Load MERA-Files

Run it yourself

This page is also an executable Jupyter notebook: open / download 07_multi_Mera_Files.ipynb. The notebooks run end-to-end and double as part of Mera's test suite.

The RAMSES simulation data is stored in JLD2 file format and can be accessed from these files. Our high-resolution galaxy simulations, run on over 5,000 cores, show that using compressed Mera files greatly decreases storage requirements and accelerates data loading compared to standard RAMSES files. Refer to the Benchmarks section.

Quick Reference

Essential Functions

# Convert from RAMSES files multiple data to JLD2
convertdata(output_num, path="ramses_path", fpath="jld2_path")
convertdata(output_num, [:hydro, :particles], path="ramses_path", fpath="jld2_path")

# Save individual loaded datasets
savedata(data_object, "output_path", fmode=:write)   # Create new file
savedata(data_object, "output_path", fmode=:append)  # Add to existing file

# Load from JLD2
loaddata(output_num, "jld2_path", :hydro)
loaddata(output_num, "jld2_path", :particles)
loaddata(output_num, "jld2_path", :gravity)

# Load with spatial selection
loaddata(output_num, "jld2_path", :hydro,
         xrange=[-10,10], yrange=[-10,10], zrange=[-2,2],
         center=[:boxcenter], range_unit=:kpc)

# View and inspect stored data
viewdata(output_num, "jld2_path")                    # Show file contents
infodata(output_num, "jld2_path", :hydro)           # Data type info

Key File Modes

:write - Create new file or overwrite existing (use for first save)
:append - Add data types to existing file (safe for additional data)

Data Types

:hydro - Gas density, velocity, pressure, temperature
:particles - Stellar/DM particles: position, velocity, mass, age
:gravity - Gravitational potential and force fields
:clumps - Structure identification data
:rt - Radiative-transfer photon densities and fluxes
:sinks - Sink particles: accreting point masses (see Sink Data: First Inspection)

using Mera

*__   __ _______ ______   _______
|  |_|  |       |    _ | |   _   |
|       |    ___|   | || |  |_|  |
|       |   |___|   |_||_|       |
|       |    ___|    __  |       |
| ||_|| |   |___|   |  | |   _   |
|_|   |_|_______|___|  |_|__| |__|
Mera v1.8.3 | Julia 1.12.7 | 4 threads

Load the Data From Ramses

# Example-data root. Point this at your own simulation folder, or set the
# MERA_EXAMPLES environment variable; every path below is built from it.
MERA_EXAMPLES = get(ENV, "MERA_EXAMPLES", "/Volumes/FASTStorage/Simulations/Mera-Tests");

info = getinfo(300,  "$MERA_EXAMPLES/RAMSES/mw_L10");
gas  = gethydro(info, verbose=false, show_progress=false);
part = getparticles(info, verbose=false, show_progress=false);
grav = getgravity(info, verbose=false, show_progress=false);
# the same applies for clump-data...

[Mera]: 2026-10-25T09:12:15.112
Code: RAMSES
output [300] summary:
mtime: 2023-04-09T05:34:09
ctime: 2025-06-21T18:31:24.020
=======================================================
simulation time: 445.89 [Myr]
boxlen: 48.0 [kpc]
ncpu: 640
ndim: 3
cosmological:  false
-------------------------------------------------------
amr:           true
level(s): 6 - 10 --> cellsize(s): 750.0 [pc] - 46.88 [pc]
-------------------------------------------------------
hydro:         true
hydro-variables:  7  --> (:rho, :vx, :vy, :vz, :p, :scalar_00, :scalar_01)
hydro-descriptor: (:density, :velocity_x, :velocity_y, :velocity_z, :pressure, :scalar_00, :scalar_01)
γ: 1.6667
-------------------------------------------------------
gravity:       true
gravity-variables: (:epot, :ax, :ay, :az)
-------------------------------------------------------
particles:     true
- Nstars:   5.445150e+05
particle-variables: 7  --> (:vx, :vy, :vz, :mass, :family, :tag, :birth)
particle-descriptor: (:position_x, :position_y, :position_z, :velocity_x, :velocity_y, :velocity_z, :mass, :identity, :levelp, :family, :tag, :birth_time)
-------------------------------------------------------
rt:            false
clumps:           false
-------------------------------------------------------
namelist-file: ("&COOLING_PARAMS", "&SF_PARAMS", "&AMR_PARAMS", "&BOUNDARY_PARAMS", "&OUTPUT_PARAMS", "&POISSON_PARAMS", "&RUN_PARAMS", "&FEEDBACK_PARAMS", "&HYDRO_PARAMS", "&INIT_PARAMS", "&REFINE_PARAMS")
-------------------------------------------------------
boundaries:       not periodic (&BOUNDARY_PARAMS closes x, y, z)
timer-file:       true
compilation-file: false
makefile:         true
patchfile:        true
=======================================================

Store the Data Into JLD2 Files

The running number is taken from the original RAMSES outputs.

