------
+Description: Qualifies virtual device type.
+Supported values: cdrom
+Deprecated values: None
+Mandatory: No
-Deprecated values are acceptable and will work the way they did
-earlier except with a warning message printed to the xl log or screen.
-However it's usage is supported purely for backward compatibility
-purpose and not recommended. Also, support for these deprecated
-attributes are likely to be dropped in future versions of xl.
-The block-dev-type and access-type deprecated attributes described
-below are prepended to the format attribute. Example - tap:aio:qcow
-Further details on the deprecated attributes are below:
+cdrom
+-----
-block-dev-type:
---------------
+Convenience alias for "devtype=cdrom".
-Description: Specifies the block device type.
-Supported values: phy,file, tap, tap2
-access-type:
------------
+script=<script>
+---------------
-Description: Backend implementation option to choose from
- while accessing block device.
- Example: tap:aio:vhd:/path/to/file
-Supported values: 'aio', 'tapdisk', 'ioemu'
+Specifies that <target> is not a normal host path, but rather
+information to be interpreted by /etc/xen/scripts/block-<script>.
----------------------
-Impementation Details
----------------------
-Backend Details:
----------------
-For 'phy' block device type, blkback is always used as the backend.
-When the running dom0 instance does not support blkback, block device
-access will fail. For block device type 'file' with format raw or
-when no format specfied, tapdisk2 is used when present otherwise qemu
-fallback option is used. For 'file', 'tap' or 'tap2' block device
-type with format 'vhd', only tapdisk2 is used as qemu does not support
-vhd format. Absence of tapdisk2 support in this case will result in
-failure. When it comes to image format and how that affects the
-backend choice, for qcow/qcow2 qemu backend is used as tapdisk2 does
-not work with these formats. For raw format image file, tapdisk2 is
-used and when not available qemu backend is used as fallback. For vhd
-format, as mentioned earlier tapdisk2 is used and tapdisk2
-unavailability will result in failure as qemu fallback option does not
-support vhd file format.
+==================================
+DEPRECATED PARAMETERS AND PREFIXES
+==================================
+
+Deprecated values are acceptable and are intended work compatibly with
+xend and xl from xen 4.1. In future they may print a warning.
+Support for deprecated parameters and syntaxes are likely to be
+dropped in future versions of xl.
+
+There is also support for a deprecated old syntax for <diskspec>:
+
+ [<format>:][<target>],<vdev>[:<devtype>],<access> (deprecated)
+
+This syntax also supports deprecated prefixes, described below. These
+are found prepended to the format parameter - eg "tap:aio:qcow:".
+
+
+<format>:
+---------
+
+Description: Specifies the format (deprecated)
+Supported values: raw: qcow2: vhd:
+
+In xend and old versions of libxl it was necessary to specify the
+format with a prefix. For compatibility, these three prefixes are
+recognised as specifying the corresponding format. They are
+equivalent to "format=<format>" or the specification of <format>
+(without a colon) as a positional parameter.
+
+
+<script>:
+---------
+Description: Specifies the script (deprecated)
+Supported values: iscsi: nbd: enbd: drbd:
+In xend and old versions of libxl it was necessary to specify the
+"script" (see above) with a prefix. For compatibility, these four
+prefixes are recognised as specifying the corresponding script. They
+are equivalent to "script=<script>".
+<deprecated-prefix>:
+--------------------
+Description; Deprecated prefix, ignored
+Supported values: tapdisk: tap2: aio: ioemu: file: phy:
+Various prefixes were required by xend and older versions of libxl to
+make the block devices work. In some cases these options would
+override the backend type, but in other cases they would be ignored in
+favour of "making it work"; in yet other cases it would be necessary
+to specify several of these, for example:
+ "tap:aio:/some/path..."
+All of these prefixes are now stripped and ignored.
--
1.5.6.5
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