qemu-e2k/qemu-nbd.texi

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@example
@c man begin SYNOPSIS
@command{qemu-nbd} [OPTION]... @var{filename}
qemu-nbd: Add --list option We want to be able to detect whether a given qemu NBD server is exposing the right export(s) and dirty bitmaps, at least for regression testing. We could use 'nbd-client -l' from the upstream NBD project to list exports, but it's annoying to rely on out-of-tree binaries; furthermore, nbd-client doesn't necessarily know about all of the qemu NBD extensions. Thus, it is time to add a new mode to qemu-nbd that merely sniffs all possible information from the server during handshake phase, then disconnects and dumps the information. This patch actually implements --list/-L, while reusing other options such as --tls-creds for now designating how to connect as the client (rather than their non-list usage of how to operate as the server). I debated about adding this functionality to something akin to 'qemu-img info' - but that tool does not readily lend itself to connecting to an arbitrary NBD server without also tying to a specific export (I may, however, still add ImageInfoSpecificNBD for reporting the bitmaps available when connecting to a single export). And, while it may feel a bit odd that normally qemu-nbd is a server but 'qemu-nbd -L' is a client, we are not really making the qemu-nbd binary that much larger, because 'qemu-nbd -c' has to operate as both server and client simultaneously across two threads when feeding the kernel module for /dev/nbdN access. Sample output: $ qemu-nbd -L exports available: 1 export: '' size: 65536 flags: 0x4ed ( flush fua trim zeroes df cache ) min block: 512 opt block: 4096 max block: 33554432 available meta contexts: 1 base:allocation Note that the output only lists sizes if the server sent NBD_FLAG_HAS_FLAGS, because a newstyle server does not give the size otherwise. It has the side effect that for really old servers that did not send any flags, the size is not output even though it was available. However, I'm not too concerned about that - oldstyle servers are (rightfully) getting less common to encounter (qemu 3.0 was the last version where we even serve it), and most existing servers that still even offer oldstyle negotiation (such as nbdkit) still send flags (since that was added to the NBD protocol in 2007 to permit read-only connections). Not done here, but maybe worth future experiments: capture the meat of NBDExportInfo into a QAPI struct, and use the generated QAPI pretty-printers instead of hand-rolling our output loop. It would also permit us to add a JSON output mode for machine parsing. Signed-off-by: Eric Blake <eblake@redhat.com> Reviewed-by: Richard W.M. Jones <rjones@redhat.com> Message-Id: <20190117193658.16413-20-eblake@redhat.com> Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
2019-01-17 20:36:56 +01:00
@command{qemu-nbd} @option{-L} [OPTION]...
@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.
Other uses:
@itemize
@item
Bind a /dev/nbdX block device to a QEMU server (on Linux).
qemu-nbd: Add --list option We want to be able to detect whether a given qemu NBD server is exposing the right export(s) and dirty bitmaps, at least for regression testing. We could use 'nbd-client -l' from the upstream NBD project to list exports, but it's annoying to rely on out-of-tree binaries; furthermore, nbd-client doesn't necessarily know about all of the qemu NBD extensions. Thus, it is time to add a new mode to qemu-nbd that merely sniffs all possible information from the server during handshake phase, then disconnects and dumps the information. This patch actually implements --list/-L, while reusing other options such as --tls-creds for now designating how to connect as the client (rather than their non-list usage of how to operate as the server). I debated about adding this functionality to something akin to 'qemu-img info' - but that tool does not readily lend itself to connecting to an arbitrary NBD server without also tying to a specific export (I may, however, still add ImageInfoSpecificNBD for reporting the bitmaps available when connecting to a single export). And, while it may feel a bit odd that normally qemu-nbd is a server but 'qemu-nbd -L' is a client, we are not really making the qemu-nbd binary that much larger, because 'qemu-nbd -c' has to operate as both server and client simultaneously across two threads when feeding the kernel module for /dev/nbdN access. Sample output: $ qemu-nbd -L exports available: 1 export: '' size: 65536 flags: 0x4ed ( flush fua trim zeroes df cache ) min block: 512 opt block: 4096 max block: 33554432 available meta contexts: 1 base:allocation Note that the output only lists sizes if the server sent NBD_FLAG_HAS_FLAGS, because a newstyle server does not give the size otherwise. It has the side effect that for really old servers that did not send any flags, the size is not output even though it was available. However, I'm not too concerned about that - oldstyle servers are (rightfully) getting less common to encounter (qemu 3.0 was the last version where we even serve it), and most existing servers that still even offer oldstyle negotiation (such as nbdkit) still send flags (since that was added to the NBD protocol in 2007 to permit read-only connections). Not done here, but maybe worth future experiments: capture the meat of NBDExportInfo into a QAPI struct, and use the generated QAPI pretty-printers instead of hand-rolling our output loop. It would also permit us to add a JSON output mode for machine parsing. Signed-off-by: Eric Blake <eblake@redhat.com> Reviewed-by: Richard W.M. Jones <rjones@redhat.com> Message-Id: <20190117193658.16413-20-eblake@redhat.com> Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
2019-01-17 20:36:56 +01:00
@item
As a client to query exports of a remote NBD server.
