[Nbd] NBD_CMD_FLAG_FUA

[Alex Bligh <alex@...872...>]

>From proto.md:

> If the NBD_FLAG_SEND_FUA flag ("Force Unit Access") was set in the export 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.

NBD_CMD_FLAG_FUA itself is not defined in proto.md

nbd.h has:

#define NBD_CMD_FLAG_FUA (1<<16)

This is a bit odd as the command flags are only 16 bits long!

I think I may be the only person who has ever implemented a server side FUA, but I think this is a legacy of when commands were originally 32 bits long and we broke 16 bits off for flags.

Archeological evidence for this is as follows. In my server I have:

  rc->fua = !!(ntohl (req->command) & NBD_CMD_FLAG_FUA);

Where req is:

struct nbd_request
{
  uint32_t magic;
  uint32_t command;
  uint64_t handle;
  uint64_t offset;
  uint32_t len;
} __attribute__ ((packed));


Now flags are a separate field (and command is 16 bits), I think we should have in nbd.h

#define NBD_CMD_FLAG_FUA (1<<0)

and this should also be in proto.md.

Wouter: would you take a patch?

-- 
Alex Bligh