savedata(gas, "$MERA_EXAMPLES/MERA-FILES/JLD2_files/");

[Mera]: 2026-10-25T09:13:36.448
Not existing file: output_00300.jld2
Directory: /Volumes/FASTStorage/Simulations/Mera-Tests/RAMSES/mw_L10
-----------------------------------
merafile_version: 1.0  -  Simulation code: RAMSES
-----------------------------------
DataType: hydro  -  Data variables: (:level, :cx, :cy, :cz, :rho, :vx, :vy, :vz, :p, :scalar_00, :scalar_01)
-----------------------------------
I/O mode: nothing  -  Compression: false
-----------------------------------
-----------------------------------
Memory size: 2.321 GB (uncompressed)
-----------------------------------

<div class="alert alert-block alert-info"> <b>NOTE</b> The hydro data was not written into the file to prevent overwriting existing files.

The following argument is mandatory: fmode=:write </div>

savedata(gas, "$MERA_EXAMPLES/MERA-FILES/JLD2_files/", fmode=:write);

[Mera]: 2026-10-25T09:13:38.204
Create file: output_00300.jld2
Directory: /Volumes/FASTStorage/Simulations/Mera-Tests/RAMSES/mw_L10
-----------------------------------
merafile_version: 1.0  -  Simulation code: RAMSES
-----------------------------------
DataType: hydro  -  Data variables: (:level, :cx, :cy, :cz, :rho, :vx, :vy, :vz, :p, :scalar_00, :scalar_01)
-----------------------------------
I/O mode: write  -  Compression: JLD2Lz4.Lz4Filter(0x40000000)
-----------------------------------
JLD2  0.6.5
CodecBzip2  0.8.5
CodecZlib  0.7.8
CodecLz4  0.4.6
Mera  1.8.3
[Mera] Hint: output_00300.jld2.meta.json records where output_00300.jld2 came from.
             readsidecar("output_00300.jld2") shows it; sidecar(false) stops writing them.
             (shown once per session; verbose(false) silences Mera's messages)
-----------------------------------
Memory size: 2.321 GB (uncompressed)
Total file size: 1.275 GB
-----------------------------------

Next to output_00300.jld2, Mera also writes output_00300.jld2.meta.json, its sidecar: a small readable record of what the file holds and where it came from, with the snapshot, the SHA-256 of the file and the Julia and Mera that ran. readsidecar(file) prints it; an :append below adds each new data type to it. Sharing Results & Sidecars explains it in full.

Add/Append further datatypes:

savedata(part, "$MERA_EXAMPLES/MERA-FILES/JLD2_files/", fmode=:append);
savedata(grav, "$MERA_EXAMPLES/MERA-FILES/JLD2_files/", fmode=:append);

[Mera]: 2026-10-25T09:13:53.020
Create file: output_00300.jld2
Directory: /Volumes/FASTStorage/Simulations/Mera-Tests/RAMSES/mw_L10
-----------------------------------
merafile_version: 1.0  -  Simulation code: RAMSES
-----------------------------------
DataType: particles  -  Data variables: (:level, :x, :y, :z, :id, :family, :tag, :vx, :vy, :vz, :mass, :birth)
-----------------------------------
I/O mode: append  -  Compression: JLD2Lz4.Lz4Filter(0x40000000)
-----------------------------------
JLD2  0.6.5
CodecBzip2  0.8.5
CodecZlib  0.7.8
CodecLz4  0.4.6
Mera  1.8.3
-----------------------------------
Memory size: 38.512 MB (uncompressed)
Total file size: 1.306 GB
-----------------------------------
[Mera]: 2026-10-25T09:13:58.345
Create file: output_00300.jld2
Directory: /Volumes/FASTStorage/Simulations/Mera-Tests/RAMSES/mw_L10
-----------------------------------
merafile_version: 1.0  -  Simulation code: RAMSES
-----------------------------------
DataType: gravity  -  Data variables: (:level, :cx, :cy, :cz, :epot, :ax, :ay, :az)
-----------------------------------
I/O mode: append  -  Compression: JLD2Lz4.Lz4Filter(0x40000000)
-----------------------------------
JLD2  0.6.5
CodecBzip2  0.8.5
CodecZlib  0.7.8
CodecLz4  0.4.6
Mera  1.8.3
-----------------------------------
Memory size: 1.688 GB (uncompressed)
Total file size: 2.158 GB
-----------------------------------

<div class="alert alert-block alert-info"> <b>NOTE</b> It is not possible to exchange stored data; only writing into a new file or appending is supported. </div>

Overview of Stored Data

vd = viewdata(300, "$MERA_EXAMPLES/MERA-FILES/JLD2_files/")