@end itemize
@c man end
@c man begin OPTIONS
@var{filename} is a disk image filename, or a set of block
driver options if @option{--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
qemu-nbd: Add --list option We want to be able to detect whether a given qemu NBD server is exposing the right export(s) and dirty bitmaps, at least for regression testing. We could use 'nbd-client -l' from the upstream NBD project to list exports, but it's annoying to rely on out-of-tree binaries; furthermore, nbd-client doesn't necessarily know about all of the qemu NBD extensions. Thus, it is time to add a new mode to qemu-nbd that merely sniffs all possible information from the server during handshake phase, then disconnects and dumps the information. This patch actually implements --list/-L, while reusing other options such as --tls-creds for now designating how to connect as the client (rather than their non-list usage of how to operate as the server). I debated about adding this functionality to something akin to 'qemu-img info' - but that tool does not readily lend itself to connecting to an arbitrary NBD server without also tying to a specific export (I may, however, still add ImageInfoSpecificNBD for reporting the bitmaps available when connecting to a single export). And, while it may feel a bit odd that normally qemu-nbd is a server but 'qemu-nbd -L' is a client, we are not really making the qemu-nbd binary that much larger, because 'qemu-nbd -c' has to operate as both server and client simultaneously across two threads when feeding the kernel module for /dev/nbdN access. Sample output: $ qemu-nbd -L exports available: 1 export: '' size: 65536 flags: 0x4ed ( flush fua trim zeroes df cache ) min block: 512 opt block: 4096 max block: 33554432 available meta contexts: 1 base:allocation Note that the output only lists sizes if the server sent NBD_FLAG_HAS_FLAGS, because a newstyle server does not give the size otherwise. It has the side effect that for really old servers that did not send any flags, the size is not output even though it was available. However, I'm not too concerned about that - oldstyle servers are (rightfully) getting less common to encounter (qemu 3.0 was the last version where we even serve it), and most existing servers that still even offer oldstyle negotiation (such as nbdkit) still send flags (since that was added to the NBD protocol in 2007 to permit read-only connections). Not done here, but maybe worth future experiments: capture the meat of NBDExportInfo into a QAPI struct, and use the generated QAPI pretty-printers instead of hand-rolling our output loop. It would also permit us to add a JSON output mode for machine parsing. Signed-off-by: Eric Blake <eblake@redhat.com> Reviewed-by: Richard W.M. Jones <rjones@redhat.com> Message-Id: <20190117193658.16413-20-eblake@redhat.com> Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
2019-01-17 20:36:56 +01:00
credentials for the qemu-nbd server or client.
