On 04/05/2016 09:30 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 | 59 ++++++++++++++++++++++++++++++++++++++++++++--------------- > 1 file changed, 44 insertions(+), 15 deletions(-) > > > -- 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 > + `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`), however note that existing clients are known to set this > + bit on other commands. Subject to that, and provided `NBD_FLAG_SEND_FUA` > + is negotiated, the client MAY set this bit on all, no or some commands > + as it wishes (see the section on Ordering of messages and writes for > + details). If the server 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 s/command command/command/ > + completed and persisted to non-volatile storage. If the command does > + not in fact write data (for instance on an `NBD_CMD_TRIM` in a situation > + where the command as a whole is ignored), the server MAY ignore this bit > + being set on such a command. With the duplicate word removed, Reviewed-by: Eric Blake <eblake@...696...> -- Eric Blake eblake redhat com +1-919-301-3266 Libvirt virtualization library http://libvirt.org
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