qemu-doc.texi

\input texinfo @c -*- texinfo -*-
@c %**start of header
@setfilename qemu-doc.info

@documentlanguage en
@documentencoding UTF-8

@settitle QEMU Emulator User Documentation
@exampleindent 0
@paragraphindent 0
@c %**end of header

@ifinfo
@direntry
* QEMU: (qemu-doc).    The QEMU Emulator User Documentation.
@end direntry
@end ifinfo

@iftex
@titlepage
@sp 7
@center @titlefont{QEMU Emulator}
@sp 1
@center @titlefont{User Documentation}
@sp 3
@end titlepage
@end iftex

@ifnottex
@node Top
@top

@menu
* Introduction::
* Installation::
* QEMU PC System emulator::
* QEMU System emulator for non PC targets::
* QEMU User space emulator::
* compilation:: Compilation from the sources
* License::
* Index::
@end menu
@end ifnottex

@contents

@node Introduction
@chapter Introduction

@menu
* intro_features:: Features
@end menu

@node intro_features
@section Features

QEMU is a FAST! processor emulator using dynamic translation to
achieve good emulation speed.

QEMU has two operating modes:

@itemize
@cindex operating modes

@item
@cindex system emulation
Full system emulation. In this mode, QEMU emulates a full system (for
example a PC), including one or several processors and various
peripherals. It can be used to launch different Operating Systems
without rebooting the PC or to debug system code.

@item
@cindex user mode emulation
User mode emulation. In this mode, QEMU can launch
processes compiled for one CPU on another CPU. It can be used to
launch the Wine Windows API emulator (@url{http://www.winehq.org}) or
to ease cross-compilation and cross-debugging.

@end itemize

QEMU can run without a host kernel driver and yet gives acceptable
performance.

For system emulation, the following hardware targets are supported:
@itemize
@cindex emulated target systems
@cindex supported target systems
@item PC (x86 or x86_64 processor)
@item ISA PC (old style PC without PCI bus)
@item PREP (PowerPC processor)
@item G3 Beige PowerMac (PowerPC processor)
@item Mac99 PowerMac (PowerPC processor, in progress)
@item Sun4m/Sun4c/Sun4d (32-bit Sparc processor)
@item Sun4u/Sun4v (64-bit Sparc processor, in progress)
@item Malta board (32-bit and 64-bit MIPS processors)
@item MIPS Magnum (64-bit MIPS processor)
@item ARM Integrator/CP (ARM)
@item ARM Versatile baseboard (ARM)
@item ARM RealView Emulation/Platform baseboard (ARM)
@item Spitz, Akita, Borzoi, Terrier and Tosa PDAs (PXA270 processor)
@item Luminary Micro LM3S811EVB (ARM Cortex-M3)
@item Luminary Micro LM3S6965EVB (ARM Cortex-M3)
@item Freescale MCF5208EVB (ColdFire V2).
@item Arnewsh MCF5206 evaluation board (ColdFire V2).
@item Palm Tungsten|E PDA (OMAP310 processor)
@item N800 and N810 tablets (OMAP2420 processor)
@item MusicPal (MV88W8618 ARM processor)
@item Gumstix "Connex" and "Verdex" motherboards (PXA255/270).
@item Siemens SX1 smartphone (OMAP310 processor)
@item AXIS-Devboard88 (CRISv32 ETRAX-FS).
@item Petalogix Spartan 3aDSP1800 MMU ref design (MicroBlaze).
@item Avnet LX60/LX110/LX200 boards (Xtensa)
@end itemize

@cindex supported user mode targets
For user emulation, x86 (32 and 64 bit), PowerPC (32 and 64 bit),
ARM, MIPS (32 bit only), Sparc (32 and 64 bit),
Alpha, ColdFire(m68k), CRISv32 and MicroBlaze CPUs are supported.

@node Installation
@chapter Installation

If you want to compile QEMU yourself, see @ref{compilation}.

