[Libguestfs] [PATCH nbdkit v2 08/11] file: Return NBD_FLAG_CAN_MULTI_CONN for the file plugin.

[Eric Blake]

On 1/5/19 8:50 AM, Richard W.M. Jones wrote:
> This allows multiple connections from a single client, and should be
> safe assuming flush/FUA has been implemented correctly in the previous
> commit.

Looks safe to me between the two patches.  The NBD spec also states:

* A client which uses multiple connections to a server to parallelize
  commands MUST NOT issue an `NBD_CMD_FLUSH` request until it has
  received the reply for all write commands which it expects to be
  covered by the flush.

which implies that even if we encounter this race:

client A:        client B:
                 issue write
issue flush
flush completes
                 write completes

we don't have to worry about that case, because client B didn't wait for
the response before client A requested the flush.  Kernel fdatasync()
semantics match what we want (all writes that any client has received a
reply to will be included in the flush, even if the flush is on a
different client).

ACK

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

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