@item -p, --port=@var{port}
qemu-nbd: Add --list option We want to be able to detect whether a given qemu NBD server is exposing the right export(s) and dirty bitmaps, at least for regression testing. We could use 'nbd-client -l' from the upstream NBD project to list exports, but it's annoying to rely on out-of-tree binaries; furthermore, nbd-client doesn't necessarily know about all of the qemu NBD extensions. Thus, it is time to add a new mode to qemu-nbd that merely sniffs all possible information from the server during handshake phase, then disconnects and dumps the information. This patch actually implements --list/-L, while reusing other options such as --tls-creds for now designating how to connect as the client (rather than their non-list usage of how to operate as the server). I debated about adding this functionality to something akin to 'qemu-img info' - but that tool does not readily lend itself to connecting to an arbitrary NBD server without also tying to a specific export (I may, however, still add ImageInfoSpecificNBD for reporting the bitmaps available when connecting to a single export). And, while it may feel a bit odd that normally qemu-nbd is a server but 'qemu-nbd -L' is a client, we are not really making the qemu-nbd binary that much larger, because 'qemu-nbd -c' has to operate as both server and client simultaneously across two threads when feeding the kernel module for /dev/nbdN access. Sample output: $ qemu-nbd -L exports available: 1 export: '' size: 65536 flags: 0x4ed ( flush fua trim zeroes df cache ) min block: 512 opt block: 4096 max block: 33554432 available meta contexts: 1 base:allocation Note that the output only lists sizes if the server sent NBD_FLAG_HAS_FLAGS, because a newstyle server does not give the size otherwise. It has the side effect that for really old servers that did not send any flags, the size is not output even though it was available. However, I'm not too concerned about that - oldstyle servers are (rightfully) getting less common to encounter (qemu 3.0 was the last version where we even serve it), and most existing servers that still even offer oldstyle negotiation (such as nbdkit) still send flags (since that was added to the NBD protocol in 2007 to permit read-only connections). Not done here, but maybe worth future experiments: capture the meat of NBDExportInfo into a QAPI struct, and use the generated QAPI pretty-printers instead of hand-rolling our output loop. It would also permit us to add a JSON output mode for machine parsing. Signed-off-by: Eric Blake <eblake@redhat.com> Reviewed-by: Richard W.M. Jones <rjones@redhat.com> Message-Id: <20190117193658.16413-20-eblake@redhat.com> Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
2019-01-17 20:36:56 +01:00
The TCP port to listen on as a server, or connect to as a client
(default @samp{10809}).
@item -o, --offset=@var{offset}
The offset into the image.
@item -b, --bind=@var{iface}
qemu-nbd: Add --list option We want to be able to detect whether a given qemu NBD server is exposing the right export(s) and dirty bitmaps, at least for regression testing. We could use 'nbd-client -l' from the upstream NBD project to list exports, but it's annoying to rely on out-of-tree binaries; furthermore, nbd-client doesn't necessarily know about all of the qemu NBD extensions. Thus, it is time to add a new mode to qemu-nbd that merely sniffs all possible information from the server during handshake phase, then disconnects and dumps the information. This patch actually implements --list/-L, while reusing other options such as --tls-creds for now designating how to connect as the client (rather than their non-list usage of how to operate as the server). I debated about adding this functionality to something akin to 'qemu-img info' - but that tool does not readily lend itself to connecting to an arbitrary NBD server without also tying to a specific export (I may, however, still add ImageInfoSpecificNBD for reporting the bitmaps available when connecting to a single export). And, while it may feel a bit odd that normally qemu-nbd is a server but 'qemu-nbd -L' is a client, we are not really making the qemu-nbd binary that much larger, because 'qemu-nbd -c' has to operate as both server and client simultaneously across two threads when feeding the kernel module for /dev/nbdN access. Sample output: $ qemu-nbd -L exports available: 1 export: '' size: 65536 flags: 0x4ed ( flush fua trim zeroes df cache ) min block: 512 opt block: 4096 max block: 33554432 available meta contexts: 1 base:allocation Note that the output only lists sizes if the server sent NBD_FLAG_HAS_FLAGS, because a newstyle server does not give the size otherwise. It has the side effect that for really old servers