[Mera]: 2026-10-25T09:14:11.740
Mera-file output_00300.jld2 contains:
Datatype: particles
merafile_version: 1.0
Compression: JLD2Lz4.Lz4Filter(0x40000000)
CodecZlib: VersionNumber[v"0.7.8"]
merafile_version: 1.0
JLD2: VersionNumber[v"0.6.5"]
CodecBzip2: VersionNumber[v"0.8.5"]
JLD2compatible_versions: (lower = v"0.1.0", upper = v"0.3.0")
CodecLz4: VersionNumber[v"0.4.6"]
Mera: VersionNumber[v"1.8.3"]
-------------------------
Memory: 38.511982917785645 MB (uncompressed)
Datatype: gravity
merafile_version: 1.0
Compression: JLD2Lz4.Lz4Filter(0x40000000)
CodecZlib: VersionNumber[v"0.7.8"]
merafile_version: 1.0
JLD2: VersionNumber[v"0.6.5"]
CodecBzip2: VersionNumber[v"0.8.5"]
JLD2compatible_versions: (lower = v"0.1.0", upper = v"0.3.0")
CodecLz4: VersionNumber[v"0.4.6"]
Mera: VersionNumber[v"1.8.3"]
-------------------------
Memory: 1.6881439564749599 GB (uncompressed)
Datatype: hydro
merafile_version: 1.0
Compression: JLD2Lz4.Lz4Filter(0x40000000)
CodecZlib: VersionNumber[v"0.7.8"]
merafile_version: 1.0
JLD2: VersionNumber[v"0.6.5"]
CodecBzip2: VersionNumber[v"0.8.5"]
JLD2compatible_versions: (lower = v"0.1.0", upper = v"0.3.0")
CodecLz4: VersionNumber[v"0.4.6"]
Mera: VersionNumber[v"1.8.3"]
-------------------------
Memory: 2.321167817339301 GB (uncompressed)
-----------------------------------
convert stat: false
-----------------------------------
Total file size: 2.158 GB
-----------------------------------

Dict{Any, Any} with 4 entries:
  "particles" => Dict{Any, Any}("versions"=>Dict{Any, Any}("CodecZlib"=>Version…
  "FileSize"  => (2.158, "GB")
  "gravity"   => Dict{Any, Any}("versions"=>Dict{Any, Any}("CodecZlib"=>Version…
  "hydro"     => Dict{Any, Any}("versions"=>Dict{Any, Any}("CodecZlib"=>Version…

Information about the content, etc. is returned in a dictionary.

Get a detailed tree-view of the data-file:

vd = viewdata(300, "$MERA_EXAMPLES/MERA-FILES/JLD2_files/", showfull=true)

[Mera]: 2026-10-25T09:14:12.426
Mera-file output_00300.jld2 contains:
 ├─📂 hydro
 │  ├─🔢 data
 │  ├─🔢 info
 │  └─📂 information
 │     ├─🔢 compression
 │     ├─🔢 comments
 │     ├─🔢 storage
 │     ├─🔢 memory
 │     └─📂 versions
 │        ├─🔢 merafile_version
 │        ├─🔢 JLD2compatible_versions
 │        ├─🔢 JLD2
 │        ├─🔢 CodecBzip2
 │        ├─🔢 CodecZlib
 │        ├─🔢 CodecLz4
 │        └─🔢 Mera
 ├─📂 particles
 │  ├─🔢 data
 │  ├─🔢 info
 │  └─📂 information
 │     ├─🔢 compression
 │     ├─🔢 comments
 │     ├─🔢 storage
 │     ├─🔢 memory
 │     └─📂 versions
 │        ├─🔢 merafile_version
 │        ├─🔢 JLD2compatible_versions
 │        ├─🔢 JLD2
 │        ├─🔢 CodecBzip2
 │        ├─🔢 CodecZlib
 │        ├─🔢 CodecLz4
 │        └─🔢 Mera
 └─📂 gravity
    ├─🔢 data
    ├─🔢 info
    └─📂 information
       ├─🔢 compression
       ├─🔢 comments
       ├─🔢 storage
       ├─🔢 memory
       └─📂 versions
          ├─🔢 merafile_version
          ├─🔢 JLD2compatible_versions
          ├─🔢 JLD2
          ├─🔢 CodecBzip2
          ├─🔢 CodecZlib
          ├─🔢 CodecLz4
          └─🔢 Mera
Datatype: particles
merafile_version: 1.0
Compression: JLD2Lz4.Lz4Filter(0x40000000)
CodecZlib: VersionNumber[v"0.7.8"]
merafile_version: 1.0
JLD2: VersionNumber[v"0.6.5"]
CodecBzip2: VersionNumber[v"0.8.5"]
JLD2compatible_versions: (lower = v"0.1.0", upper = v"0.3.0")
CodecLz4: VersionNumber[v"0.4.6"]
Mera: VersionNumber[v"1.8.3"]
-------------------------
Memory: 38.511982917785645 MB (uncompressed)
Datatype: gravity
merafile_version: 1.0
Compression: JLD2Lz4.Lz4Filter(0x40000000)
CodecZlib: VersionNumber[v"0.7.8"]
merafile_version: 1.0
JLD2: VersionNumber[v"0.6.5"]
CodecBzip2: VersionNumber[v"0.8.5"]
JLD2compatible_versions: (lower = v"0.1.0", upper = v"0.3.0")
CodecLz4: VersionNumber[v"0.4.6"]
Mera: VersionNumber[v"1.8.3"]
-------------------------
Memory: 1.6881439564749599 GB (uncompressed)
Datatype: hydro
merafile_version: 1.0
Compression: JLD2Lz4.Lz4Filter(0x40000000)
CodecZlib: VersionNumber[v"0.7.8"]
merafile_version: 1.0
JLD2: VersionNumber[v"0.6.5"]
CodecBzip2: VersionNumber[v"0.8.5"]
JLD2compatible_versions: (lower = v"0.1.0", upper = v"0.3.0")
CodecLz4: VersionNumber[v"0.4.6"]
Mera: VersionNumber[v"1.8.3"]
-------------------------
Memory: 2.321167817339301 GB (uncompressed)
-----------------------------------
convert stat: false
-----------------------------------
Total file size: 2.158 GB
-----------------------------------

