Xorg

The X.Org project provides an open source implementation of the X Window System. The development work is being done in conjunction with the freedesktop.org community. The X.Org Foundation is the educational non-profit corporation whose Board serves this effort, and whose Members lead this work.

Xorg is the public, open-source implementation of the X window system version 11. Since Xorg is the most popular choice among Linux users, its ubiquity has led to making it an ever-present requisite for GUI applications, resulting in massive adoption from most distributions. See the Xorg Wikipedia article or visit the Xorg website for more details.

Installation

You will need to install the essential package xorg-server, available in the Official Repositories.

Additionally, some packages from the xorg-apps group are useful for certain configuration tasks, they are pointed out in the relevant section/page.

Tip: The default X environment is rather bare, and you will typically seek to install a window manager or a desktop environment to supplement X.

Driver installation

The Linux kernel includes open-source video drivers and support for hardware accelerated framebuffers. However, userland support is required for OpenGL and 2D acceleration in X11.

First, identify your card:

$ lspci | grep VGA

Then install an appropriate driver. You can search the package database for a complete list of open-source video drivers:

$ pacman -Ss xf86-video

The default graphics driver is vesa (package xf86-video-vesa), which handles a large number of chipsets but does not include any 2D or 3D acceleration. If a better driver cannot be found or fails to load, Xorg will fall back to vesa.

In order for video acceleration to work, and often to expose all the modes that the GPU can set, a proper video driver is required:

Brand	Type	Driver	Multilib Package (for 32-bit applications on Arch x86_64)	Documentation
AMD/ATI	Open source	xf86-video-ati	lib32-ati-dri	ATI
AMD/ATI	Proprietary	catalyst-dkms^AUR	lib32-catalyst-utils^AUR	AMD Catalyst
Intel	Open source	xf86-video-intel	lib32-intel-dri	Intel Graphics
Nvidia	Open source	xf86-video-nouveau	lib32-nouveau-dri	Nouveau
	Open source	xf86-video-nv	–	(legacy driver)
	Proprietary	nvidia	lib32-nvidia-libgl	NVIDIA
		nvidia-304xx	lib32-nvidia-304xx-utils
		nvidia-173xx^AUR	lib32-nvidia-173xx-utils^AUR
		nvidia-96xx^AUR	lib32-nvidia-96xx-utils^AUR
VIA	Open source	xf86-video-openchrome	–	VIA

Xorg should run smoothly without closed source drivers, which are typically needed only for advanced features such as fast 3D-accelerated rendering for games, dual-screen setups, and TV-out.

Running

See also: Start X at Login

Tip: The easiest way to start X is by using a display manager such as GDM, KDM or SLiM.

If you want to start X without a display manager, install the package xorg-xinit. Optionally, the packages xorg-twm, xorg-xclock and xterm allows for a default environment, as described below.

The startx and xinit commands will start the X server and clients (the startx script is merely a front end to the xinit command). To determine the client to run, startx/xinit will first look to parse a ~/.xinitrc file in the user's home directory. In the absence of ~/.xinitrc, it defaults to the global file /etc/X11/xinit/xinitrc, which defaults to starting a basic environment with the Twm window manager, Xclock and Xterm. For more information, see xinitrc.

Note:

X must always be run on the same tty where the login occurred, to preserve the logind session. This is handled by the default /etc/X11/xinit/xserverrc. See General Troubleshooting#Session permissions for details.
If a problem occurs, then view the log at /var/log/Xorg.0.log. Be on the lookout for any lines beginning with (EE), which represent errors, and also (WW), which are warnings that could indicate other issues.
If there is an empty .xinitrc file in your $HOME, either delete or edit it in order for X to start properly. If you do not do this X will show a blank screen with what appears to be no errors in your Xorg.0.log. Simply deleting it will get it running with a default X environment.

Warning: If you choose to use xinit instead of startx, you are responsible for passing -nolisten tcp and ensuring the session does not break by starting X on a different tty.

Configuration

Note: Arch supplies default configuration files in /etc/X11/xorg.conf.d, and no extra configuration is necessary for most setups.

Xorg uses a configuration file called xorg.conf and files ending in the suffix .conf for its initial setup: the complete list of the folders where these files are searched can be found at [1] or by running man xorg.conf, together with a detailed explanation of all the available options.