that did not send any flags, the size is not output even though it was available. However, I'm not too concerned about that - oldstyle servers are (rightfully) getting less common to encounter (qemu 3.0 was the last version where we even serve it), and most existing servers that still even offer oldstyle negotiation (such as nbdkit) still send flags (since that was added to the NBD protocol in 2007 to permit read-only connections). Not done here, but maybe worth future experiments: capture the meat of NBDExportInfo into a QAPI struct, and use the generated QAPI pretty-printers instead of hand-rolling our output loop. It would also permit us to add a JSON output mode for machine parsing. Signed-off-by: Eric Blake <eblake@redhat.com> Reviewed-by: Richard W.M. Jones <rjones@redhat.com> Message-Id: <20190117193658.16413-20-eblake@redhat.com> Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
2019-01-17 20:36:56 +01:00
The interface to bind to as a server, or connect to as a client
(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}
qemu-nbd: Deprecate qemu-nbd --partition The existing qemu-nbd --partition code claims to handle logical partitions up to 8, since its introduction in 2008 (commit 7a5ca86). However, the implementation is bogus (actual MBR logical partitions form a sort of linked list, with one partition per extended table entry, rather than four logical partitions in a single extended table), making the code unlikely to work for anything beyond -P5 on actual guest images. What's more, the code does not support GPT partitions, which are becoming more popular, and maintaining device subsetting in both NBD and the raw device is unnecessary duplication of effort (even if it is not too difficult). Note that obtaining the offsets of a partition (MBR or GPT) can be learned by using 'qemu-nbd -c /dev/nbd0 file.qcow2 && sfdisk --dump /dev/nbd0', but by the time you've done that, you might as well just mount /dev/nbd0p1 that the kernel creates for you instead of bothering with qemu exporting a subset. Or, keeping to just user-space code, use nbdkit's partition filter, which has already known both GPT and primary MBR partitions for a while, and was just recently enhanced to support arbitrary logical MBR parititions. Start the clock on the deprecation cycle, with examples of how to accomplish device subsetting without using -P. Signed-off-by: Eric Blake <eblake@redhat.com> Message-Id: <20190125234837.2272-1-eblake@redhat.com> Reviewed-by: Richard W.M. Jones <rjones@redhat.com> Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
2019-01-26 00:48:37 +01:00
Deprecated: Only expose MBR partition @var{num}. Understands physical
partitions 1-4 and logical partition 5. New code should instead use
@option{--image-opts} with the raw driver wrapping a subset of the
original image.
@item -B, --bitmap=@var{name}
If @var{filename} has a qcow2 persistent bitmap @var{name}, expose
that bitmap via the ``qemu:dirty-bitmap:@var{name}'' context
accessible through NBD_OPT_SET_META_CONTEXT.
@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} (Linux only).
@item -d, --disconnect
Disconnect the device @var{dev} (Linux only).
@item -e, --shared=@var{num}
Allow up to @var{num} clients to share the device (default
@samp{1}). Safe for readers, but for now, consistency is not
guaranteed between multiple writers.
@item -t, --persistent
Don't exit on the last connection.
@item -x, --export-name=@var{name}
Set the NBD volume export name (default of a zero-length string).
@item -D, --description=@var{description}
Set the NBD volume export description, as a human-readable
string.
qemu-nbd: Add --list option We want to be able to detect whether a given qemu NBD server is exposing the right export(s) and dirty bitmaps, at least for regression testing. We could use 'nbd-client -l' from the upstream NBD project to list exports, but it's annoying to rely on out-of-tree binaries; furthermore, nbd-client doesn't necessarily know about all of the qemu NBD extensions. Thus, it is time to add a new mode to qemu-nbd that merely sniffs all possible information from the server during handshake phase, then disconnects and dumps the information. This patch actually implements --list/-L, while reusing other options such as --tls-creds for now designating how to connect as the client (rather than their non-list usage of how to operate as the server). I debated about adding this functionality to something akin to 'qemu-img info' - but that tool does not readily lend itself to connecting to an arbitrary NBD server without also tying to a specific export (I may, however, still add ImageInfoSpecificNBD for reporting the bitmaps available when connecting to a single export). And, while it may feel a bit odd that