file sectors into the image file.
@end table
-By default, writethrough caching is used for all block device. This means that
-the host page cache will be used to read and write data but write notification
-will be sent to the guest only when the data has been reported as written by
-the storage subsystem.
-
-Writeback caching will report data writes as completed as soon as the data is
-present in the host page cache. This is safe as long as you trust your host.
-If your host crashes or loses power, then the guest may experience data
-corruption.
+By default, the @option{cache=writeback} mode is used. It will report data
+writes as completed as soon as the data is present in the host page cache.
+This is safe as long as your guest OS makes sure to correctly flush disk caches
+where needed. If your guest OS does not handle volatile disk write caches
+correctly and your host crashes or loses power, then the guest may experience
+data corruption.
+
+For such guests, you should consider using @option{cache=writethrough}. This
+means that the host page cache will be used to read and write data, but write
+notification will be sent to the guest only after QEMU has made sure to flush
+each write to the disk. Be aware that this has a major impact on performance.
The host page cache can be avoided entirely with @option{cache=none}. This will
-attempt to do disk IO directly to the guests memory. QEMU may still perform
-an internal copy of the data.
+attempt to do disk IO directly to the guest's memory. QEMU may still perform
+an internal copy of the data. Note that this is considered a writeback mode and
+the guest OS must handle the disk write cache correctly in order to avoid data
+corruption on host crashes.
The host page cache can be avoided while only sending write notifications to
-the guest when the data has been reported as written by the storage subsystem
-using @option{cache=directsync}.
-
-Some block drivers perform badly with @option{cache=writethrough}, most notably,
-qcow2. If performance is more important than correctness,
-@option{cache=writeback} should be used with qcow2.
+the guest when the data has been flushed to the disk using
+@option{cache=directsync}.
In case you don't care about data integrity over host failures, use
-cache=unsafe. This option tells QEMU that it never needs to write any data
-to the disk but can instead keeps things in cache. If anything goes wrong,
+@option{cache=unsafe}. This option tells QEMU that it never needs to write any
+data to the disk but can instead keep things in cache. If anything goes wrong,
like your host losing power, the disk storage getting disconnected accidentally,
-etc. you're image will most probably be rendered unusable. When using
+etc. your image will most probably be rendered unusable. When using
the @option{-snapshot} option, unsafe caching is always used.
Copy-on-read avoids accessing the same backing file sectors repeatedly and is
connections (@var{host}:@var{d},@code{reverse}), the @var{d} argument
is a TCP port number, not a display number.
+@item websocket
+
+Opens an additional TCP listening port dedicated to VNC Websocket connections.
+By defintion the Websocket port is 5700+@var{display}. If @var{host} is
+specified connections will only be allowed from this host.
+As an alternative the Websocket port could be specified by using
+@code{websocket}=@var{port}.
+
@item password
Require that password based authentication is used for client connections.
" connect the host TAP network interface to VLAN 'n'\n"
#else
"-net tap[,vlan=n][,name=str][,fd=h][,ifname=name][,script=file][,downscript=dfile][,helper=helper][,sndbuf=nbytes][,vnet_hdr=on|off][,vhost=on|off][,vhostfd=h][,vhostforce=on|off]\n"
- " connect the host TAP network interface to VLAN 'n' \n"
+ " connect the host TAP network interface to VLAN 'n'\n"
" use network scripts 'file' (default=" DEFAULT_NETWORK_SCRIPT ")\n"
" to configure it and 'dfile' (default=" DEFAULT_NETWORK_DOWN_SCRIPT ")\n"
" to deconfigure it\n"
#endif
#if defined(__linux__) || defined(__sun__) || defined(__FreeBSD__) \
|| defined(__NetBSD__) || defined(__OpenBSD__) || defined(__DragonFly__)
+ "-chardev serial,id=id,path=path[,mux=on|off]\n"
"-chardev tty,id=id,path=path[,mux=on|off]\n"
#endif
#if defined(__linux__) || defined(__FreeBSD__) || defined(__DragonFly__)
+ "-chardev parallel,id=id,path=path[,mux=on|off]\n"
"-chardev parport,id=id,path=path[,mux=on|off]\n"
#endif
#if defined(CONFIG_SPICE)
"-chardev spicevmc,id=id,name=name[,debug=debug]\n"
+ "-chardev spiceport,id=id,name=name[,debug=debug]\n"
#endif
, QEMU_ARCH_ALL
)
@option{stdio},
@option{braille},
@option{tty},
+@option{parallel},
@option{parport},
@option{spicevmc}.
+@option{spiceport}.
The specific backend will determine the applicable options.
All devices must have an id, which can be any string up to 127 characters long.
Send traffic from the guest to a serial device on the host.
-@option{serial} is
-only available on Windows hosts.
+On Unix hosts serial will actually accept any tty device,
+not only serial lines.
@option{path} specifies the name of the serial device to open.
@item -chardev tty ,id=@var{id} ,path=@var{path}
-Connect to a local tty device.
-
@option{tty} is only available on Linux, Sun, FreeBSD, NetBSD, OpenBSD and
-DragonFlyBSD hosts.
+DragonFlyBSD hosts. It is an alias for -serial.
@option{path} specifies the path to the tty. @option{path} is required.
+@item -chardev parallel ,id=@var{id} ,path=@var{path}
@item -chardev parport ,id=@var{id} ,path=@var{path}
-@option{parport} is only available on Linux, FreeBSD and DragonFlyBSD hosts.
+@option{parallel} is only available on Linux, FreeBSD and DragonFlyBSD hosts.
Connect to a local parallel port.
Connect to a spice virtual machine channel, such as vdiport.
+@item -chardev spiceport ,id=@var{id} ,debug=@var{debug}, name=@var{name}
+
+@option{spiceport} is only available when spice support is built in.
+
+@option{debug} debug level for spicevmc
+
+@option{name} name of spice port to connect to
+
+Connect to a spice port, allowing a Spice client to handle the traffic
+identified by a name (preferably a fqdn).
+
@end table
ETEXI
projects / qemu.git / blobdiff