On 04/05/2016 06:07 AM, Alex Bligh wrote: > Improve the documentation of NBD_CMD_FLUSH and NBD_CMD_FLAG_FUA. Specifically > the latter may be set on any command, and its semantics on commands other > than NBD_CMD_WRITE need explaining. Further, explain how these relate to > reordering of commands. > > Signed-off-by: Alex Bligh <alex@...872...> > --- > doc/proto.md | 51 +++++++++++++++++++++++++++++++++++++++++---------- > 1 file changed, 41 insertions(+), 10 deletions(-) > close, but needs a v4 > +++ b/doc/proto.md > @@ -217,6 +217,33 @@ handle as was sent by the client in the corresponding request. In > this way, the client can correlate which request is receiving a > response. > > +#### Ordering of messages and writes > + > +The server MAY process commands out of order, and MAY reply out of > +order, save that: maybe s/save/except/ > + > +* All write commands (that includes both `NBD_CMD_WRITE` and > + `NBD_CMD_TRIM`) that the server completes (i.e. replies to) should we also list NBD_CMD_WRITE_ZEROES? > + prior to processing to a `NBD_CMD_FLUSH` MUST be written to non-volatile > + storage prior to replying to that `NBD_CMD_FLUSH`. This > + paragraph only applies if `NBD_FLAG_SEND_FLUSH` is set within > + the transmission flags, as otherwise `NBD_CMD_FLUSH` will never > + be sent by the client to the server. > + > +* A server MUST NOT reply to a command that has `NBD_CMD_FLAG_FUA` set > + in its command flags until the data (if any) written by that command > + is persisted to non-volatile storage. This only applies if > + `NBD_FLAG_SEND_FLUSH` is set within the transmission flags, as otherwise s/FLUSH/FUA/ > + `NBD_CMD_FLAG_FUA` will not be set on any commands sent to the server > + by the client. > + > +`NBD_CMD_FLUSH` is modelled on the Linux kernel empty bio with > +`REQ_FLUSH` set. `NBD_CMD_FLAG_FUA` is modelled on the Linux > +kernel bio with `REQ_FUA` set. In case of ambiguity in this > +specification, the > +[kernel documentation](https://www.kernel.org/doc/Documentation/block/writeback_cache_control.txt) > +may be useful. > + > #### Request message > > The request message, sent by the client, looks as follows: > @@ -483,10 +510,20 @@ affects a particular command. Clients MUST NOT set a command flag bit > that is not documented for the particular command; and whether a flag is > valid may depend on negotiation during the handshake phase. > > -- bit 0, `NBD_CMD_FLAG_FUA`; valid during `NBD_CMD_WRITE` and > - `NBD_CMD_WRITE_ZEROES` commands. SHOULD be set to 1 if the client requires > - "Force Unit Access" mode of operation. MUST NOT be set unless transmission > - flags included `NBD_FLAG_SEND_FUA`. > +- bit 0, `NBD_CMD_FLAG_FUA`; This flag is valid for all commands provided s/commands/commands,/ > + `NBD_FLAG_SEND_FUA` has been negotiated, in which case the server MUST > + accept all commands with this bit set (even by ignoring the bit). The > + client SHOULD NOT set this bit unless the command has the potential of > + writing data (current commands are `NBD_CMD_WRITE`, `NBD_CMD_WRITE_ZEROES` > + and `NBD_CMD_TRIM`); existing clients are known to set this bit on maybe: s/existing/but existing/ > + other commands; subject to that, provided `NBD_FLAG_SEND_FUA` is > + negotiated, the client MAY set this bit as it wishes. If the server Sounds redundant. I'd strike 'subject to that, ... as it wishes'. > + receives a command with `NBD_CMD_FLAG_FUA` set it MUST NOT send its > + reply to that command until all write operations (if any) associated with > + that command command have been completed and persisted to non-volatile > + storage. If the command does not in fact write data (for instance on an > + `NBD_CMD_TRIM` which does is ignored), the server MAY ignore this bit s/does // > + being set on such a command. > - bit 1, `NBD_CMD_NO_HOLE`; defined by the experimental `WRITE_ZEROES` > extension; see below. > - bit 2, `NBD_CMD_FLAG_DF`; defined by the experimental `STRUCTURED_REPLY` > @@ -535,12 +572,6 @@ The following request types exist: > message. The server MAY send the reply message before the data has > reached permanent storage. > > - If the `NBD_FLAG_SEND_FUA` flag ("Force Unit Access") was set in the > - transmission flags field, the client MAY set the flag `NBD_CMD_FLAG_FUA` in > - the command flags field. If this flag was set, the server MUST NOT send > - the reply until it has ensured that the newly-written data has reached > - permanent storage. > - You should drop this paragraph from both NBD_CMD_WRITE and NBD_CMD_WRITE_ZEROES, now that the flag is globally described (here, you only dropped it from one of the two). -- Eric Blake eblake redhat com +1-919-301-3266 Libvirt virtualization library http://libvirt.org
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