Re: Write atomicity guarantees

[Chris Mason <clm@fb.com>]

On 04/24/2014 01:39 PM, Matthew Wilcox wrote:
>
> NVMe allows the drive to tell the host what atomicity guarantees it
> provides for a write command.  At the moment, I don't think Linux has
> a way for the driver to pass that information up to the filesystem.
>
> The value that is most interesting to report is Atomic Write Unit Power
> Fail ("if you send a write no larger than this, the drive guarantees to
> write all of it or none of it"), minimum value 1 sector. [1]
>
> There's a proposal before the NVMe workgroup to add a boundary size/offset
> to modify AWUPF ("except if you cross this boundary, then AWUPF is not
> guaranteed").  Think RAID stripe crossing.
>
> So, three questions.  Is there somewhere already to pass boundary
> information up to the filesystem?  Can filesystems make use of a larger
> atomic write unit than a single sector?  And, if the device is internally
> a RAID device, is knowing the boundary size/offset useful?
>
>
> [1] There is also Atomic Write Unit Normal ("if you send two writes,
> neither of which is larger than this, subsequent reads will get either
> one or the other, not a mixture of both"), which I don't think we care
> about because the page cache prevents us from sending two writes which
> overlap with each other.

I think we really need the atomics to be vectored.  Send N writes which 
as a unit are not larger than X, but which may span anywhere on device. 
  An array with writeback cache, or a log structured squirrel in the FTL 
should be able to provide this pretty easily?

The immediate use case is mysql (16K writes) on a fragmented filesystem. 
  The FS needs to be able to collect a single atomic write made up of N 
4K sectors.

-chris