@menu
* install_linux::   Linux
* install_windows:: Windows
* install_mac::     Macintosh
@end menu

@node install_linux
@section Linux
@cindex installation (Linux)

If a precompiled package is available for your distribution - you just
have to install it. Otherwise, see @ref{compilation}.

@node install_windows
@section Windows
@cindex installation (Windows)

Download the experimental binary installer at
@url{http://www.free.oszoo.org/@/download.html}.
TODO (no longer available)

@node install_mac
@section Mac OS X

Download the experimental binary installer at
@url{http://www.free.oszoo.org/@/download.html}.
TODO (no longer available)

@node QEMU PC System emulator
@chapter QEMU PC System emulator
@cindex system emulation (PC)

@menu
* pcsys_introduction:: Introduction
* pcsys_quickstart::   Quick Start
* sec_invocation::     Invocation
* pcsys_keys::         Keys in the graphical frontends
* mux_keys::           Keys in the character backend multiplexer
* pcsys_monitor::      QEMU Monitor
* disk_images::        Disk Images
* pcsys_network::      Network emulation
* pcsys_other_devs::   Other Devices
* direct_linux_boot::  Direct Linux Boot
* pcsys_usb::          USB emulation
* vnc_security::       VNC security
* gdb_usage::          GDB usage
* pcsys_os_specific::  Target OS specific information
@end menu

@node pcsys_introduction
@section Introduction

@c man begin DESCRIPTION

The QEMU PC System emulator simulates the
following peripherals:

@itemize @minus
@item
i440FX host PCI bridge and PIIX3 PCI to ISA bridge
@item
Cirrus CLGD 5446 PCI VGA card or dummy VGA card with Bochs VESA
extensions (hardware level, including all non standard modes).
@item
PS/2 mouse and keyboard
@item
2 PCI IDE interfaces with hard disk and CD-ROM support
@item
Floppy disk
@item
PCI and ISA network adapters
@item
Serial ports
@item
IPMI BMC, either and internal or external one
@item
Creative SoundBlaster 16 sound card
@item
ENSONIQ AudioPCI ES1370 sound card
@item
Intel 82801AA AC97 Audio compatible sound card
@item
Intel HD Audio Controller and HDA codec
@item
Adlib (OPL2) - Yamaha YM3812 compatible chip
@item
Gravis Ultrasound GF1 sound card
@item
CS4231A compatible sound card
@item
PCI UHCI USB controller and a virtual USB hub.
@end itemize

SMP is supported with up to 255 CPUs.

QEMU uses the PC BIOS from the Seabios project and the Plex86/Bochs LGPL
VGA BIOS.

QEMU uses YM3812 emulation by Tatsuyuki Satoh.

QEMU uses GUS emulation (GUSEMU32 @url{http://www.deinmeister.de/gusemu/})
by Tibor "TS" Schütz.

Note that, by default, GUS shares IRQ(7) with parallel ports and so
QEMU must be told to not have parallel ports to have working GUS.

@example
qemu-system-i386 dos.img -soundhw gus -parallel none
@end example

Alternatively:
@example
qemu-system-i386 dos.img -device gus,irq=5
@end example

Or some other unclaimed IRQ.

CS4231A is the chip used in Windows Sound System and GUSMAX products

@c man end

@node pcsys_quickstart
@section Quick Start
@cindex quick start

Download and uncompress the linux image (@file{linux.img}) and type:

@example
qemu-system-i386 linux.img
@end example

Linux should boot and give you a prompt.

@node sec_invocation
@section Invocation

@example
@c man begin SYNOPSIS
@command{qemu-system-i386} [@var{options}] [@var{disk_image}]
@c man end
@end example

@c man begin OPTIONS
@var{disk_image} is a raw hard disk image for IDE hard disk 0. Some
targets do not need a disk image.

