# Input and output¶

PISM is a program that reads NetCDF files and then outputs NetCDF files. Table Table 26 summarizes command-line options controlling the most basic ways to input and output NetCDF files when starting and ending PISM runs.

Table 26 Basic NetCDF input and output options
Option Description
-i Chooses a PISM output file (NetCDF format) to initialize or restart from. See section Initialization and bootstrapping.
-bootstrap Bootstrap from the file set using -i using heuristics to “fill in” missing fields. See section Initialization and bootstrapping.
-ssa_read_initial_guess false Turns off reading the ubar_ssa and vbar_ssa velocities saved by a previous run using the ssa or ssa+sia stress balance (see section Choosing the stress balance).
-o Chooses the output file name. Default name is unnamed.nc.
-o_size size_keyword

Chooses the size of the output file to produce. Possible sizes are

• none (no output file at all),
• small (only variables necessary to restart PISM),
• medium (the default, includes diagnostic quantities listed in the configuration parameter output.sizes.medium, if they are available in the current PISM setup),
• big_2d (same as medium, plus variables listed in output.sizes.big_2d), and
• big (same as big_2d, plus variables listed in output.sizes.big).

Table 27 lists the controls on what is printed to the standard output. Note the -help and -usage options for getting help at the command line.

Table 27 Options controlling PISM’s standard output
Option Description
-help Brief descriptions of the many PISM and PETSc options. The run occurs as usual according to the other options. (The option documentation does not get listed if the run didn’t get started properly.) Use with a pipe into grep to get usefully-filtered information on options, for example pisms -help | grep cold.
-info Gives information about PETSc operations during the run.
-list_diagnostics Prints a list of all available diagnostic outputs (time series and spatial) for the run with the given options. Stops run after printing the list.
-log_summary At the end of the run gives a performance summary and also a synopsis of the PETSc configuration in use.
-options_left At the end of the run shows an options table which will indicate if a user option was not read or was misspelled.
-usage Short summary of PISM executable usage, without listing all the options, and without doing the run.
-verbose Increased verbosity of standard output. Usually given with an integer level; 0,1,2,3,4,5 are allowed. If given without argument then sets level 3, while -verbose 2 is the default (i.e. equivalent to no option). At the extremes, -verbose 0 produces no stdout at all, -verbose 1 prints only warnings and a few high priority messages, and -verbose 5 spews a lot of usually-undesirable stuff. -verbose 3 output regarding initialization may be useful.
-version Show version numbers of PETSc and PISM.

The following sections describe more input and output options, especially related to saving quantities during a run, or adding to the “diagnostic” outputs of PISM.

## PISM file I/O performance¶

When working with fine grids[1], the time PISM spends writing output files, spatially-varying diagnostic files, or backup files can become significant.

It turns out that it is a lot faster to read and write files using the t,y,x,z storage order, as opposed to the more convenient (e.g. for NetCDF tools) t,z,y,x order. The reason is that PISM uses the y,x,z order internally,[2] and therefore writing an array in a different order is an inherently-expensive operation.

You can, however, choose any one of the three supported output orders using the -o_order option with one of xyz, yxz, and zyx as the argument.

To transpose dimensions in an existing file, use the ncpdq (“permute dimensions quickly”) tool from the NCO suite. For example, run

ncpdq -a t,z,zb,y,x bad.nc good.nc


to turn bad.nc (with any inconvenient storage order) into good.nc using the t,z,y,x order.

PISM also supports NetCDF-4 parallel I/O, which gives better performance in high-resolution runs and avoids NetCDF-3 file format limitations. (In a NetCDF-3 file a variable record cannot exceed 4 gigabytes.) Build PISM with parallel NetCDF-4 and use -o_format netcdf4_parallel to enable this code.

In addition to -o_format netcdf4_parallel and netcdf3 (default) modes, PISM can be built with PnetCDF for best I/O performance. The option -o_format pnetcdf turns “on” PnetCDF I/O code. (PnetCDF seems to be somewhat fragile, though, so use at your own risk.)

Footnotes

 [1] For example, resolutions of 2km and higher on the whole-Greenland scale.
 [2] This is not likely to change.

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