normally qemu-nbd is a server but 'qemu-nbd -L' is a client, we are not really making the qemu-nbd binary that much larger, because 'qemu-nbd -c' has to operate as both server and client simultaneously across two threads when feeding the kernel module for /dev/nbdN access. Sample output: $ qemu-nbd -L exports available: 1 export: '' size: 65536 flags: 0x4ed ( flush fua trim zeroes df cache ) min block: 512 opt block: 4096 max block: 33554432 available meta contexts: 1 base:allocation Note that the output only lists sizes if the server sent NBD_FLAG_HAS_FLAGS, because a newstyle server does not give the size otherwise. It has the side effect that for really old servers that did not send any flags, the size is not output even though it was available. However, I'm not too concerned about that - oldstyle servers are (rightfully) getting less common to encounter (qemu 3.0 was the last version where we even serve it), and most existing servers that still even offer oldstyle negotiation (such as nbdkit) still send flags (since that was added to the NBD protocol in 2007 to permit read-only connections). Not done here, but maybe worth future experiments: capture the meat of NBDExportInfo into a QAPI struct, and use the generated QAPI pretty-printers instead of hand-rolling our output loop. It would also permit us to add a JSON output mode for machine parsing. Signed-off-by: Eric Blake <eblake@redhat.com> Reviewed-by: Richard W.M. Jones <rjones@redhat.com> Message-Id: <20190117193658.16413-20-eblake@redhat.com> Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
2019-01-17 20:36:56 +01:00
@item -L, --list
Connect as a client and list all details about the exports exposed by
a remote NBD server. This enables list mode, and is incompatible
with options that change behavior related to a specific export (such as
@option{--export-name}, @option{--offset}, ...).
@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
qemu-nbd: Add --list option We want to be able to detect whether a given qemu NBD server is exposing the right export(s) and dirty bitmaps, at least for regression testing. We could use 'nbd-client -l' from the upstream NBD project to list exports, but it's annoying to rely on out-of-tree binaries; furthermore, nbd-client doesn't necessarily know about all of the qemu NBD extensions. Thus, it is time to add a new mode to qemu-nbd that merely sniffs all possible information from the server during handshake phase, then disconnects and dumps the information. This patch actually implements --list/-L, while reusing other options such as --tls-creds for now designating how to connect as the client (rather than their non-list usage of how to operate as the server). I debated about adding this functionality to something akin to 'qemu-img info' - but that tool does not readily lend itself to connecting to an arbitrary NBD server without also tying to a specific export (I may, however, still add ImageInfoSpecificNBD for reporting the bitmaps available when connecting to a single export). And, while it may feel a bit odd that normally qemu-nbd is a server but 'qemu-nbd -L' is a client, we are not really making the qemu-nbd binary that much larger, because 'qemu-nbd -c' has to operate as both server and client simultaneously across two threads when feeding the kernel module for /dev/nbdN access. Sample output: $ qemu-nbd -L exports available: 1 export: '' size: 65536 flags: 0x4ed ( flush fua trim zeroes df cache ) min block: 512 opt block: 4096 max block: 33554432 available meta contexts: 1 base:allocation Note that the output only lists sizes if the server sent NBD_FLAG_HAS_FLAGS, because a newstyle server does not give the size otherwise. It has the side effect that for really old servers that did not send any flags, the size is not output even though it was available. However, I'm not too concerned about that - oldstyle servers are (rightfully) getting less common to encounter (qemu 3.0 was the last version where we even serve it), and most existing servers that still even offer oldstyle negotiation (such as nbdkit) still send flags (since that was added to the NBD protocol in 2007 to permit read-only connections). Not done here, but maybe worth future experiments: capture the meat of NBDExportInfo into a QAPI struct, and use the generated QAPI pretty-printers instead of hand-rolling our output loop. It would also permit us to add a JSON output mode for machine parsing. Signed-off-by: Eric Blake <eblake@redhat.com> Reviewed-by: Richard W.M. Jones <rjones@redhat.com> Message-Id: <20190117193658.16413-20-eblake@redhat.com> Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
2019-01-17 20:36:56 +01:00
option; or provide the credentials needed for connecting as a client
in list mode.