@include qemu-options.texi

@c man end

@node pcsys_keys
@section Keys in the graphical frontends

@c man begin OPTIONS

During the graphical emulation, you can use special key combinations to change
modes. The default key mappings are shown below, but if you use @code{-alt-grab}
then the modifier is Ctrl-Alt-Shift (instead of Ctrl-Alt) and if you use
@code{-ctrl-grab} then the modifier is the right Ctrl key (instead of Ctrl-Alt):

@table @key
@item Ctrl-Alt-f
@kindex Ctrl-Alt-f
Toggle full screen

@item Ctrl-Alt-+
@kindex Ctrl-Alt-+
Enlarge the screen

@item Ctrl-Alt--
@kindex Ctrl-Alt--
Shrink the screen

@item Ctrl-Alt-u
@kindex Ctrl-Alt-u
Restore the screen's un-scaled dimensions

@item Ctrl-Alt-n
@kindex Ctrl-Alt-n
Switch to virtual console 'n'. Standard console mappings are:
@table @emph
@item 1
Target system display
@item 2
Monitor
@item 3
Serial port
@end table

@item Ctrl-Alt
@kindex Ctrl-Alt
Toggle mouse and keyboard grab.
@end table

@kindex Ctrl-Up
@kindex Ctrl-Down
@kindex Ctrl-PageUp
@kindex Ctrl-PageDown
In the virtual consoles, you can use @key{Ctrl-Up}, @key{Ctrl-Down},
@key{Ctrl-PageUp} and @key{Ctrl-PageDown} to move in the back log.

@c man end

@node mux_keys
@section Keys in the character backend multiplexer

@c man begin OPTIONS

During emulation, if you are using a character backend multiplexer
(which is the default if you are using @option{-nographic}) then
several commands are available via an escape sequence. These
key sequences all start with an escape character, which is @key{Ctrl-a}
by default, but can be changed with @option{-echr}. The list below assumes
you're using the default.

@table @key
@item Ctrl-a h
@kindex Ctrl-a h
Print this help
@item Ctrl-a x
@kindex Ctrl-a x
Exit emulator
@item Ctrl-a s
@kindex Ctrl-a s
Save disk data back to file (if -snapshot)
@item Ctrl-a t
@kindex Ctrl-a t
Toggle console timestamps
@item Ctrl-a b
@kindex Ctrl-a b
Send break (magic sysrq in Linux)
@item Ctrl-a c
@kindex Ctrl-a c
Rotate between the frontends connected to the multiplexer (usually
this switches between the monitor and the console)
@item Ctrl-a Ctrl-a
@kindex Ctrl-a Ctrl-a
Send the escape character to the frontend
@end table
@c man end

@ignore

@c man begin SEEALSO
The HTML documentation of QEMU for more precise information and Linux
user mode emulator invocation.
@c man end

@c man begin AUTHOR
Fabrice Bellard
@c man end

@end ignore

@node pcsys_monitor
@section QEMU Monitor
@cindex QEMU monitor

The QEMU monitor is used to give complex commands to the QEMU
emulator. You can use it to:

@itemize @minus

@item
Remove or insert removable media images
(such as CD-ROM or floppies).

@item
Freeze/unfreeze the Virtual Machine (VM) and save or restore its state
from a disk file.

@item Inspect the VM state without an external debugger.

@end itemize

@subsection Commands

The following commands are available:

@include qemu-monitor.texi

@include qemu-monitor-info.texi

@subsection Integer expressions

The monitor understands integers expressions for every integer
argument. You can use register names to get the value of specifics
CPU registers by prefixing them with @emph{$}.

@node disk_images
@section Disk Images

Since version 0.6.1, QEMU supports many disk image formats, including
growable disk images (their size increase as non empty sectors are
written), compressed and encrypted disk images. Version 0.8.3 added
the new qcow2 disk image format which is essential to support VM
snapshots.

