Re: Write atomicity guarantees

[Chris Mason <clm@fb.com>]

On 04/24/2014 02:25 PM, Matthew Wilcox wrote:
> On Thu, Apr 24, 2014 at 11:10:09AM -0700, Purush Gupta wrote:
>> That would require a new NVMe write command semantic with descriptors
>> currently its not there.
>
> Yes ... I don't want to start defining such a command on linux-fsdevel.
> What I'm hearing is that there's no benefit to a device that can guarantee
> to write multiple contiguous sectors in a non-torn manner over a device
> that can write a single sector in a non-torn manner.
>
> For any real benefit, filesystems need (and Linux needs to introduce
> plumbing for) vectored atomic writes.
>

That's my feeling.  The non-vectored use case is pretty limited, mostly 
to help get a contiguous log entry on disk as a full unit.  But most of 
the time the filesystem log commits are pretty big.  Workloads with a 
very small number of latency sensitive writers would see improvements.

We could also the contiguous atomics for something like mysql by using a 
16K sector size in the filesystem.  At that point all of the db 16K 
units will be contig and the existing atomics proposals become interesting.

But I was really hoping for the vectors ;)

-chris