Dict{Any, Any} with 4 entries:
  "particles" => Dict{Any, Any}("versions"=>Dict{Any, Any}("CodecZlib"=>Version…
  "FileSize"  => (2.158, "GB")
  "gravity"   => Dict{Any, Any}("versions"=>Dict{Any, Any}("CodecZlib"=>Version…
  "hydro"     => Dict{Any, Any}("versions"=>Dict{Any, Any}("CodecZlib"=>Version…

Get Info

The following function infodata is comparable to getinfo() used for the RAMSES files and loads detailed information about the simulation output:

info = infodata(300, "$MERA_EXAMPLES/MERA-FILES/JLD2_files/");

[Mera]: 2026-10-25T09:14:12.588
Use datatype: hydro
Code: RAMSES
output [300] summary:
mtime: 2023-04-09T05:34:09
ctime: 2025-06-21T18:31:24.020
=======================================================
simulation time: 445.89 [Myr]
boxlen: 48.0 [kpc]
ncpu: 640
ndim: 3
cosmological:  false
-------------------------------------------------------
amr:           true
level(s): 6 - 10 --> cellsize(s): 750.0 [pc] - 46.88 [pc]
-------------------------------------------------------
hydro:         true
hydro-variables:  7  --> (:rho, :vx, :vy, :vz, :p, :scalar_00, :scalar_01)
hydro-descriptor: (:density, :velocity_x, :velocity_y, :velocity_z, :pressure, :scalar_00, :scalar_01)
γ: 1.6667
-------------------------------------------------------
gravity:       true
gravity-variables: (:epot, :ax, :ay, :az)
-------------------------------------------------------
particles:     true
- Nstars:   5.445150e+05
particle-variables: 7  --> (:vx, :vy, :vz, :mass, :family, :tag, :birth)
particle-descriptor: (:position_x, :position_y, :position_z, :velocity_x, :velocity_y, :velocity_z, :mass, :identity, :levelp, :family, :tag, :birth_time)
-------------------------------------------------------
rt:            false
clumps:           false
-------------------------------------------------------
namelist-file: ("&COOLING_PARAMS", "&HYDRO_PARAMS", "&SF_PARAMS", "&AMR_PARAMS", "&BOUNDARY_PARAMS", "&OUTPUT_PARAMS", "&POISSON_PARAMS", "&INIT_PARAMS", "&RUN_PARAMS", "&FEEDBACK_PARAMS", "&REFINE_PARAMS")
-------------------------------------------------------
boundaries:       not periodic (&BOUNDARY_PARAMS closes x, y, z)
timer-file:       true
compilation-file: false
makefile:         true
patchfile:        true
=======================================================

In this case, it loaded the InfoDataType from the hydro data. Choose a different stored datatype to get the info from:

info = infodata(300, "$MERA_EXAMPLES/MERA-FILES/JLD2_files/", :particles);

[Mera]: 2026-10-25T09:14:13.272
Use datatype: particles
Code: RAMSES
output [300] summary:
mtime: 2023-04-09T05:34:09
ctime: 2025-06-21T18:31:24.020
=======================================================
simulation time: 445.89 [Myr]
boxlen: 48.0 [kpc]
ncpu: 640
ndim: 3
cosmological:  false
-------------------------------------------------------
amr:           true
level(s): 6 - 10 --> cellsize(s): 750.0 [pc] - 46.88 [pc]
-------------------------------------------------------
hydro:         true
hydro-variables:  7  --> (:rho, :vx, :vy, :vz, :p, :scalar_00, :scalar_01)
hydro-descriptor: (:density, :velocity_x, :velocity_y, :velocity_z, :pressure, :scalar_00, :scalar_01)
γ: 1.6667
-------------------------------------------------------
gravity:       true
gravity-variables: (:epot, :ax, :ay, :az)
-------------------------------------------------------
particles:     true
- Nstars:   5.445150e+05
particle-variables: 7  --> (:vx, :vy, :vz, :mass, :family, :tag, :birth)
particle-descriptor: (:position_x, :position_y, :position_z, :velocity_x, :velocity_y, :velocity_z, :mass, :identity, :levelp, :family, :tag, :birth_time)
-------------------------------------------------------
rt:            false
clumps:           false
-------------------------------------------------------
namelist-file: ("&COOLING_PARAMS", "&HYDRO_PARAMS", "&SF_PARAMS", "&AMR_PARAMS", "&BOUNDARY_PARAMS", "&OUTPUT_PARAMS", "&POISSON_PARAMS", "&INIT_PARAMS", "&RUN_PARAMS", "&FEEDBACK_PARAMS", "&REFINE_PARAMS")
-------------------------------------------------------
boundaries:       not periodic (&BOUNDARY_PARAMS closes x, y, z)
timer-file:       true
compilation-file: false
makefile:         true
patchfile:        true
=======================================================