@menu
* disk_images_quickstart::    Quick start for disk image creation
* disk_images_snapshot_mode:: Snapshot mode
* vm_snapshots::              VM snapshots
* qemu_img_invocation::       qemu-img Invocation
* qemu_nbd_invocation::       qemu-nbd Invocation
* qemu_ga_invocation::        qemu-ga Invocation
* disk_images_formats::       Disk image file formats
* host_drives::               Using host drives
* disk_images_fat_images::    Virtual FAT disk images
* disk_images_nbd::           NBD access
* disk_images_sheepdog::      Sheepdog disk images
* disk_images_iscsi::         iSCSI LUNs
* disk_images_gluster::       GlusterFS disk images
* disk_images_ssh::           Secure Shell (ssh) disk images
@end menu

@node disk_images_quickstart
@subsection Quick start for disk image creation

You can create a disk image with the command:
@example
qemu-img create myimage.img mysize
@end example
where @var{myimage.img} is the disk image filename and @var{mysize} is its
size in kilobytes. You can add an @code{M} suffix to give the size in
megabytes and a @code{G} suffix for gigabytes.

See @ref{qemu_img_invocation} for more information.

@node disk_images_snapshot_mode
@subsection Snapshot mode

If you use the option @option{-snapshot}, all disk images are
considered as read only. When sectors in written, they are written in
a temporary file created in @file{/tmp}. You can however force the
write back to the raw disk images by using the @code{commit} monitor
command (or @key{C-a s} in the serial console).

@node vm_snapshots
@subsection VM snapshots

VM snapshots are snapshots of the complete virtual machine including
CPU state, RAM, device state and the content of all the writable
disks. In order to use VM snapshots, you must have at least one non
removable and writable block device using the @code{qcow2} disk image
format. Normally this device is the first virtual hard drive.

Use the monitor command @code{savevm} to create a new VM snapshot or
replace an existing one. A human readable name can be assigned to each
snapshot in addition to its numerical ID.

Use @code{loadvm} to restore a VM snapshot and @code{delvm} to remove
a VM snapshot. @code{info snapshots} lists the available snapshots
with their associated information:

@example
(qemu) info snapshots
Snapshot devices: hda
Snapshot list (from hda):
ID        TAG                 VM SIZE                DATE       VM CLOCK
1         start                   41M 2006-08-06 12:38:02   00:00:14.954
2                                 40M 2006-08-06 12:43:29   00:00:18.633
3         msys                    40M 2006-08-06 12:44:04   00:00:23.514
@end example

A VM snapshot is made of a VM state info (its size is shown in
@code{info snapshots}) and a snapshot of every writable disk image.
The VM state info is stored in the first @code{qcow2} non removable
and writable block device. The disk image snapshots are stored in
every disk image. The size of a snapshot in a disk image is difficult
to evaluate and is not shown by @code{info snapshots} because the
associated disk sectors are shared among all the snapshots to save
disk space (otherwise each snapshot would need a full copy of all the
disk images).

When using the (unrelated) @code{-snapshot} option
(@ref{disk_images_snapshot_mode}), you can always make VM snapshots,
but they are deleted as soon as you exit QEMU.

VM snapshots currently have the following known limitations:
@itemize
@item
They cannot cope with removable devices if they are removed or
inserted after a snapshot is done.
@item
A few device drivers still have incomplete snapshot support so their
state is not saved or restored properly (in particular USB).
@end itemize

@node qemu_img_invocation
@subsection @code{qemu-img} Invocation

@include qemu-img.texi

@node qemu_nbd_invocation
@subsection @code{qemu-nbd} Invocation

@include qemu-nbd.texi

@node qemu_ga_invocation
@subsection @code{qemu-ga} Invocation

@include qemu-ga.texi

@node disk_images_formats
@subsection Disk image file formats

QEMU supports many image file formats that can be used with VMs as well as with
any of the tools (like @code{qemu-img}). This includes the preferred formats
raw and qcow2 as well as formats that are supported for compatibility with
older QEMU versions or other hypervisors.