Using .conf files

The /etc/X11/xorg.conf.d/ directory stores user-specific configuration. You are free to add configuration files there, but they must have a .conf suffix: the files are read in ASCII order, and by convention their names start with XX- (two digits and a hyphen, so that for example 10 is read before 20). These files are parsed by the X server upon startup and are treated like part of the traditional xorg.conf configuration file. The X server essentially treats the collection of configuration files as one big file with entries from xorg.conf at the end.

Using xorg.conf

Xorg can also be configured via /etc/X11/xorg.conf or /etc/xorg.conf. You can also generate a skeleton for xorg.conf with:

# Xorg :0 -configure

This should create a xorg.conf.new file in /root/ that you can copy over to /etc/X11/xorg.conf.

Alternatively, your proprietary video card drivers may come with a tool to automatically configure Xorg: see the article of your video driver, NVIDIA or AMD Catalyst, for more details.

Note: Configuration file keywords are case insensitive, and "_" characters are ignored. Most strings (including Option names) are also case insensitive, and insensitive to white space and "_" characters.

Sample configurations

xorg.conf

Sample 1
- Note: Sample configuration file using /etc/X11/xorg.conf.d/10-evdev.conf for the keyboard layouts. Note the commented out InputDevice sections.

/etc/X11/xorg.conf.d/10-evdev.conf

Sample 1
- Note: This is the 10-evdev.conf file that goes with the xorg.conf sample 1.

/etc/X11/xorg.conf.d/10-monitor.conf

VMware
KVM
NVIDIA
- Note: nvidia-ck binary drivers (v325); Dual GPU, Dual Monitor, Dual Screen; No Twinview, No Xinerama; Rotated and vertically placed screen1 above screen0.

Input devices

Udev will detect your hardware and evdev will act as the hotplugging input driver for almost all devices. Udev is provided by systemd and xf86-input-evdev is required by xorg-server, so there is no need to explicitly install those packages. If evdev does not support your device, install the needed driver from the xorg-drivers group.

You should have 10-evdev.conf in the /etc/X11/xorg.conf.d directory, which manages the keyboard, the mouse, the touchpad and the touchscreen.

Mouse acceleration

See the main page: Mouse acceleration

Extra mouse buttons

See the main page: All Mouse Buttons Working

Touchpad Synaptics

See the main page: Touchpad Synaptics

Keyboard settings

See the main page: Keyboard Configuration in Xorg

InputClasses

This article or section needs language, wiki syntax or style improvements.

Reason: Duplication of external resources (perhaps an old revision): [2]. (Discuss in Talk:Xorg#)

Taken from: https://fedoraproject.org/wiki/Input_device_configuration

InputClasses are a new type of configuration section that does not apply to a single device but rather to a class of devices, including hotplugged devices. An InputClass section's scope is limited by the matches specified – to apply to an input device, all matches must apply to a device. An example InputClass section is provided below:

Section "InputClass"
    Identifier             "touchpad catchall"
    MatchIsTouchpad        "on"
    Driver                 "synaptics"
EndSection

The next snippet might also be helpful:

Section "InputClass"
    Identifier             "evdev touchpad catchall"
    MatchIsTouchpad        "on"
    MatchDevicePath        "/dev/input/event*"
    Driver                 "evdev"
EndSection

If this snippet is present in the xorg.conf or xorg.conf.d, any touchpad present in the system is assigned the synaptics driver. Note that due to precedence order (alphanumeric sorting of xorg.conf.d snippets) the Driver setting overwrites previously set driver options – the more generic the class, the earlier it should be listed. The default snippet shipped with the xorg-x11-drv-Xorg package is 00-evdev.conf and applies the evdev driver to all input devices.

The match options specify which devices a section may apply to. To match a device, all match lines must apply. The following match lines are supported (with examples):

MatchIsPointer, MatchIsKeyboard, MatchIsTouchpad, MatchIsTouchscreen, MatchIsJoystick – boolean options to apply to a group of devices.
MatchProduct "foo|bar": match any device with a product name containing either "foo" or "bar"
MatchVendor "foo|bar|baz": match any device with a vendor string containing either "foo", "bar", or "baz"
MatchDevicePath "/dev/input/event*": match any device with a device path matching the given patch (see fnmatch(3) for the allowed pattern)
MatchTag "foo|bar": match any device with a tag of either "foo" or "bar". Tags may be assigned by the config backend – udev in our case – to label devices that need quirks or special configuration.

