Re: [Nbd] [RESEND][PATCH 0/5] nbd improvements

On Thu, Sep 15, 2016 at 04:52:17AM -0700, Christoph Hellwig wrote:

On Thu, Sep 15, 2016 at 12:46:07PM +0100, Alex Bligh wrote:

quoted

Essentially NBD does supports FLUSH/FUA like this:

https://www.kernel.org/doc/Documentation/block/writeback_cache_control.txt

IE supports the same FLUSH/FUA primitives as other block drivers (AIUI).

Link to protocol (per last email) here:

https://github.com/yoe/nbd/blob/master/doc/proto.md#ordering-of-messages-and-writes

Flush as defined by the Linux block layer (and supported that way in
SCSI, ATA, NVMe) only requires to flush all already completed writes
to non-volatile media.

That is precisely what FLUSH in nbd does, too.

It does not impose any ordering unlike the nbd spec.

If you read the spec differently, then that's a bug in the spec. Can you
clarify which part of it caused that confusion? We should fix it, then.

FUA as defined by the Linux block layer (and supported that way in SCSI,
ATA, NVMe) only requires the write operation the FUA bit is set on to be
on non-volatile media before completing the write operation.  It does
not impose any ordering, which seems to match the nbd spec.  Unlike the
NBD spec Linux does not allow FUA to be set on anything by WRITE
commands.  Some other storage protocols allow a FUA bit on READ
commands or other commands that write data to the device, though.

Yes. There was some discussion on that part, and we decided that setting
the flag doesn't hurt, but the spec also clarifies that using it on READ
does nothing, semantically.

The problem is that there are clients in the wild which do set it on
READ, so it's just a matter of "be liberal in what you accept".

-- 
< ron> I mean, the main *practical* problem with C++, is there's like a dozen
       people in the world who think they really understand all of its rules,
       and pretty much all of them are just lying to themselves too.
 -- #debian-devel, OFTC, 2016-02-12