Re: RDMA power failure write atomicity

Vladislav Bolkhovitin
2016-03-12T00:26:47Z

I'm aware of this proposal. Unfortunately, it is quite orthogonal to my question,
because it is about how to ensure persistence of RDMA writes. Atomicity it is
mentioning as well as general RDMA atomicity is atomicity with regard of parallel
commands acting on the same locations. However, I'm asking about power failure
atomicity, which is something different.

For instance, you are doing RDMA WRITE of 10 bytes of data. If a power failure happen
while this operation is in progress, what data will end up on the target location? All
10 bytes new? All 10 bytes old? Or mix of 5 bytes new and five bytes old? Power failure
atomicity I mean is guarantee that the data either old, or new, never mix of old and
new data.

Thanks,
Vlad

Asgeir Eiriksson wrote on 03/10/2016 05:33 PM:
> Vladislav,
>
> This is an area of active R&D
>
> You might be interested in the following (at ietf.org):
>
> Title : RDMA Durable Write Commit
> Authors : Tom Talpey
> Jim Pinkerton
> <>
> Filename : draft-talpey-rdma-commit-00.txt
> Pages : 24
> Date : 2016-02-19
>
> Regards,
>
> ‘Asgeir
>
>
>> On Mar 10, 2016, at 3:45 PM, Vladislav Bolkhovitin <vst-d+Crzxg7Rs0@public.gmane.org> wrote:
>>
>> Hello,
>>
>> I'm currently considering to use NVDIMM behind RDMA and wonder what is RDMA power
>> failure write atomicity? I mean, what is minimal size and alignment guaranteed to be
>> written atomically in face of power failure (or some other similar failure), i.e.
>> either written in full, or not written at all?
>>
>> For memory writes on Intel it is 8 bytes with 8 bytes alignment. Is there anything like
>> this for RDMA? Or different vendors/implementation have so different expectations and
>> promises, so you can not assume anything >1 byte?
>>
>> I can't find such info anywhere.
>>
>> Thanks,
>> Vlad