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According to the NBD spec, a server that advertises
NBD_FLAG_CAN_MULTI_CONN promises that multiple client connections will
not see any cache inconsistencies: when properly separated by a single
flush, actions performed by one client will be visible to another
client, regardless of which client did the flush.
We always satisfy these conditions in qemu - even when we support
multiple clients, ALL clients go through a single point of reference
into the block layer, with no local caching. The effect of one client
is instantly visible to the next client. Even if our backend were a
network device, we argue that any multi-path caching effects that
would cause inconsistencies in back-to-back actions not seeing the
effect of previous actions would be a bug in that backend, and not the
fault of caching in qemu. As such, it is safe to unconditionally
advertise CAN_MULTI_CONN for any qemu NBD server situation that
supports parallel clients.
Note, however, that we don't want to advertise CAN_MULTI_CONN when we
know that a second client cannot connect (for historical reasons,
qemu-nbd defaults to a single connection while nbd-server-add and QMP
commands default to unlimited connections; but we already have
existing means to let either style of NBD server creation alter those
defaults). This is visible by no longer advertising MULTI_CONN for
'qemu-nbd -r' without -e, as in the iotest nbd-qemu-allocation.
The harder part of this patch is setting up an iotest to demonstrate
behavior of multiple NBD clients to a single server. It might be
possible with parallel qemu-io processes, but I found it easier to do
in python with the help of libnbd, and help from Nir and Vladimir in
writing the test.
Signed-off-by: Eric Blake <eblake@redhat.com>
Suggested-by: Nir Soffer <nsoffer@redhat.com>
Suggested-by: Vladimir Sementsov-Ogievskiy <v.sementsov-og@mail.ru>
Message-Id: <20220512004924.417153-3-eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
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Replace Qemu -> QEMU.
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20211118143401.4101497-1-philmd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
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'qemu-img map' provides a way to determine which extents of an image
come from the top layer vs. inherited from a backing chain. This is
useful information worth exposing over NBD. There is a proposal to
add a QMP command block-dirty-bitmap-populate which can create a dirty
bitmap that reflects allocation information, at which point the
qemu:dirty-bitmap:NAME metadata context can expose that information
via the creation of a temporary bitmap, but we can shorten the effort
by adding a new qemu:allocation-depth metadata context that does the
same thing without an intermediate bitmap (this patch does not
eliminate the need for that proposal, as it will have other uses as
well).
While documenting things, remember that although the NBD protocol has
NBD_OPT_SET_META_CONTEXT, the rest of its documentation refers to
'metadata context', which is a more apt description of what is
actually being used by NBD_CMD_BLOCK_STATUS: the user is requesting
metadata by passing one or more context names. So I also touched up
some existing wording to prefer the term 'metadata context' where it
makes sense.
Note that this patch does not actually enable any way to request a
server to enable this context; that will come in the next patch.
Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20201027050556.269064-10-eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
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I found that there are many spelling errors in the comments of qemu,
so I used the spellcheck tool to check the spelling errors
and finally found some spelling errors in the docs folder.
Signed-off-by: zhaolichang <zhaolichang@huawei.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20200917075029.313-4-zhaolichang@huawei.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>
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Commit fe0480d6 and friends added BDRV_REQ_NO_FALLBACK as a way to
avoid wasting time on a preliminary write-zero request that will later
be rewritten by actual data, if it is known that the write-zero
request will use a slow fallback; but in doing so, could not optimize
for NBD. The NBD specification is now considering an extension that
will allow passing on those semantics; this patch updates the new
protocol bits and 'qemu-nbd --list' output to recognize the bit, as
well as the new errno value possible when using the new flag; while
upcoming patches will improve the client to use the feature when
present, and the server to advertise support for it.
The NBD spec recommends (but not requires) that ENOTSUP be avoided for
all but failures of a fast zero (the only time it is mandatory to
avoid an ENOTSUP failure is when fast zero is supported but not
requested during write zeroes; the questionable use is for ENOTSUP to
other actions like a normal write request). However, clients that get
an unexpected ENOTSUP will either already be treating it the same as
EINVAL, or may appreciate the extra bit of information. We were
equally loose for returning EOVERFLOW in more situations than
recommended by the spec, so if it turns out to be a problem in
practice, a later patch can tighten handling for both error codes.
Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20190823143726.27062-3-eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
[eblake: tweak commit message, also handle EOPNOTSUPP]
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The NBD specification defines NBD_FLAG_CAN_MULTI_CONN, which can be
advertised when the server promises cache consistency between
simultaneous clients (basically, rules that determine what FUA and
flush from one client are able to guarantee for reads from another
client). When we don't permit simultaneous clients (such as qemu-nbd
without -e), the bit makes no sense; and for writable images, we
probably have a lot more work before we can declare that actions from
one client are cache-consistent with actions from another. But for
read-only images, where flush isn't changing any data, we might as
well advertise multi-conn support. What's more, advertisement of the
bit makes it easier for clients to determine if 'qemu-nbd -e' was in
use, where a second connection will succeed rather than hang until the
first client goes away.
This patch affects qemu as server in advertising the bit. We may want
to consider patches to qemu as client to attempt parallel connections
for higher throughput by spreading the load over those connections
when a server advertises multi-conn, but for now sticking to one
connection per nbd:// BDS is okay.
See also: https://bugzilla.redhat.com/1708300
Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20190815185024.7010-1-eblake@redhat.com>
[eblake: tweak blockdev-nbd.c to not request shared when writable,
fix iotest 233]
Reviewed-by: John Snow <jsnow@redhat.com>
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It can be useful to figure out which NBD protocol features are
exposed by a server, as well as what features a client will
take advantage of if available, for a given qemu release. It's
not always precise to base features on version numbers (thanks
to downstream backports), but any documentation is better than
making users search through git logs themselves.
This patch originally stemmed from a request to document that
pristine 3.0 has a known bug where NBD_OPT_LIST_META_CONTEXT
with 0 queries forgot to advertise an available
"qemu:dirty-bitmap" context, but documenting bugs like this (or
the fact that 3.0 also botched NBD_CMD_CACHE) gets to be too
much details, especially since buggy releases will be less
likely connection targets over time. Instead, I chose to just
remind users to check stable release branches.
Suggested-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20181215135324.152629-3-eblake@redhat.com>
Reviewed-by: Richard W.M. Jones <rjones@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
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Describe new metadata namespace: "qemu".
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20180609151758.17343-7-vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
[eblake: grammar tweaks]
Signed-off-by: Eric Blake <eblake@redhat.com>
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