Re: [PATCH 1/2] block: Add support for atomic writes

[Matthew Wilcox <willy@linux.intel.com>]

On Thu, Nov 07, 2013 at 08:52:20AM -0500, Chris Mason wrote:
> Unfortunately, it's hard to say.  I think the fusionio cards are the
> only shipping devices that support this, but I've definitely heard that
> others plan to support it as well.  mariadb/percona already support the
> atomics via fusionio specific ioctls, and turning that into a real
> O_ATOMIC is a priority so other hardware can just hop on the train.
> 
> This feature in general is pretty natural for the log structured squirrels
> they stuff inside flash, so I'd expect everyone to support it.  Matthew,
> how do you feel about all of this?

NVMe doesn't have support for this functionality.  I know what stories I've
heard from our internal device teams about what they can and can't support
in the way of this kind of thing, but I obviously can't repeat them here!

I took a look at the SCSI Block Command spec.  If I understand it
correctly, SCSI would implement this with the WRITE USING TOKEN command.
I don't see why it couldn't implement this API, though it seems like
SCSI would prefer a separate setup step before the write comes in.  I'm
not sure that's a reasonable request to make of the application (nor
am I sure I understand SBC correctly).

I like the API, but I'm a little confused not to see a patch saying "Oh,
and here's how we implemented it in btrfs without any hardware support"
;-)  It seems to me that the concept is just as good a match for an
advanced filesystem that supports snapshots as it is for the FTL inside
a drive.

> With the fusionio drivers, we've recently increased the max atomic size.
> It's basically 1MB, disjoint or contig doesn't matter.  We're powercut
> safe at 1MB.