Depending on the image format, different options can be passed to
@code{qemu-img create} and @code{qemu-img convert} using the @code{-o} option.
This section describes each format and the options that are supported for it.

@table @option
@item raw

Raw disk image format. This format has the advantage of
being simple and easily exportable to all other emulators. If your
file system supports @emph{holes} (for example in ext2 or ext3 on
Linux or NTFS on Windows), then only the written sectors will reserve
space. Use @code{qemu-img info} to know the real size used by the
image or @code{ls -ls} on Unix/Linux.

Supported options:
@table @code
@item preallocation
Preallocation mode (allowed values: @code{off}, @code{falloc}, @code{full}).
@code{falloc} mode preallocates space for image by calling posix_fallocate().
@code{full} mode preallocates space for image by writing zeros to underlying
storage.
@end table

@item qcow2
QEMU image format, the most versatile format. Use it to have smaller
images (useful if your filesystem does not supports holes, for example
on Windows), zlib based compression and support of multiple VM
snapshots.

Supported options:
@table @code
@item compat
Determines the qcow2 version to use. @code{compat=0.10} uses the
traditional image format that can be read by any QEMU since 0.10.
@code{compat=1.1} enables image format extensions that only QEMU 1.1 and
newer understand (this is the default). Amongst others, this includes
zero clusters, which allow efficient copy-on-read for sparse images.

@item backing_file
File name of a base image (see @option{create} subcommand)
@item backing_fmt
Image format of the base image
@item encryption
If this option is set to @code{on}, the image is encrypted with 128-bit AES-CBC.

The use of encryption in qcow and qcow2 images is considered to be flawed by
modern cryptography standards, suffering from a number of design problems:

@itemize @minus
@item The AES-CBC cipher is used with predictable initialization vectors based
on the sector number. This makes it vulnerable to chosen plaintext attacks
which can reveal the existence of encrypted data.
@item The user passphrase is directly used as the encryption key. A poorly
chosen or short passphrase will compromise the security of the encryption.
@item In the event of the passphrase being compromised there is no way to
change the passphrase to protect data in any qcow images. The files must
be cloned, using a different encryption passphrase in the new file. The
original file must then be securely erased using a program like shred,
though even this is ineffective with many modern storage technologies.
@end itemize

Use of qcow / qcow2 encryption with QEMU is deprecated, and support for
it will go away in a future release.  Users are recommended to use an
alternative encryption technology such as the Linux dm-crypt / LUKS
system.

@item cluster_size
Changes the qcow2 cluster size (must be between 512 and 2M). Smaller cluster
sizes can improve the image file size whereas larger cluster sizes generally
provide better performance.

@item preallocation
Preallocation mode (allowed values: @code{off}, @code{metadata}, @code{falloc},
@code{full}). An image with preallocated metadata is initially larger but can
improve performance when the image needs to grow. @code{falloc} and @code{full}
preallocations are like the same options of @code{raw} format, but sets up
metadata also.

@item lazy_refcounts
If this option is set to @code{on}, reference count updates are postponed with
the goal of avoiding metadata I/O and improving performance. This is
particularly interesting with @option{cache=writethrough} which doesn't batch
metadata updates. The tradeoff is that after a host crash, the reference count
tables must be rebuilt, i.e. on the next open an (automatic) @code{qemu-img
check -r all} is required, which may take some time.

This option can only be enabled if @code{compat=1.1} is specified.

@item nocow
If this option is set to @code{on}, it will turn off COW of the file. It's only
valid on btrfs, no effect on other file systems.

Btrfs has low performance when hosting a VM image file, even more when the guest
on the VM also using btrfs as file system. Turning off COW is a way to mitigate
this bad performance. Generally there are two ways to turn off COW on btrfs:
a) Disable it by mounting with nodatacow, then all newly created files will be
NOCOW. b) For an empty file, add the NOCOW file attribute. That's what this option
does.

