=head1 NAME

xl-disk-configuration - XL Disk Configuration Syntax

=head1 SYNTAX

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

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

where each C<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.

=head1 Positional Parameters

=over 4

=item B<target>

=over 4

=item Description

Block device or image file path.  When this is used as a path, F</dev>
will be prepended if the path doesn't start with a '/'.

=item Supported values

N/A

=item Deprecated values

N/A

=item 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.

=item Special syntax

When this parameter is specified by name, ie with the C<target=>
syntax in the configuration file, it consumes the whole rest of the
C<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.

=back

=item B<format>

=over 4

=item Description

Specifies the format of image file.

=item Supported values

raw, qcow, qcow2, vhd, qed

=item Deprecated values

None

=item Default value

raw

=back

=item B<vdev>

=over 4

=item Description

Virtual device as seen by the guest (also referred to as guest drive
designation in some specifications).  See L<xen-vbd-interface(7)>.

=item Supported values

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

=item Deprecated values

None

=item Default Value

None, this parameter is mandatory.

=back

=item B<access>

=over 4

=item 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.

=item Supported values

C<ro>, C<r>   (specifies read-only)

C<rw>, C<w>   (specifies read/write)

=item Deprecated values

None

=item Default value

C<rw> unless devtype=cdrom, in which case C<r>

=back

=back

=head1 Other Parameters And Flags

=over 4

=item B<devtype>=I<DEVTYPE>

=over 4

=item Description

Qualifies virtual device type.

=item Supported values

cdrom

=item Deprecated values

None

=item Mandatory

No

=back

=item B<cdrom>

Convenience alias for "devtype=cdrom".


=item B<backend>=I<DOMAIN-NAME>

=over 4

=item Description

Designates a backend domain for the device

=item Supported values

Valid domain names

=item Mandatory

No

=back

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.


=item B<backendtype>=I<TYPE>

=over 4

=item Description

Specifies the backend implementation to use

=item Supported values

phy, qdisk, standalone, tap

=item Mandatory

No

=item Default value

Automatically determine which backend to use.

=back

It controls which software implementation of the backend driver is used.
Depending on the "specification" option this may affect the guest's view
of the device.

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

"tap" needs blktap's tapback to be running.


=item B<script>=I<SCRIPT>

Specifies that B<target> is not a normal host path, but rather
information to be interpreted by the executable program I<SCRIPT>,
(looked for in F<@XEN_SCRIPT_DIR@>, if it doesn't contain a slash).

These scripts are normally called "block-I<SCRIPT>".


=item B<direct-io-safe>

=over 4

=item Description

Disables non-O_DIRECT workaround

=item Supported values

absent, present

=item Mandatory

No

=item Default value

absent (workaround may be enabled)

=back

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:
 L<https://lists.xenproject.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 (C<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.


=item B<discard> / B<no-discard>

=over 4

=item Description

Request that backend advertise discard support to frontend

=item Supported values

discard, no-discard

=item Mandatory

No

=item Default value

discard

=back

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.

=item B<trusted> / B<untrusted>

=over 4

=item Description

Reports whether the backend should be trusted by the frontend

=item Supported values

trusted, untrusted

=item Mandatory

No

=item Default value

trusted

=back

An advisory setting for the frontend driver on whether the backend should be
trusted.  The frontend should deploy whatever protections it has available to
prevent an untrusted backend from accessing guest data not related to the I/O
processing or causing malfunction to the frontend or the whole domain.

Note frontends can ignore such recommendation.

=item B<specification>=I<SPECIFICATION>

=over 4

=item Description

Specifies the communication protocol (specification) to use for the chosen
"backendtype" option

=item Supported values

xen, virtio

=item Mandatory

No

=item Default value

xen

=back

Besides forcing toolstack to use specific backend implementation, this also
affects the guest's view of the device. For example, "virtio" requires
Virtio frontend driver (virtio-blk) to be used. Please note, the virtual
device (vdev) is not passed to the guest in that case, but it still must be
specified for the internal purposes.

=item B<grant_usage=BOOLEAN>

=over 4

=item Description

Specifies the usage of Xen grants for accessing guest memory. Only applicable
to specification "virtio".

=item Supported values

1, 0

=item Mandatory

No

=item Default value

If this option is missing, then the default grant setting will be used,
i.e. "grant_usage=1" if backend-domid != 0 or "grant_usage=0" otherwise.

=back

=back

=head1 COLO Parameters

=over 4

=item B<colo>

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


=item B<colo-host>

=over 4

=item Description

Secondary host's address

=item Mandatory

Yes when COLO enabled

=back

=item B<colo-port>

=over 4

=item Description

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

=item Mandatory

Yes when COLO enabled

=back

=item B<colo-export>

=over 4

=item Description

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

=item Mandatory

Yes when COLO enabled

=back

=item B<active-disk>

=over 4

=item Description

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

=item Mandatory

Yes when COLO enabled

=back

=item B<hidden-disk>

=over 4

=item Description

This is used by secondary. It buffers the original content that is modified
by the primary VM.

=item Mandatory

Yes when COLO enabled

=back

=back


=head1 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 C<DISKSPEC>:

  [<format>:][<target>],<vdev>[:<devtype>],<access>   (deprecated)

This syntax also supports deprecated prefixes, described below.  These
are found prepended to the format parameter - eg C<tap:aio:qcow:>.

=over 4

=item B<format>

=over 4

=item Description

Specifies the format (deprecated)

=item Supported values

raw:  qcow2:  vhd:

=back

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 C<format=FORMAT> or the specification of B<format>
(without a colon) as a positional parameter.


=item B<script>

=over 4

=item Description

Specifies the script (deprecated)

=item Supported values

iscsi:  nbd:  enbd:  drbd:

=back

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 C<script=block-SCRIPT>.


=item B<deprecated-prefix>

=over 4

=item Description

Deprecated prefix, ignored

=item Supported values

tapdisk:  tap2:  aio:  ioemu:  file:  phy:

=back

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.

=back

=head2 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) C<,hdc:cdrom,r>.
