# Mdrun -rerun output file sizes do not make sense

**URL:** <https://gromacs.bioexcel.eu/t/mdrun-rerun-output-file-sizes-do-not-make-sense/4122>\
**Category:** User discussions\
**Created:** [May 6, 2022, 2:51pm UTC](https://gromacs.bioexcel.eu/t/mdrun-rerun-output-file-sizes-do-not-make-sense/4122 "2022-05-06T14:51:12Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![sodiumnitrate](https://avatars.discourse-cdn.com/v4/letter/s/eada6e/32.png) [@sodiumnitrate](https://gromacs.bioexcel.eu/u/sodiumnitrate)\
**Post date:** [May 6, 2022, 2:51pm UTC](https://gromacs.bioexcel.eu/t/mdrun-rerun-output-file-sizes-do-not-make-sense/4122/1 "2022-05-06T14:51:12Z")

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GROMACS version: 2022  
GROMACS modification: No

I’m trying to calculate potential energies from an `.xtc` file and the original `.tpr` file. Since I don’t have an `.edr` file, I’m doing `gmx mdrun -rerun`. The `.xtc` file has ~500 snapshots that are spaced 1ns apart. With these files, when I do the rerun, it says I get the following warning:

`WARNING: This run will generate roughly 3411371164616200192 Mb of data`

This must be a bug, right?

If I do run it, it seems to only process every 1ns, as expected. However, the `.trr` file it produces is `1.1G`, compared to the `.xtc` file that was `127M`. Does this make sense? What could the `.trr` have that the `.xtc` file didn’t? Neighbor list information, etc.?

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<div class="post-metadata">

**Author:** ![hess](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/hess/32/416_2.png) [@hess](https://gromacs.bioexcel.eu/u/hess)\
**Post date:** [May 9, 2022, 2:01pm UTC](https://gromacs.bioexcel.eu/t/mdrun-rerun-output-file-sizes-do-not-make-sense/4122/2 "2022-05-09T14:01:09Z")

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Yes, mdrun -rerun does not know at startup how many frames it will read.

The trr write full precision binary output, whereas xtc is compressed.