An example section for user-specific configuration is:

Section "InputClass"
    Identifier             "lasermouse slowdown"
    MatchIsPointer         "on"
    MatchProduct           "Lasermouse"
    MatchVendor            "LaserMouse Inc."
    Option                 "ConstantDeceleration" 20
EndSection

This section would match a pointer device containing "Lasermouse" from "Lasermouse Inc." and apply a constant deceleration of 20 on this device – slowing it down by factor 20.

Some devices may get picked up by the X server when they really should not be. These devices can be configured to be ignored:

Section "InputClass"
    Identifier            "no need for accelerometers in X"
    MatchProduct          "accelerometer"
    Option                "Ignore" "on"
EndSection

Example configurations

The following subsections describe example configurations for commonly used configuration options. Note that if you use a desktop environment such as GNOME or KDE, options you set in the xorg.conf may get overwritten with user-specific options upon login.

Example: wheel emulation (for a trackpoint)

If you own a computer with a Trackpoint (a Thinkpad, for example) you can add the following to the xorg.conf to use the middle button to emulate a mouse wheel:

Section "InputClass"
    Identifier            "Wheel Emulation"
    MatchIsPointer        "on"
    MatchProduct          "TrackPoint"
    Option                "EmulateWheelButton" "2"
    Option                "EmulateWheel" "on"
EndSection

For full support of TrackPoints (including horizontal scrolling) you can use the following:

Section "InputClass"
    Identifier            "Trackpoint Wheel Emulation"
    MatchProduct          "TPPS/2 IBM TrackPoint|DualPoint Stick|Synaptics Inc. Composite TouchPad / TrackPoint|ThinkPad USB Keyboard with TrackPoint|USB Trackpoint pointing device"
    MatchDevicePath       "/dev/input/event*"
    Option                "EmulateWheel"                "true"
    Option                "EmulateWheelButton"          "2"
    Option                "Emulate3Buttons"             "false"
    Option                "XAxisMapping"                "6 7"
    Option                "YAxisMapping"                "4 5"
EndSection

Example: tap-to-click

Tap-to-click can be enabled in the mouse configuration dialog (in the touchpad tab) but if you need tapping enabled at gdm already, the following snippet does it for you:

Section "InputClass"
    Identifier            "tap-by-default"
    MatchIsTouchpad       "on"
    Option                "TapButton1" "1"
EndSection

Example: keyboard layout and model on Acer 5920G laptop

Keyboard model and layout may be set in the file /etc/X11/xorg.conf.d/keyboard.conf or any other .conf file in the same directory.

MatchIsKeyboard "yes": set the input device to a keyboard
Option "XkbModel" "acer_laptop": set the keyboard model to an Acer laptop keyboard. You may replace acer_laptop with your actual keyboard layout.
Option "XkbLayout" "be": set the keyboard layout to belgian. You may replace be with whatever layout you have.
Option "XkbVariant" "sundeadkeys": set the layout variant to Sun dead keys. You may omit the XkbVariant option if you stick with the default variant.

Note that a list of keyboard layouts and models can be found in /usr/share/X11/xkb/rules/base.lst

Section "InputClass"
    Identifier             "Keyboard Defaults"
    MatchIsKeyboard        "yes"
    Option                 "XkbModel" "acer_laptop"
    Option                 "XkbLayout" "be"
    Option                 "XkbVariant" "sundeadkeys"
EndSection

Example: third button emulation (for all attached mice)

Third button emulation allows you to use the 1 and 2 buttons (left and right click) together to produce a button 3 event (middle-click), which is really useful for copy and paste in X.

We will modify the mouse input catchall in /etc/X11/xorg.conf.d/10-evdev.conf

Section "InputClass"
        Identifier "evdev pointer catchall"
        MatchIsPointer "on"
        MatchDevicePath "/dev/input/event*"
        Driver "evdev"
        Option "Emulate3Buttons"     "True"
        Option "Emulate3Timeout"     "25"
EndSection

Here you see the Option "Emulate3Buttons" "True" and Option "Emulate3Timeout" "25" options that specify that 3 buttons should be emulated and the delay to recognize both buttons down as a middle click is 25ms.

Monitor settings

Getting started

Note: Newer versions of Xorg are auto-configuring, you should not need to use this.

