[Libguestfs] [PATCH 4/6] v2v: rhv-upload-plugin: Support multiple connections

[Eric Blake]

On 1/26/21 9:03 AM, Richard W.M. Jones wrote:

>>>   https://github.com/NetworkBlockDevice/nbd/blob/master/doc/proto.md
>>>
>>>   "bit 8, NBD_FLAG_CAN_MULTI_CONN: Indicates that the server operates
>>>   entirely without cache, or that the cache it uses is shared among
>>>   all connections to the given device. In particular, if this flag is
>>>   present, then the effects of NBD_CMD_FLUSH and NBD_CMD_FLAG_FUA MUST
>>>   be visible across all connections when the server sends its reply to
>>>   that command to the client. In the absence of this flag, clients
>>>   SHOULD NOT multiplex their commands over more than one connection to
>>>   the export."
> 
> Although the text only mentions flush & FUA, I wonder if there's
> another case where multi-conn should not be advertised or used: That
> is where the block or sector size is larger than the size of writes,
> so writes are being turned into r/m/w cycles.  Multiple adjacent (even
> non-overlapping) writes could then interfere with each other.

That's the complication that explains why qemu-nbd does not advertise it
on writeable connections.  For proper cross-thread consistency when the
bit is set, and assuming a situation where there are 2 writers and 1
reader client, if the two writers both issue an overlapping write
request with CMD_FLAG_FUA or followed by NBD_CMD_FLUSH, then the reader
must see exactly one of five states: the pre-write state, the state
after just writer 1, the state after just writer 2, the state after both
writers with writer 1 completing before writer 2 starts, or the state
after both writers with writer 2 completing before writer 1 starts.  Any
other result would indicate write shredding (where the two writers were
not properly interlocked, and therefore the second writer partially
undoes some of the effects of the first).

> This is vanishingly unlikely to affect nbdcopy which uses a huge block
> size, but would it be a problem in the NBD protocol itself?

I'm not convinced it is a hole in the protocol, so much as a reason that
CAN_MULTI_CONN was added in the first place (if you don't advertise the
bit, clients can't assume consistency at the server, and so have to do
their own locking or otherwise avoid overlapping writes client-side to
ensure consistency; if you do advertise the bits, clients can take
shortcuts).  But maybe you do have a point that when a server advertises
block sizes, a client should pay attention to the recommended block
size, and assume anything smaller than that may trigger rmw that might
not be safe even if CAN_MULTI_CONN was advertised.  Should we ask for
clarification on the NBD mailing list?

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org