VeloC and MPI IO

[denisbertini]

Hi
Just beginner question: is it possible to adapt a code using MPI- collective IO for its
checkpointing files to VeloC?
Thanks
Denis

[gonsie]

Hey @denisbertini

You might be able to use the file-based mode that veloc offers. You call VELOC_Route_file with the file path that you would normally write to and instead use the passed-back file path. Veloc then handles the files and moves them around / provides fault tolerance.

[denisbertini]

Hi @gonsie
Thanks a lot, i will try to use VeloC to leverage the checkpointing IO on our plasma physics simulation.
This is using extensively MPI-IO, i hope to make it runnuing. Do you know examples already that i can look at ?
Thanks
Denis

[gonsie]

They only examples I know of are provided in the documentation (at the bottom of the page).

[denisbertini]

These example are using fwrite/fread but i suppose one can substitute the corresponding MPI_WRITE/MPI_READ functions in my case ...

[bnicolae]

Is there a particular reason why you need MPI-IO? It should be easier for you to protect the memory regions directly instead of writing them into a file using MPI-IO.

[denisbertini]

Well the program that i use is already using MPI-IO to do chekpointing and to dump the data files.
Is there some limitation in this case ? I mean combining MPI IO and VeloC ?

[denisbertini]

When you say "protecting memory region directly" you mean what is done in the heatdis_mem example ?

[bnicolae]

Yes, this is what I mean. If you are using MPI-IO, then you are already writing to a parallel file system so there is no point in using VELOC. The idea of using VELOC is to checkpoint asynchronously and avoid paying for expensive I/O (which you do if you wait for MPI-IO to finish).

[bnicolae]

Also, please note that we have a mailing list you can subscribe to: veloc-users@lists.mcs.anl.gov.
This is the right place to discuss such considerations. We use github issues primarily for bug reports.