Load The Data from JLD2

Full Data

gas = loaddata(300, "$MERA_EXAMPLES/MERA-FILES/JLD2_files/", :hydro);

[Mera]: 2026-10-25T09:14:13.328
Open Mera-file output_00300.jld2:
domain:
xmin::xmax: 0.0 :: 1.0  	==> 0.0 [kpc] :: 48.0 [kpc]
ymin::ymax: 0.0 :: 1.0  	==> 0.0 [kpc] :: 48.0 [kpc]
zmin::zmax: 0.0 :: 1.0  	==> 0.0 [kpc] :: 48.0 [kpc]
Memory used for data table :2.321167668327689 GB
-------------------------------------------------------

typeof(gas)

HydroDataType

part = loaddata(300, "$MERA_EXAMPLES/MERA-FILES/JLD2_files/", :particles);

[Mera]: 2026-10-25T09:14:14.487
Open Mera-file output_00300.jld2:
domain:
xmin::xmax: 0.0 :: 1.0  	==> 0.0 [kpc] :: 48.0 [kpc]
ymin::ymax: 0.0 :: 1.0  	==> 0.0 [kpc] :: 48.0 [kpc]
zmin::zmax: 0.0 :: 1.0  	==> 0.0 [kpc] :: 48.0 [kpc]
Memory used for data table :38.51209735870361 MB
-------------------------------------------------------

typeof(part)

PartDataType

Data Range

Complete data is loaded, and the selected subregion is returned:

gas = loaddata(300, "$MERA_EXAMPLES/MERA-FILES/JLD2_files/", :hydro,
                    xrange=[-10,10],
                    yrange=[-10,10], zrange=[-2,2],
                    center=[:boxcenter],
                    range_unit=:kpc);

[Mera]: 2026-10-25T09:14:14.739
Open Mera-file output_00300.jld2:
center: [0.5, 0.5, 0.5] ==> [24.0 [kpc] :: 24.0 [kpc] :: 24.0 [kpc]]
domain:
xmin::xmax: 0.2916667 :: 0.7083333  	==> 14.0 [kpc] :: 34.0 [kpc]
ymin::ymax: 0.2916667 :: 0.7083333  	==> 14.0 [kpc] :: 34.0 [kpc]
zmin::zmax: 0.4583333 :: 0.5416667  	==> 22.0 [kpc] :: 26.0 [kpc]
Memory used for data table :580.3606014251709 MB
-------------------------------------------------------

Convert RAMSES Output Into JLD2

Existing AMR, hydro, gravity, particle, and clump data is sequentially stored in a JLD2 file. The individual loading/writing processes are timed, and the memory usage is returned in a dictionary:

Full Data

cvd = convertdata(300, path="$MERA_EXAMPLES/RAMSES/mw_L10",
                  fpath="$MERA_EXAMPLES/MERA-FILES/JLD2_files/");

[Mera]: 2026-10-25T09:14:16.941
Requested datatypes: [:hydro, :gravity, :particles, :clumps, :rt, :sinks]
Max threads: 4 of 4 available
Threading applied to: hydro, gravity, particles
Threading NOT applied to: clumps (single-threaded by design)
domain:
xmin::xmax: 0.0 :: 1.0  	==> 0.0 [kpc] :: 48.0 [kpc]
ymin::ymax: 0.0 :: 1.0  	==> 0.0 [kpc] :: 48.0 [kpc]
zmin::zmax: 0.0 :: 1.0  	==> 0.0 [kpc] :: 48.0 [kpc]
reading/writing lmax: 10 of 10
-----------------------------------
Compression: JLD2Lz4.Lz4Filter(0x40000000)
-----------------------------------
- hydro (threaded: max_threads=4)
Processing files: 100%|██████████████████████████████████████████████████| Time: 0:00:18 (28.29 ms/it)
✓ File processing complete! Combining results...
- gravity (threaded: max_threads=4)
Processing files: 100%|██████████████████████████████████████████████████| Time: 0:00:13 (21.81 ms/it)
✓ File processing complete! Combining results...
- particles (threaded: max_threads=4)
Final Statistics:
================
- total folder size: 5.682 GB
- selected data size: 5.68 GB
- peak memory used: 4.047 GB
- compressed file size: 2.158 GB
- compression ratio: 0.38
- data reduction: 62.0%
- total processing time: 108.61 seconds
- effective threads: 4