First, create a new config file, such as /etc/X11/xorg.conf.d/10-monitor.conf.

/etc/X11/xorg.conf.d/10-monitor.conf

Section "Monitor"
    Identifier             "Monitor0"
EndSection

Section "Device"
    Identifier             "Device0"
    Driver                 "vesa" #Choose the driver used for this monitor
EndSection

Section "Screen"
    Identifier             "Screen0"  #Collapse Monitor and Device section to Screen section
    Device                 "Device0"
    Monitor                "Monitor0"
    DefaultDepth            16 #Choose the depth (16||24)
    SubSection             "Display"
        Depth               16
        Modes              "1024x768_75.00" #Choose the resolution
    EndSubSection
EndSection

Multiple monitors

See main article Multihead for general information.

More than one graphics card

You must define the correct driver to use and put the bus ID of your graphic cards.

Section "Device"
    Identifier             "Screen0"
    Driver                 "nouveau"
    BusID                  "PCI:0:12:0"
EndSection

Section "Device"
    Identifier             "Screen1"
    Driver                 "radeon"
    BusID                  "PCI:1:0:0"
EndSection

To get your bus ID:

$ lspci | grep VGA

01:00.0 VGA compatible controller: nVidia Corporation G96 [GeForce 9600M GT] (rev a1)

The bus ID here is 1:0:0.

Display size and DPI

The factual accuracy of this article or section is disputed.

Reason: Xorg always sets dpi to 96. See this, this and finally this. (Discuss in Talk:Xorg#)

The DPI of the X server is determined in the following manner:

The -dpi command line option has highest priority.
If this is not used, the DisplaySize setting in the X config file is used to derive the DPI, given the screen resolution.
If no DisplaySize is given, the monitor size values from DDC are used to derive the DPI, given the screen resolution.
If DDC does not specify a size, 75 DPI is used by default.

In order to get correct dots per inch (DPI) set, the display size must be recognized or set. Having the correct DPI is especially necessary where fine detail is required (like font rendering). Previously, manufacturers tried to create a standard for 96 DPI (a 10.3" diagonal monitor would be 800x600, a 13.2" monitor 1024x768). These days, screen DPIs vary and may not be equal horizontally and vertically. For example, a 19" widescreen LCD at 1440x900 may have a DPI of 89x87. To be able to set the DPI, the Xorg server attempts to auto-detect your monitor's physical screen size through the graphic card with DDC. ~~When the Xorg server knows the physical screen size, it will be able to set the correct DPI depending on resolution size.~~

To see if your display size and DPI are detected/calculated correctly:

$ xdpyinfo | grep -B2 resolution

Check that the dimensions match your display size. If the Xorg server is not able to correctly calculate the screen size, it will default to 75x75 DPI and you will have to calculate it yourself.

If you have specifications on the physical size of the screen, they can be entered in the Xorg configuration file so that the proper DPI is calculated:

Section "Monitor"
    Identifier             "Monitor0"
    DisplaySize             286 179    # In millimeters
EndSection

If you only want to enter the specification of your monitor without creating a full xorg.conf create a new config file. For example (/etc/X11/xorg.conf.d/90-monitor.conf):

Section "Monitor"
    Identifier             "<default monitor>"
    DisplaySize            286 179    # In millimeters
EndSection

If you do not have specifications for physical screen width and height (most specifications these days only list by diagonal size), you can use the monitor's native resolution (or aspect ratio) and diagonal length to calculate the horizontal and vertical physical dimensions. Using the Pythagorean theorem on a 13.3" diagonal length screen with a 1280x800 native resolution (or 16:10 aspect ratio):

$ echo 'scale=5;sqrt(1280^2+800^2)' | bc  # 1509.43698

This will give the pixel diagonal length and with this value you can discover the physical horizontal and vertical lengths (and convert them to millimeters):

$ echo 'scale=5;(13.3/1509)*1280*25.4' | bc  # 286.43072
$ echo 'scale=5;(13.3/1509)*800*25.4'  | bc  # 179.01920

Note: This calculation works for monitors with square pixels; however, there is the seldom monitor that may compress aspect ratio (e.g 16:10 aspect resolution to a 16:9 monitor). If this is the case, you should measure your screen size manually.