Note: this option is only valid to new or empty files. If there is an existing
file which is COW and has data blocks already, it couldn't be changed to NOCOW
by setting @code{nocow=on}. One can issue @code{lsattr filename} to check if
the NOCOW flag is set or not (Capital 'C' is NOCOW flag).

@end table

@item qed
Old QEMU image format with support for backing files and compact image files
(when your filesystem or transport medium does not support holes).

When converting QED images to qcow2, you might want to consider using the
@code{lazy_refcounts=on} option to get a more QED-like behaviour.

Supported options:
@table @code
@item backing_file
File name of a base image (see @option{create} subcommand).
@item backing_fmt
Image file format of backing file (optional).  Useful if the format cannot be
autodetected because it has no header, like some vhd/vpc files.
@item cluster_size
Changes the cluster size (must be power-of-2 between 4K and 64K). Smaller
cluster sizes can improve the image file size whereas larger cluster sizes
generally provide better performance.
@item table_size
Changes the number of clusters per L1/L2 table (must be power-of-2 between 1
and 16).  There is normally no need to change this value but this option can be
used for performance benchmarking.
@end table

@item qcow
Old QEMU image format with support for backing files, compact image files,
encryption and compression.

Supported options:
@table @code
@item backing_file
File name of a base image (see @option{create} subcommand)
@item encryption
If this option is set to @code{on}, the image is encrypted.
@end table

@item vdi
VirtualBox 1.1 compatible image format.
Supported options:
@table @code
@item static
If this option is set to @code{on}, the image is created with metadata
preallocation.
@end table

@item vmdk
VMware 3 and 4 compatible image format.

Supported options:
@table @code
@item backing_file
File name of a base image (see @option{create} subcommand).
@item compat6
Create a VMDK version 6 image (instead of version 4)
@item subformat
Specifies which VMDK subformat to use. Valid options are
@code{monolithicSparse} (default),
@code{monolithicFlat},
@code{twoGbMaxExtentSparse},
@code{twoGbMaxExtentFlat} and
@code{streamOptimized}.
@end table

@item vpc
VirtualPC compatible image format (VHD).
Supported options:
@table @code
@item subformat
Specifies which VHD subformat to use. Valid options are
@code{dynamic} (default) and @code{fixed}.
@end table

@item VHDX
Hyper-V compatible image format (VHDX).
Supported options:
@table @code
@item subformat
Specifies which VHDX subformat to use. Valid options are
@code{dynamic} (default) and @code{fixed}.
@item block_state_zero
Force use of payload blocks of type 'ZERO'.  Can be set to @code{on} (default)
or @code{off}.  When set to @code{off}, new blocks will be created as
@code{PAYLOAD_BLOCK_NOT_PRESENT}, which means parsers are free to return
arbitrary data for those blocks.  Do not set to @code{off} when using
@code{qemu-img convert} with @code{subformat=dynamic}.
@item block_size
Block size; min 1 MB, max 256 MB.  0 means auto-calculate based on image size.
@item log_size
Log size; min 1 MB.
@end table
@end table

@subsubsection Read-only formats
More disk image file formats are supported in a read-only mode.
@table @option
@item bochs
Bochs images of @code{growing} type.
@item cloop
Linux Compressed Loop image, useful only to reuse directly compressed
CD-ROM images present for example in the Knoppix CD-ROMs.
@item dmg
Apple disk image.
@item parallels
Parallels disk image format.
@end table


@node host_drives
@subsection Using host drives

In addition to disk image files, QEMU can directly access host
devices. We describe here the usage for QEMU version >= 0.8.3.

@subsubsection Linux

On Linux, you can directly use the host device filename instead of a
disk image filename provided you have enough privileges to access
it. For example, use @file{/dev/cdrom} to access to the CDROM.