Timer

Get a view of the timers:

using Mera.TimerOutputs

cvd

Dict{Any, Any} with 5 entries:
  "threading"    => Dict{Any, Any}("max_threads_requested"=>4, "julia_version"=…
  "viewdata"     => Dict{Any, Any}("particles"=>Dict{Any, Any}("versions"=>Dict…
  "size"         => Dict{Any, Any}("folder"=>Any[6101111412, "Bytes"], "selecte…
  "benchmark"    => Dict{Any, Any}("xrange"=>[missing, missing], "subset"=>fals…
  "TimerOutputs" => Dict{Any, Any}("writing"=>─────────────────────────────────…

cvd["TimerOutputs"]["reading"]

──────────────────────────────────────────────────────────────────────
                             Time                    Allocations
                    ───────────────────────   ────────────────────────
 Tot / % measured:        109s /  67.4%            101GiB /  95.1%

Section     ncalls     time    %tot     avg     alloc    %tot      avg
──────────────────────────────────────────────────────────────────────
hydro            1    55.0s   74.9%   55.0s   76.1GiB   79.6%  76.1GiB
gravity          1    16.9s   23.0%   16.9s   17.8GiB   18.6%  17.8GiB
particles        1    1.60s    2.2%   1.60s   1.71GiB    1.8%  1.71GiB
──────────────────────────────────────────────────────────────────────

cvd["TimerOutputs"]["writing"]

──────────────────────────────────────────────────────────────────────
                             Time                    Allocations
                    ───────────────────────   ────────────────────────
 Tot / % measured:        109s /  31.8%            101GiB /   4.8%

Section     ncalls     time    %tot     avg     alloc    %tot      avg
──────────────────────────────────────────────────────────────────────
gravity          1    18.2s   52.3%   18.2s   1.95GiB   40.5%  1.95GiB
hydro            1    9.44s   27.2%   9.44s   2.81GiB   58.4%  2.81GiB
particles        1    7.11s   20.5%   7.11s   54.2MiB    1.1%  54.2MiB
──────────────────────────────────────────────────────────────────────

# prep timer
to = TimerOutput();

@timeit to "MERA" begin
    @timeit to "hydro"     gas = loaddata(300, "$MERA_EXAMPLES/MERA-FILES/JLD2_files/", :hydro, )
    @timeit to "particles" part= loaddata(300, "$MERA_EXAMPLES/MERA-FILES/JLD2_files/", :particles)
end;

[Mera]: 2026-10-25T09:16:06.186
Open Mera-file output_00300.jld2:
domain:
xmin::xmax: 0.0 :: 1.0  	==> 0.0 [kpc] :: 48.0 [kpc]
ymin::ymax: 0.0 :: 1.0  	==> 0.0 [kpc] :: 48.0 [kpc]
zmin::zmax: 0.0 :: 1.0  	==> 0.0 [kpc] :: 48.0 [kpc]
Memory used for data table :2.321167668327689 GB
-------------------------------------------------------
[Mera]: 2026-10-25T09:16:06.959
Open Mera-file output_00300.jld2:
domain:
xmin::xmax: 0.0 :: 1.0  	==> 0.0 [kpc] :: 48.0 [kpc]
ymin::ymax: 0.0 :: 1.0  	==> 0.0 [kpc] :: 48.0 [kpc]
zmin::zmax: 0.0 :: 1.0  	==> 0.0 [kpc] :: 48.0 [kpc]
Memory used for data table :38.51209735870361 MB
-------------------------------------------------------

to

────────────────────────────────────────────────────────────────────────
                               Time                    Allocations
                      ───────────────────────   ────────────────────────
  Tot / % measured:        866ms /  91.0%           5.32GiB /  99.7%

Section       ncalls     time    %tot     avg     alloc    %tot      avg
────────────────────────────────────────────────────────────────────────
MERA               1    788ms  100.0%   788ms   5.30GiB  100.0%  5.30GiB
  hydro            1    773ms   98.0%   773ms   5.22GiB   98.5%  5.22GiB
  particles        1   15.6ms    2.0%  15.6ms   81.2MiB    1.5%  81.2MiB
────────────────────────────────────────────────────────────────────────

<div class="alert alert-block alert-info"> <b>NOTE</b> The reading from JLD2 files is multiple times faster than from the original RAMSES files. </div>

Used Memory

cvd["size"]

Dict{Any, Any} with 4 entries:
  "folder"   => Any[6101111412, "Bytes"]
  "selected" => Any[6.09885e9, "Bytes"]
  "ondisc"   => Any[2317580946, "Bytes"]
  "used"     => Any[4.34535e9, "Bytes"]

