Revision e35bdc123a4ace9f4d3fccaaf88907014e2438cd authored by Kevin Wolf on 05 October 2018, 16:57:40 UTC, committed by Kevin Wolf on 05 November 2018, 14:09:55 UTC
If a management application builds the block graph node by node, the protocol layer doesn't inherit its read-only option from the format layer any more, so it must be set explicitly. Backing files should work on read-only storage, but at the same time, a block job like commit should be able to reopen them read-write if they are on read-write storage. However, without option inheritance, reopen only changes the read-only option for the root node (typically the format layer), but not the protocol layer, so reopening fails (the format layer wants to get write permissions, but the protocol layer is still read-only). A simple workaround for the problem in the management tool would be to open the protocol layer always read-write and to make only the format layer read-only for backing files. However, sometimes the file is actually stored on read-only storage and we don't know whether the image can be opened read-write (for example, for NBD it depends on the server we're trying to connect to). This adds an option that makes QEMU try to open the image read-write, but allows it to degrade to a read-only mode without returning an error. The documentation for this option is consciously phrased in a way that allows QEMU to switch to a better model eventually: Instead of trying when the image is first opened, making the read-only flag dynamic and changing it automatically whenever the first BLK_PERM_WRITE user is attached or the last one is detached would be much more useful behaviour. Unfortunately, this more useful behaviour is also a lot harder to implement, and libvirt needs a solution now before it can switch to -blockdev, so let's start with this easier approach for now. Instead of adding a new auto-read-only option, turning the existing read-only into an enum (with a bool alternate for compatibility) was considered, but it complicated the implementation to the point that it didn't seem to be worth it. Signed-off-by: Kevin Wolf <kwolf@redhat.com> Reviewed-by: Eric Blake <eblake@redhat.com>
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qemu-nbd.texi
@example
@c man begin SYNOPSIS
@command{qemu-nbd} [OPTION]... @var{filename}
@command{qemu-nbd} @option{-d} @var{dev}
@c man end
@end example
@c man begin DESCRIPTION
Export a QEMU disk image using the NBD protocol.
@c man end
@c man begin OPTIONS
@var{filename} is a disk image filename, or a set of block
driver options if @var{--image-opts} is specified.
@var{dev} is an NBD device.
@table @option
@item --object type,id=@var{id},...props...
Define a new instance of the @var{type} object class identified by @var{id}.
See the @code{qemu(1)} manual page for full details of the properties
supported. The common object types that it makes sense to define are the
@code{secret} object, which is used to supply passwords and/or encryption
keys, and the @code{tls-creds} object, which is used to supply TLS
credentials for the qemu-nbd server.
@item -p, --port=@var{port}
The TCP port to listen on (default @samp{10809})
@item -o, --offset=@var{offset}
The offset into the image
@item -b, --bind=@var{iface}
The interface to bind to (default @samp{0.0.0.0})
@item -k, --socket=@var{path}
Use a unix socket with path @var{path}
@item --image-opts
Treat @var{filename} as a set of image options, instead of a plain
filename. If this flag is specified, the @var{-f} flag should
not be used, instead the '@code{format=}' option should be set.
@item -f, --format=@var{fmt}
Force the use of the block driver for format @var{fmt} instead of
auto-detecting
@item -r, --read-only
Export the disk as read-only
@item -P, --partition=@var{num}
Only expose partition @var{num}
@item -s, --snapshot
Use @var{filename} as an external snapshot, create a temporary
file with backing_file=@var{filename}, redirect the write to
the temporary one
@item -l, --load-snapshot=@var{snapshot_param}
Load an internal snapshot inside @var{filename} and export it
as an read-only device, @var{snapshot_param} format is
'snapshot.id=[ID],snapshot.name=[NAME]' or '[ID_OR_NAME]'
@item -n, --nocache
@itemx --cache=@var{cache}
The cache mode to be used with the file. See the documentation of
the emulator's @code{-drive cache=...} option for allowed values.
@item --aio=@var{aio}
Set the asynchronous I/O mode between @samp{threads} (the default)
and @samp{native} (Linux only).
@item --discard=@var{discard}
Control whether @dfn{discard} (also known as @dfn{trim} or @dfn{unmap})
requests are ignored or passed to the filesystem. @var{discard} is one of
@samp{ignore} (or @samp{off}), @samp{unmap} (or @samp{on}). The default is
@samp{ignore}.
@item --detect-zeroes=@var{detect-zeroes}
Control the automatic conversion of plain zero writes by the OS to
driver-specific optimized zero write commands. @var{detect-zeroes} is one of
@samp{off}, @samp{on} or @samp{unmap}. @samp{unmap}
converts a zero write to an unmap operation and can only be used if
@var{discard} is set to @samp{unmap}. The default is @samp{off}.
@item -c, --connect=@var{dev}
Connect @var{filename} to NBD device @var{dev}
@item -d, --disconnect
Disconnect the device @var{dev}
@item -e, --shared=@var{num}
Allow up to @var{num} clients to share the device (default @samp{1})
@item -t, --persistent
Don't exit on the last connection
@item -x, --export-name=@var{name}
Set the NBD volume export name. This switches the server to use
the new style NBD protocol negotiation
@item -D, --description=@var{description}
Set the NBD volume export description, as a human-readable
string. Requires the use of @option{-x}
@item --tls-creds=ID
Enable mandatory TLS encryption for the server by setting the ID
of the TLS credentials object previously created with the --object
option.
@item --fork
Fork off the server process and exit the parent once the server is running.
@item -v, --verbose
Display extra debugging information
@item -h, --help
Display this help and exit
@item -V, --version
Display version information and exit
@item -T, --trace [[enable=]@var{pattern}][,events=@var{file}][,file=@var{file}]
@findex --trace
@include qemu-option-trace.texi
@end table
@c man end
@ignore
@setfilename qemu-nbd
@settitle QEMU Disk Network Block Device Server
@c man begin AUTHOR
Copyright (C) 2006 Anthony Liguori <anthony@codemonkey.ws>.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
@c man end
@c man begin SEEALSO
qemu(1), qemu-img(1)
@c man end
@end ignore
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