@table @code
@item CD
You can specify a CDROM device even if no CDROM is loaded. QEMU has
specific code to detect CDROM insertion or removal. CDROM ejection by
the guest OS is supported. Currently only data CDs are supported.
@item Floppy
You can specify a floppy device even if no floppy is loaded. Floppy
removal is currently not detected accurately (if you change floppy
without doing floppy access while the floppy is not loaded, the guest
OS will think that the same floppy is loaded).
Use of the host's floppy device is deprecated, and support for it will
be removed in a future release.
@item Hard disks
Hard disks can be used. Normally you must specify the whole disk
(@file{/dev/hdb} instead of @file{/dev/hdb1}) so that the guest OS can
see it as a partitioned disk. WARNING: unless you know what you do, it
is better to only make READ-ONLY accesses to the hard disk otherwise
you may corrupt your host data (use the @option{-snapshot} command
line option or modify the device permissions accordingly).
@end table

@subsubsection Windows

@table @code
@item CD
The preferred syntax is the drive letter (e.g. @file{d:}). The
alternate syntax @file{\\.\d:} is supported. @file{/dev/cdrom} is
supported as an alias to the first CDROM drive.

Currently there is no specific code to handle removable media, so it
is better to use the @code{change} or @code{eject} monitor commands to
change or eject media.
@item Hard disks
Hard disks can be used with the syntax: @file{\\.\PhysicalDrive@var{N}}
where @var{N} is the drive number (0 is the first hard disk).

WARNING: unless you know what you do, it is better to only make
READ-ONLY accesses to the hard disk otherwise you may corrupt your
host data (use the @option{-snapshot} command line so that the
modifications are written in a temporary file).
@end table


@subsubsection Mac OS X

@file{/dev/cdrom} is an alias to the first CDROM.

Currently there is no specific code to handle removable media, so it
is better to use the @code{change} or @code{eject} monitor commands to
change or eject media.

@node disk_images_fat_images
@subsection Virtual FAT disk images

QEMU can automatically create a virtual FAT disk image from a
directory tree. In order to use it, just type:

@example
qemu-system-i386 linux.img -hdb fat:/my_directory
@end example

Then you access access to all the files in the @file{/my_directory}
directory without having to copy them in a disk image or to export
them via SAMBA or NFS. The default access is @emph{read-only}.

Floppies can be emulated with the @code{:floppy:} option:

@example
qemu-system-i386 linux.img -fda fat:floppy:/my_directory
@end example

A read/write support is available for testing (beta stage) with the
@code{:rw:} option:

@example
qemu-system-i386 linux.img -fda fat:floppy:rw:/my_directory
@end example

What you should @emph{never} do:
@itemize
@item use non-ASCII filenames ;
@item use "-snapshot" together with ":rw:" ;
@item expect it to work when loadvm'ing ;
@item write to the FAT directory on the host system while accessing it with the guest system.
@end itemize

@node disk_images_nbd
@subsection NBD access

QEMU can access directly to block device exported using the Network Block Device
protocol.

@example
qemu-system-i386 linux.img -hdb nbd://my_nbd_server.mydomain.org:1024/
@end example

If the NBD server is located on the same host, you can use an unix socket instead
of an inet socket:

@example
qemu-system-i386 linux.img -hdb nbd+unix://?socket=/tmp/my_socket
@end example

In this case, the block device must be exported using qemu-nbd:

@example
qemu-nbd --socket=/tmp/my_socket my_disk.qcow2
@end example

The use of qemu-nbd allows sharing of a disk between several guests:
@example
qemu-nbd --socket=/tmp/my_socket --share=2 my_disk.qcow2
@end example

@noindent
and then you can use it with two guests:
@example
qemu-system-i386 linux1.img -hdb nbd+unix://?socket=/tmp/my_socket
qemu-system-i386 linux2.img -hdb nbd+unix://?socket=/tmp/my_socket
@end example

If the nbd-server uses named exports (supported since NBD 2.9.18, or with QEMU's
own embedded NBD server), you must specify an export name in the URI:
@example
qemu-system-i386 -cdrom nbd://localhost/debian-500-ppc-netinst
qemu-system-i386 -cdrom nbd://localhost/openSUSE-11.1-ppc-netinst
@end example