<div class="alert alert-block alert-info"> <b>NOTE</b> The compressed JLD2 file takes a significantly smaller disk space than the original RAMSES folder.</div>

factor = cvd["size"]["folder"][1] / cvd["size"]["ondisc"][1]
println("==============================================================================")
println("In this example, the disk space is reduced by a factor of $factor !!")
println("==============================================================================")

==============================================================================
In this example, the disk space is reduced by a factor of 2.632534333926993 !!
==============================================================================

Selected Datatypes

cvd = convertdata(300, [:hydro, :particles],
                  path="$MERA_EXAMPLES/RAMSES/mw_L10",
                  fpath="$MERA_EXAMPLES/MERA-FILES/JLD2_files/");

[Mera]: 2026-10-25T09:16:07.237
Requested datatypes: [:hydro, :particles]
Max threads: 4 of 4 available
Threading applied to: hydro, gravity, particles
Threading NOT applied to: clumps (single-threaded by design)
domain:
xmin::xmax: 0.0 :: 1.0  	==> 0.0 [kpc] :: 48.0 [kpc]
ymin::ymax: 0.0 :: 1.0  	==> 0.0 [kpc] :: 48.0 [kpc]
zmin::zmax: 0.0 :: 1.0  	==> 0.0 [kpc] :: 48.0 [kpc]
reading/writing lmax: 10 of 10
-----------------------------------
Compression: JLD2Lz4.Lz4Filter(0x40000000)
-----------------------------------
- hydro (threaded: max_threads=4)
Processing files: 100%|██████████████████████████████████████████████████| Time: 0:00:26 (41.59 ms/it)
✓ File processing complete! Combining results...
- particles (threaded: max_threads=4)
Final Statistics:
================
- total folder size: 5.682 GB
- selected data size: 4.002 GB
- peak memory used: 2.359 GB
- compressed file size: 1.306 GB
- compression ratio: 0.326
- data reduction: 77.0%
- total processing time: 79.61 seconds
- effective threads: 4

What survives the round trip

A mera file stores the data table, not a reduced summary, so every column comes back exactly as it was read from RAMSES, including the ones that identify what a particle is. :family and :tag are ordinary columns and are written and read back unchanged, so particle-type selection works identically on a mera file and on the original output:

parts = loaddata(3, "path/to/mera_files", :particles)
getparticlemask(parts, :tracer)      # same result as on the RAMSES output
getmask(parts, :family, ==(2.0))     # :family behaves like any other quantity

This is worth knowing before converting a large run: the conversion is lossless for selection purposes, so you do not have to keep the original output around in order to separate dark matter from stars, or tracers from either. The same applies to the values themselves, a round trip reproduces the RAMSES read bit-for-bit, which is one of the properties Mera's own test suite asserts on every snapshot of a public fixture.

Compression

Mera files are LZ4-compressed. This build runs on JLD2 0.6, whose compression is provided by JLD2Lz4, LZ4 is the only codec available. Passing compress=true (or leaving it out) gives you LZ4; passing compress=false writes uncompressed.

Other compressors are accepted but substituted

For backwards compatibility, ZlibCompressor and Bzip2Compressor are still accepted as arguments, but Mera warns and writes LZ4 instead, so do not choose one expecting a smaller file. The cells below demonstrate exactly that: watch for the warning in the output.

Argument	What you get
`compress=true` (or omitted)	LZ4, the default
`compress=false`	no compression
`ZlibCompressor()` / `Bzip2Compressor()`	accepted, but substituted with LZ4 and a warning
any other JLD2-accepted filter	passed through to JLD2 unchanged

Passing a compressor Mera cannot use shows what happens, the call succeeds, the file is written, and the warning tells you the codec was swapped:

using Mera.CodecZlib
cvd = convertdata(300, [:hydro, :particles], compress=ZlibCompressor(),
                  path="$MERA_EXAMPLES/RAMSES/mw_L10",
                  fpath="$MERA_EXAMPLES/MERA-FILES/JLD2_files/");

[Mera]: 2026-10-25T09:17:27.225
Requested datatypes: [:hydro, :particles]
Max threads: 4 of 4 available
Threading applied to: hydro, gravity, particles
Threading NOT applied to: clumps (single-threaded by design)
domain:
xmin::xmax: 0.0 :: 1.0  	==> 0.0 [kpc] :: 48.0 [kpc]
ymin::ymax: 0.0 :: 1.0  	==> 0.0 [kpc] :: 48.0 [kpc]
zmin::zmax: 0.0 :: 1.0  	==> 0.0 [kpc] :: 48.0 [kpc]
┌ Warning: This Mera build (JLD2 0.6) supports LZ4 compression only — using LZ4 instead of ZlibCompressor.
└ @ Mera ~/code-github/Mera.jl/src/functions/data/data_save.jl:321
reading/writing lmax: 10 of 10
-----------------------------------
Compression: JLD2Lz4.Lz4Filter(0x40000000)
-----------------------------------
- hydro (threaded: max_threads=4)
Processing files: 100%|██████████████████████████████████████████████████| Time: 0:00:17 (27.18 ms/it)
✓ File processing complete! Combining results...
- particles (threaded: max_threads=4)
Final Statistics:
================
- total folder size: 5.682 GB
- selected data size: 4.002 GB
- peak memory used: 2.359 GB
- compressed file size: 1.306 GB
- compression ratio: 0.326
- data reduction: 77.0%
- total processing time: 70.63 seconds
- effective threads: 4

savedata(gas, "$MERA_EXAMPLES/MERA-FILES/JLD2_files/",
            fmode=:write, compress=ZlibCompressor());

