bionic (5) xl-disk-configuration.5.gz

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NAME

       xl-disk-configuration - XL Disk Configuration Syntax

SYNTAX

       This document specifies the xl config file format disk configuration option.  It has the following form:

          disk = [ 'DISKSPEC', 'DISKSPEC', ... ]

       where each "DISKSPEC" is in this form:

          [<key>=<value>|<flag>,]*,
            [<target>, [<format>, [<vdev>, [<access>]]]],
            [<key>=<value>|<flag>,]*
            [target=<target>]

       For example, these strings are equivalent:

           /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:

           /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:

           disk = [ '/dev/vg/guest-volume,,hda', '/root/image.iso,,hdc,cdrom' ]

       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

           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.

           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" including trailing whitespaces.  Therefore in that
               case it must come last.  This is permissible even if an empty value for the target was already
               specified 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, qed

           Deprecated values
               None

           Default value
               raw

       vdev
           Description
               Virtual device as seen by the guest (also referred to as guest drive designation in some
               specifications).  xen-vbd-interface(7)

           Supported values
               hd[x], xvd[x], sd[x] etc.  Please refer to the above specification for further details.

           Deprecated values
               None

           Default Value
               None, this parameter is mandatory.

       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
               "ro", "r"   (specifies read-only)

               "rw", "w"   (specifies read/write)

           Deprecated values
               None

           Default value
               "rw" unless devtype=cdrom, in which case "r"

Other Parameters And Flags

       devtype=DEVTYPE
           Description
               Qualifies virtual device type.

           Supported values
               cdrom

           Deprecated values
               None

           Mandatory
               No

       cdrom
           Convenience alias for "devtype=cdrom".

       backend=DOMAIN-NAME
           Description
               Designates a backend domain for the device

           Supported values
               Valid domain names

           Mandatory
               No

           Specifies the backend domain which this device should attach to. This defaults to domain 0.
           Specifying another domain requires setting up a driver domain which is outside the scope of this
           document.

       backendtype=TYPE
           Description
               Specifies the backend implementation to use

           Supported values
               phy, tap, qdisk

           Mandatory
               No

           Default value
               Automatically determine which backend to use.

           This does not affect the guest's view of the device.  It controls which software implementation of
           the Xen backend driver is used.

           Not all backend drivers support all combinations of other options.  For example, "phy" does not
           support formats other than "raw".  Normally this option should not be specified, in which case libxl
           will automatically determine the most suitable backend.

       script=SCRIPT
           Specifies that target is not a normal host path, but rather information to be interpreted by the
           executable program SCRIPT, (looked for in /etc/xen/scripts, if it doesn't contain a slash).

           These scripts are normally called "block-SCRIPT".

       direct-io-safe
           Description
               Disables non-O_DIRECT workaround

           Supported values
               absent, present

           Mandatory
               No

           Default value
               absent (workaround may be enabled)

           There is a memory lifetime bug in some driver domain (dom0) kernels which can cause crashes when
           using O_DIRECT.  The bug occurs due to a mismatch between the backend-visible lifetime of pages used
           for the Xen PV network protocol and that expected by the backend kernel's networking subsystem.  This
           can cause crashes when using certain backends with certain underlying storage.

           See:
            <http://lists.xen.org/archives/html/xen-devel/2012-12/msg01154.html>

           For this reason, (this version of) the Xen libxl toolstack disables O_DIRECT when using the qemu-
           based Xen PV backend ("qdisk").

           However, this workaround has performance and scaling implications, and it is only necessary if the
           underlying device is a network filesystem.  If the underlying device is not, then it is good to
           disable it; that is what this option is for.

           This option simply requests that the workaround be disabled.  (However, not all backends versions
           which use the workaround understand this option, so this is on a best effort basis.)

           It's important to note that if you are storing the VM disk on a network filesystem or a network block
           device (NFS or ISCSI) it might not be safe to use this option.  Otherwise specifying it is safe and
           can give better performances.

           If in the future the bug is fixed properly this option will then be silently ignored.

       discard / no-discard
           Description
               Request that backend advertise discard support to frontend

           Supported values
               discard, no-discard

           Mandatory
               No

           Default value
               discard

           An advisory setting for the backend driver, specifying whether to advertise discard support (TRIM,
           UNMAP) to the frontend.  The real benefit of this option is to be able to force it off rather than
           on.  It can be used to disable "hole punching" for file based backends which were intentionally
           created non-sparse to avoid fragmentation of the file.

COLO Parameters

       colo
           Enable COLO HA for disk. For better understanding block replication on QEMU, please refer to:
           <http://wiki.qemu.org/Features/BlockReplication> Note that the COLO configuration settings should be
           considered unstable.  They may change incompatibly in future versions of Xen.

       colo-host
           Description
               Secondary host's address

           Mandatory
               Yes when COLO enabled

       colo-port
           Description
               Secondary port.  We will run a nbd server on secondary host, and the nbd server will listen this
               port.

           Mandatory
               Yes when COLO enabled

       colo-export
           Description
               We will run a nbd server on secondary host, exportname is the nbd server's disk export name.

           Mandatory
               Yes when COLO enabled

       active-disk
           Description
               This is used by secondary. Secondary guest's write will be buffered in this disk.

           Mandatory
               Yes when COLO enabled

       hidden-disk
           Description
               This is used by secondary. It buffers the original content that is modified by the primary VM.

           Mandatory
               Yes when COLO enabled

Deprecated Parameters, Prefixes And Syntaxes

       Deprecated forms 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 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=block-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.

   Missing format and empty target
       The following syntax is also supported:

         ,<vdev>:<devtype>,<access>   (deprecated)

       This is solely for compatibility with xend's syntax for empty cdroms, which is (for example)
       ",hdc:cdrom,r".