The URI syntax for NBD is supported since QEMU 1.3.  An alternative syntax is
also available.  Here are some example of the older syntax:
@example
qemu-system-i386 linux.img -hdb nbd:my_nbd_server.mydomain.org:1024
qemu-system-i386 linux2.img -hdb nbd:unix:/tmp/my_socket
qemu-system-i386 -cdrom nbd:localhost:10809:exportname=debian-500-ppc-netinst
@end example

@node disk_images_sheepdog
@subsection Sheepdog disk images

Sheepdog is a distributed storage system for QEMU.  It provides highly
available block level storage volumes that can be attached to
QEMU-based virtual machines.

You can create a Sheepdog disk image with the command:
@example
qemu-img create sheepdog:///@var{image} @var{size}
@end example
where @var{image} is the Sheepdog image name and @var{size} is its
size.

To import the existing @var{filename} to Sheepdog, you can use a
convert command.
@example
qemu-img convert @var{filename} sheepdog:///@var{image}
@end example

You can boot from the Sheepdog disk image with the command:
@example
qemu-system-i386 sheepdog:///@var{image}
@end example

You can also create a snapshot of the Sheepdog image like qcow2.
@example
qemu-img snapshot -c @var{tag} sheepdog:///@var{image}
@end example
where @var{tag} is a tag name of the newly created snapshot.

To boot from the Sheepdog snapshot, specify the tag name of the
snapshot.
@example
qemu-system-i386 sheepdog:///@var{image}#@var{tag}
@end example

You can create a cloned image from the existing snapshot.
@example
qemu-img create -b sheepdog:///@var{base}#@var{tag} sheepdog:///@var{image}
@end example
where @var{base} is a image name of the source snapshot and @var{tag}
is its tag name.

You can use an unix socket instead of an inet socket:

@example
qemu-system-i386 sheepdog+unix:///@var{image}?socket=@var{path}
@end example

If the Sheepdog daemon doesn't run on the local host, you need to
specify one of the Sheepdog servers to connect to.
@example
qemu-img create sheepdog://@var{hostname}:@var{port}/@var{image} @var{size}
qemu-system-i386 sheepdog://@var{hostname}:@var{port}/@var{image}
@end example

@node disk_images_iscsi
@subsection iSCSI LUNs

iSCSI is a popular protocol used to access SCSI devices across a computer
network.

There are two different ways iSCSI devices can be used by QEMU.

The first method is to mount the iSCSI LUN on the host, and make it appear as
any other ordinary SCSI device on the host and then to access this device as a
/dev/sd device from QEMU. How to do this differs between host OSes.

The second method involves using the iSCSI initiator that is built into
QEMU. This provides a mechanism that works the same way regardless of which
host OS you are running QEMU on. This section will describe this second method
of using iSCSI together with QEMU.

In QEMU, iSCSI devices are described using special iSCSI URLs

@example
URL syntax:
iscsi://[<username>[%<password>]@@]<host>[:<port>]/<target-iqn-name>/<lun>
@end example

Username and password are optional and only used if your target is set up
using CHAP authentication for access control.
Alternatively the username and password can also be set via environment
variables to have these not show up in the process list

@example
export LIBISCSI_CHAP_USERNAME=<username>
export LIBISCSI_CHAP_PASSWORD=<password>
iscsi://<host>/<target-iqn-name>/<lun>
@end example

Various session related parameters can be set via special options, either
in a configuration file provided via '-readconfig' or directly on the
command line.

If the initiator-name is not specified qemu will use a default name
of 'iqn.2008-11.org.linux-kvm[:<name>'] where <name> is the name of the
virtual machine.


@example
Setting a specific initiator name to use when logging in to the target
-iscsi initiator-name=iqn.qemu.test:my-initiator