[Mera]: 2026-10-25T09:18:37.894
Create file: output_00300.jld2
Directory: /Volumes/FASTStorage/Simulations/Mera-Tests/RAMSES/mw_L10
-----------------------------------
merafile_version: 1.0  -  Simulation code: RAMSES
-----------------------------------
DataType: hydro  -  Data variables: (:level, :cx, :cy, :cz, :rho, :vx, :vy, :vz, :p, :scalar_00, :scalar_01)
-----------------------------------
I/O mode: write  -  Compression: JLD2Lz4.Lz4Filter(0x40000000)
-----------------------------------
JLD2  0.6.5
CodecBzip2  0.8.5
CodecZlib  0.7.8
CodecLz4  0.4.6
Mera  1.8.3
-----------------------------------
Memory size: 2.321 GB (uncompressed)
Total file size: 1.275 GB
-----------------------------------

Get more information about the parameters of the compressor:

?ZlibCompressor

search: ZlibCompressor ZlibDecompressor GzipCompressor ZlibCompressorStream

  ZlibCompressor(;level=-1, windowbits=15)

  Create a zlib compression codec.

  Arguments
  =========

    •  level (-1..9): compression level. 1 gives best speed, 9 gives best
       compression, 0 gives no compression at all (the input data is
       simply copied a block at a time). -1 requests a default compromise
       between speed and compression (currently equivalent to level 6).
    •  windowbits (9..15): size of history buffer is 2^windowbits.

  │ Warning
  │
  │  serialize and deepcopy will not work with this codec due to stored
  │  raw pointers.

Comments

Add a description to the files:

comment = "The simulation is...."
cvd = convertdata(300, [:hydro, :particles], comments=comment,
                  path="$MERA_EXAMPLES/RAMSES/mw_L10",
                  fpath="$MERA_EXAMPLES/MERA-FILES/JLD2_files/");

[Mera]: 2026-10-25T09:18:50.666
Requested datatypes: [:hydro, :particles]
Max threads: 4 of 4 available
Threading applied to: hydro, gravity, particles
Threading NOT applied to: clumps (single-threaded by design)
domain:
xmin::xmax: 0.0 :: 1.0  	==> 0.0 [kpc] :: 48.0 [kpc]
ymin::ymax: 0.0 :: 1.0  	==> 0.0 [kpc] :: 48.0 [kpc]
zmin::zmax: 0.0 :: 1.0  	==> 0.0 [kpc] :: 48.0 [kpc]
reading/writing lmax: 10 of 10
-----------------------------------
Compression: JLD2Lz4.Lz4Filter(0x40000000)
-----------------------------------
- hydro (threaded: max_threads=4)
Processing files: 100%|██████████████████████████████████████████████████| Time: 0:00:26 (41.15 ms/it)
✓ File processing complete! Combining results...
- particles (threaded: max_threads=4)
Final Statistics:
================
- total folder size: 5.682 GB
- selected data size: 4.002 GB
- peak memory used: 2.359 GB
- compressed file size: 1.306 GB
- compression ratio: 0.326
- data reduction: 77.0%
- total processing time: 79.31 seconds
- effective threads: 4

comment = "The simulation is...."
savedata(gas, "$MERA_EXAMPLES/MERA-FILES/JLD2_files/", comments=comment, fmode=:write);

[Mera]: 2026-10-25T09:20:10.009
Create file: output_00300.jld2
Directory: /Volumes/FASTStorage/Simulations/Mera-Tests/RAMSES/mw_L10
-----------------------------------
merafile_version: 1.0  -  Simulation code: RAMSES
-----------------------------------
DataType: hydro  -  Data variables: (:level, :cx, :cy, :cz, :rho, :vx, :vy, :vz, :p, :scalar_00, :scalar_01)
-----------------------------------
I/O mode: write  -  Compression: JLD2Lz4.Lz4Filter(0x40000000)
-----------------------------------
JLD2  0.6.5
CodecBzip2  0.8.5
CodecZlib  0.7.8
CodecLz4  0.4.6
Mera  1.8.3
-----------------------------------
Memory size: 2.321 GB (uncompressed)
Total file size: 1.275 GB
-----------------------------------

Load the comment (hydro) from JLD2 file:

vd = viewdata(300, "$MERA_EXAMPLES/MERA-FILES/JLD2_files/", verbose=false);

vd["hydro"]["comments"]

"The simulation is...."