# HDF5 doesn't support writing VLENs with parallel I/O? **URL:** \ **Category:** HDF5 Library\ **Created:** [May 16, 2018, 8:41am UTC](https://forum.hdfgroup.org/t/hdf5-doesnt-support-writing-vlens-with-parallel-i-o/4420 "2018-05-16T08:41:04Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![edwardjameshartnett](https://forum.hdfgroup.org/letter_avatar_proxy/v4/letter/e/bbce88/32.png) [@edwardjameshartnett](https://forum.hdfgroup.org/u/edwardjameshartnett)\ **Post date:** [May 16, 2018, 8:41am UTC](https://forum.hdfgroup.org/t/hdf5-doesnt-support-writing-vlens-with-parallel-i-o/4420/1 "2018-05-16T08:41:04Z")
Howdy! I was writing some netCDF tests for parallel I/O, trying to write an array of strings with parallel I/O. But I get this error: HDF5-DIAG: Error detected in HDF5 (1.10.1) MPI-process 2: #000: H5Dio.c line 269 in H5Dwrite(): can’t prepare for writing data major: Dataset minor: Write failed #001: H5Dio.c line 345 in H5D\_\_pre\_write(): can’t write data major: Dataset minor: Write failed #002: H5Dio.c line 656 in H5D\_\_write(): Parallel IO does not support writing VL datatypes yet major: Dataset minor: Feature is unsupported This suprised me. Will VLEN ever be supported with parallel I/O? Is this just a restriction on writing? Can I read VLENs with parallel I/O? Thanks! ---
**Author:** ![epourmal](https://forum.hdfgroup.org/letter_avatar_proxy/v4/letter/e/c67d28/32.png) [@epourmal](https://forum.hdfgroup.org/u/epourmal)\ **Post date:** [May 16, 2018, 4:43pm UTC](https://forum.hdfgroup.org/t/hdf5-doesnt-support-writing-vlens-with-parallel-i-o/4420/2 "2018-05-16T16:43:52Z")
Hi Ed, Yes, this is a known limitation of the parallel HDF5 implementation. Enabling support for VL types is on our to-do list, but there is no time estimate when it will be available. Elena ---
**Author:** ![andrich](https://forum.hdfgroup.org/letter_avatar_proxy/v4/letter/a/958977/32.png) [@andrich](https://forum.hdfgroup.org/u/andrich)\ **Post date:** [January 14, 2022, 10:30pm UTC](https://forum.hdfgroup.org/t/hdf5-doesnt-support-writing-vlens-with-parallel-i-o/4420/3 "2022-01-14T22:30:04Z")
As this is an older thread I would like to know if the situation changed, is there a time estimate now? ---
**Author:** ![gheber](https://forum.hdfgroup.org/user_avatar/forum.hdfgroup.org/gheber/32/1919_2.png) [@gheber](https://forum.hdfgroup.org/u/gheber)\ **Post date:** [January 17, 2022, 11:48pm UTC](https://forum.hdfgroup.org/t/hdf5-doesnt-support-writing-vlens-with-parallel-i-o/4420/4 "2022-01-17T23:48:27Z")
No change. The current (internal) VL representation is rather inefficient and parallelization would probably make sense only after that’d been fixed/revised. G. ---
**Author:** ![liubo19980712](https://forum.hdfgroup.org/letter_avatar_proxy/v4/letter/l/71e660/32.png) [@liubo19980712](https://forum.hdfgroup.org/u/liubo19980712)\ **Post date:** [July 11, 2023, 5:40pm UTC](https://forum.hdfgroup.org/t/hdf5-doesnt-support-writing-vlens-with-parallel-i-o/4420/5 "2023-07-11T17:40:52Z")
Hello guys, I just ran into the same situation. Would HDF5 support VLEN with parallel IO now? Thanks for your help in advance! ---
**Author:** ![gheber](https://forum.hdfgroup.org/user_avatar/forum.hdfgroup.org/gheber/32/1919_2.png) [@gheber](https://forum.hdfgroup.org/u/gheber)\ **Post date:** [July 11, 2023, 7:44pm UTC](https://forum.hdfgroup.org/t/hdf5-doesnt-support-writing-vlens-with-parallel-i-o/4420/6 "2023-07-11T19:44:37Z")
No updates, but this will be a topic of discussion at [HUG’23](https://www.hdfgroup.org/hug/hug23/). G. ---
**Author:** ![amandajansenofficial](https://forum.hdfgroup.org/letter_avatar_proxy/v4/letter/a/f19dbf/32.png) [@amandajansenofficial](https://forum.hdfgroup.org/u/amandajansenofficial)\ **Post date:** [July 17, 2023, 2:01pm UTC](https://forum.hdfgroup.org/t/hdf5-doesnt-support-writing-vlens-with-parallel-i-o/4420/7 "2023-07-17T14:01:47Z")
Thanks, my issue has been fixed.