# Re: [petsc-users] 2D vector in 3D dmda

```   Understood. We don't have any direct way with DA for storing this
information directly associated with you 3D DA. ```
```
You need to figure out how to store it so that each process has access to
the parts of the data that it needs; which may not be completely trivial. You
could possibly use some 2d DMDA on sub communicators with suitable layouts to
be accessible, you need to figure out the details., the dx[k] probably can be
just stored on every process since it is 1d.

Barry

> On Sep 17, 2016, at 2:58 AM, Aurelien Ponte <aurelien.po...@ifremer.fr> wrote:
>
>
> I guess my concern is of the second type:
> By grid metric terms I meant essentially grid spacings which look like:
> dx(i,j), dy(i,j) and dz(k) where (i,j,k) are indices running along the 3
> dimensions of the grid.
> Storing dx(i,j) into a 3D array seemed like a bit waste of memory to me but I
> must
> be wrong. The elliptic problem I am solving for is close to a poisson
> equation btw.
> I guess I can at least store dx and dy into a single dxy 3D array.
>
> Thanks again,
>
> Aurelien
>
>
>
> Le 16/09/16 à 19:43, Barry Smith a écrit :
>>> On Sep 16, 2016, at 9:29 AM, Aurelien PONTE <aurelien.po...@ifremer.fr>
>>> wrote:
>>>
>>> Hi,
>>>
>>> I've started using petsc4py in order to solve a 3D problem (inversion of
>>> elliptic operator).
>>> I would like to store 2D metric terms describing the grid
>>
>>      What do you mean by 2D metric terms describing the grid?
>>
>>      Do you want to store something like a little dense 2d array for each
>> grid point? If so create another 3D DA with a dof = the product of the
>> dimensions of the little dense 2d array and then store the little dense 2d
>> arrays at in a global vector obtained from that DA.
>>
>>     Or is the grid uniform in one dimension and not uniform in the other two
>> and hence you want to store the information about the non-uniformness in
>> only a 2d array so as to not "waste" the redundant information in the third
>> direction? Then I recommend just "waste" the redundant information in the
>> third dimension; it is trivial compared to all the data you need to solve
>> the problem.
>>
>>     Or do you mean something else?
>>
>>   Barry
>>
>>> I am working on but don't know
>>> how to do that given my domain is tiled in 3D directions:
>>>
>>> self.da = PETSc.DMDA().create([self.grid.Nx, self.grid.Ny, self.grid.Nz],
>>>                                      stencil_width=2)
>>>
>>> I create my 3D vectors with, for example:
>>>
>>> self.Q = self.da.createGlobalVec()
>>>
>>> What am I supposed to do for a 2D vector?
>>> Is it a bad idea?
>>>
>>> thanks
>>>
>>> aurelien
>>>
>
>
> --
> Aurélien Ponte
> Tel: (+33) 2 98 22 40 73
> Fax: (+33) 2 98 22 44 96
> UMR 6523, IFREMER
> ZI de la Pointe du Diable
> CS 10070
> 29280 Plouzané
>

```