Setting DPI manually

DPI can be set manually if you only plan to use one resolution (DPI calculator):

Section "Monitor"
    Identifier             "Monitor0"
    Option                 "DPI" "96 x 96"
EndSection

If you use an NVIDIA card, you can manually set the DPI adding the options bellow on /etc/X11/xorg.conf.d/20-nvidia.conf (inside Device section):

Option              "UseEdidDpi" "False"
Option              "DPI" "96 x 96"

For RandR compliant drivers, you can set it by:

$ xrandr --dpi 96

See Execute commands after X start to make it permanent.

Note: While you can set any dpi you like and applications using Qt and GTK will scale accordingly, it's recommended to set it to 96, 120 (25% higher), 144 (50% higher), 168 (75% higher), 192 (100% higher) etc., to reduce scaling artifacts to GUI that use bitmaps. Reducing it below 96 dpi may not reduce size of graphical elements of GUI as typically the lowest dpi the icons are made for is 96.

DPMS

DPMS (Display Power Management Signaling) is a technology that allows power saving behaviour of monitors when the computer is not in use. This will allow you to have your monitors automatically go into standby after a predefined period of time. See: DPMS

Composite

The Composite extension for X causes an entire sub-tree of the window hierarchy to be rendered to an off-screen buffer. Applications can then take the contents of that buffer and do whatever they like. The off-screen buffer can be automatically merged into the parent window or merged by external programs, called compositing managers. See the following page for more info.

Compiz -- The original composite/window manager from Novell
Xcompmgr -- A simple composite manager capable of drop shadows and primitive transparency
Compton -- A fork of xcompmgr with feature improves and bug fix
Cairo Composite Manager -- A versatile and extensible composite manager which uses cairo for rendering.
Wikipedia:Compositing window manager

Tips and tricks

X startup tweaking (startx)

For X's option reference see:

$ man Xserver

The following options have to be appended to the variable "defaultserverargs" in the /usr/bin/startx file:

Enable deferred glyph loading for 16 bit fonts:

-deferglyphs 16

Note: If you start X with kdm, the startx script does not seem to be executed. X options must be appended to the variable ServerArgsLocal in the /usr/share/config/kdm/kdmrc file.

Nested X session

To run a nested session of another desktop environment:

$ /usr/bin/Xnest :1 -geometry 1024x768+0+0 -ac -name Windowmaker & wmaker -display :1

This will launch a Window Maker session in a 1024 by 768 window within your current X session.

This needs the package xorg-server-xnest to be installed.

Starting GUI programs remotely

See main article: SSH#X11 forwarding.

On-demand disabling and enabling of input sources

With the help of xinput you can temporarily disable or enable input sources. This might be useful, for example, on systems that have more than one mouse, such as the ThinkPads and you would rather use just one to avoid unwanted mouse clicks.

Install the xorg-xinput package from the official repositories.

Find the ID of the device you want to disable:

$ xinput

For example in a Lenovo ThinkPad T500, the output looks like this:

$ xinput

⎡ Virtual core pointer                          id=2    [master pointer  (3)]
⎜   ↳ Virtual core XTEST pointer                id=4    [slave  pointer  (2)]
⎜   ↳ TPPS/2 IBM TrackPoint                     id=11   [slave  pointer  (2)]
⎜   ↳ SynPS/2 Synaptics TouchPad                id=10   [slave  pointer  (2)]
⎣ Virtual core keyboard                         id=3    [master keyboard (2)]
    ↳ Virtual core XTEST keyboard               id=5    [slave  keyboard (3)]
    ↳ Power Button                              id=6    [slave  keyboard (3)]
    ↳ Video Bus                                 id=7    [slave  keyboard (3)]
    ↳ Sleep Button                              id=8    [slave  keyboard (3)]
    ↳ AT Translated Set 2 keyboard              id=9    [slave  keyboard (3)]
    ↳ ThinkPad Extra Buttons                    id=12   [slave  keyboard (3)]

Disable the device with xinput --disable device_id, where device_id is the device ID you want to disable. In this example we'll disable the Synaptics Touchpad, with the ID 10:

$ xinput --disable 10

To re-enable the device, just issue the opposite command:

$ xinput --enable 10

Troubleshooting

Common problems

If Xorg will not start, the screen is completely black, the keyboard and mouse are not working, etc., first take these simple steps:

Check the log file: cat /var/log/Xorg.0.log
Check specific pages in Category:Input devices if you have issues with keyboard, mouse, touchpad etc.
Finally, search for common problems in ATI, Intel and NVIDIA articles.