@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
@c man begin EXAMPLES
Start a server listening on port 10809 that exposes only the
guest-visible contents of a qcow2 file, with no TLS encryption, and
with the default export name (an empty string). The command is
one-shot, and will block until the first successful client
disconnects:
@example
qemu-nbd -f qcow2 file.qcow2
@end example
Start a long-running server listening with encryption on port 10810,
and require clients to have a correct X.509 certificate to connect to
a 1 megabyte subset of a raw file, using the export name 'subset':
@example
qemu-nbd \
--object tls-creds-x509,id=tls0,endpoint=server,dir=/path/to/qemutls \
--tls-creds tls0 -t -x subset -p 10810 \
--image-opts driver=raw,offset=1M,size=1M,file.driver=file,file.filename=file.raw
@end example
Serve a read-only copy of just the first MBR partition of a guest
image over a Unix socket with as many as 5 simultaneous readers, with
a persistent process forked as a daemon:
@example
qemu-nbd --fork --persistent --shared=5 --socket=/path/to/sock \
--partition=1 --read-only --format=qcow2 file.qcow2
@end example
Expose the guest-visible contents of a qcow2 file via a block device
/dev/nbd0 (and possibly creating /dev/nbd0p1 and friends for
partitions found within), then disconnect the device when done.
Access to bind qemu-nbd to an /dev/nbd device generally requires root
privileges, and may also require the execution of @code{modprobe nbd}
to enable the kernel NBD client module. @emph{CAUTION}: Do not use
this method to mount filesystems from an untrusted guest image - a
malicious guest may have prepared the image to attempt to trigger
kernel bugs in partition probing or file system mounting.
@example
qemu-nbd -c /dev/nbd0 -f qcow2 file.qcow2
qemu-nbd -d /dev/nbd0
@end example
qemu-nbd: Add --list option We want to be able to detect whether a given qemu NBD server is exposing the right export(s) and dirty bitmaps, at least for regression testing. We could use 'nbd-client -l' from the upstream NBD project to list exports, but it's annoying to rely on out-of-tree binaries; furthermore, nbd-client doesn't necessarily know about all of the qemu NBD extensions. Thus, it is time to add a new mode to qemu-nbd that merely sniffs all possible information from the server during handshake phase, then disconnects and dumps the information. This patch actually implements --list/-L, while reusing other options such as --tls-creds for now designating how to connect as the client (rather than their non-list usage of how to operate as the server). I debated about adding this functionality to something akin to 'qemu-img info' - but that tool does not readily lend itself to connecting to an arbitrary NBD server without also tying to a specific export (I may, however, still add ImageInfoSpecificNBD for reporting the bitmaps available when connecting to a single export). And, while it may feel a bit odd that normally qemu-nbd is a server but 'qemu-nbd -L' is a client, we are not really making the qemu-nbd binary that much larger, because 'qemu-nbd -c' has to operate as both server and client simultaneously across two threads when feeding the kernel module for /dev/nbdN access. Sample output: $ qemu-nbd -L exports available: 1 export: '' size: 65536 flags: 0x4ed ( flush fua trim zeroes df cache ) min block: 512 opt block: 4096 max block: 33554432 available meta contexts: 1 base:allocation Note that the output only lists sizes if the server sent NBD_FLAG_HAS_FLAGS, because a newstyle server does not give the size otherwise. It has the side effect that for really old servers that did not send any flags, the size is not output even though it was available. However, I'm not too concerned about that - oldstyle servers are (rightfully) getting less common to encounter (qemu 3.0 was the last version where we even serve it), and most existing servers that still even offer oldstyle negotiation (such as nbdkit) still send flags (since that was added to the NBD protocol in 2007 to permit read-only connections). Not done here, but maybe worth future experiments: capture the meat of NBDExportInfo into a QAPI struct, and use the generated QAPI pretty-printers instead of hand-rolling our output loop. It would also permit us to add a JSON output mode for machine parsing. Signed-off-by: Eric Blake <eblake@redhat.com> Reviewed-by: Richard W.M. Jones <rjones@redhat.com> Message-Id: <20190117193658.16413-20-eblake@redhat.com> Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
2019-01-17 20:36:56 +01:00
Query a remote server to see details about what export(s) it is
serving on port 10809, and authenticating via PSK:
@example
qemu-nbd \
--object tls-creds-psk,id=tls0,dir=/tmp/keys,username=eblake,endpoint=client \
--tls-creds tls0 -L -b remote.example.com
@end example
@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