CTRL right key does not work with oss keymap

Edit as root /usr/share/X11/xkb/symbols/fr, and change the line:

include "level5(rctrl_switch)"

to

// include "level5(rctrl_switch)"

Then restart X or reboot.

X clients started with "su" fail

If you are getting "Client is not authorized to connect to server", try adding the line:

session        optional        pam_xauth.so

to /etc/pam.d/su. pam_xauth will then properly set environment variables and handle xauth keys.

Program requests "font '(null)'"

Error message: "unable to load font `(null)'."

Some programs only work with bitmap fonts. Two major packages with bitmap fonts are available, xorg-fonts-75dpi and xorg-fonts-100dpi. You do not need both; one should be enough. To find out which one would be better in your case, try this:

$ xdpyinfo | grep resolution

and use what is closer to you (75 or 100 instead of XX)

# pacman -S xorg-fonts-XXdpi

Frame-buffer mode problems

If X fails to start with the following log messages,

(WW) Falling back to old probe method for fbdev
(II) Loading sub module "fbdevhw"
(II) LoadModule: "fbdevhw"
(II) Loading /usr/lib/xorg/modules/linux//libfbdevhw.so
(II) Module fbdevhw: vendor="X.Org Foundation"
       compiled for 1.6.1, module version=0.0.2
       ABI class: X.Org Video Driver, version 5.0
(II) FBDEV(1): using default device

Fatal server error:
Cannot run in framebuffer mode. Please specify busIDs for all framebuffer devices

uninstall fbdev:

# pacman -R xf86-video-fbdev

DRI with Matrox cards stops working

If you use a Matrox card and DRI stops working after upgrading to Xorg, try adding the line:

Option "OldDmaInit" "On"

to the Device section that references the video card in xorg.conf.

Recovery: disabling Xorg before GUI login

This article or section is out of date.

Reason: systemd uses targets instead of runlevels, see systemd#Change default target to boot into (Discuss in Talk:Xorg#)

If Xorg is set to boot up automatically and for some reason you need to prevent it from starting up before the login/display manager appears (if the system is wrongly configured and Xorg does not recognize your mouse or keyboard input, for instance), you can accomplish this task with two methods.

From the GRUB menu, you can specify the runlevel in the kernel line by adding a number to the end of the kernel line specifying the run level you want. The following example sets the run level to 3:

 kernel /boot/vmlinuz-linux root=/dev/disk/by-uuid/..ro 3

If you have not only a faulty system that makes Xorg unusable, but you have also set the GRUB menu wait time to zero, or cannot otherwise use GRUB to prevent Xorg from booting, you can use the Arch Linux live CD. Boot up the live CD and log in as root. You need a mount point, such as /mnt, and you need to know the name of the partition you want to mount.

You can use the command,

# fdisk -l

to see your partitions. Usually, the one you want will be resembling /dev/sda1. Then, to mount this to /mnt, use

# mount /dev/sda1 /mnt

Then your filesystem will show up under /mnt. From here you can delete the gdm daemon to prevent Xorg from booting up normally or make any other necessary changes to the configuration.

X failed to start: Keyboard initialization failed

If your hard disk is full, startx will fail. /var/log/Xorg.0.log will end with:

(EE) Error compiling keymap (server-0)
(EE) XKB: Couldn't compile keymap
(EE) XKB: Failed to load keymap. Loading default keymap instead.
(EE) Error compiling keymap (server-0)
(EE) XKB: Couldn't compile keymap
XKB: Failed to compile keymap
Keyboard initialization failed. This could be a missing or incorrect setup of xkeyboard-config.
Fatal server error:
Failed to activate core devices.
Please consult the The X.Org Foundation support  at http://wiki.x.org
for help.
Please also check the log file at "/var/log/Xorg.0.log" for additional information.
(II) AIGLX: Suspending AIGLX clients for VT switch

Make some free space on your root partition and X will start.

Black screen, No protocol specified.., Resource temporarily unavailable for all or some users

X creates configuration and temporary files in current user's home directory. Make sure there is free disk space available on the partition your home directory resides in. Unfortunately, X server does not provide any more obvious information about lack of disk space in this case.