# JLD2: triggering the actual writes to file
**URL:** https://discourse.julialang.org/t/jld2-triggering-the-actual-writes-to-file/74981
**Category:** Data
**Tags:** jld2
**Created:** [January 21, 2022, 1:10pm UTC](https://discourse.julialang.org/t/jld2-triggering-the-actual-writes-to-file/74981 "2022-01-21T13:10:07Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@Gregstrq](https://discourse.julialang.org/u/Gregstrq)
#### Post date: [January 21, 2022, 1:10pm UTC](https://discourse.julialang.org/t/jld2-triggering-the-actual-writes-to-file/74981/1 "2022-01-21T13:10:07Z")
Let’s say I saved a dataset named “dataset” to “file.jld2”:
```julia
using JLD2, Random
jldopen("file.jld2", "w") do fd
fd["dataset"] = [randn(100,100) for i=1:100]
end
```
Now, I want to alter one of the entries in the dataset:
```julia
jldopen("file.jld2", "a") do fd
ds = fd["dataset"]
ds[1] = randn(100,100)
end
```
- **When is the actual write to the file triggered in the last case? When I do the assignment `ds[1] = randn(100,100)`? Or when the file is closed?**
- **Does it depend on the memory size of the element I am trying to write?** I mean, smaller or larger than buffer.
---
### Author:  [@JonasIsensee](https://discourse.julialang.org/u/JonasIsensee)
#### Post date: [January 21, 2022, 1:21pm UTC](https://discourse.julialang.org/t/jld2-triggering-the-actual-writes-to-file/74981/2 "2022-01-21T13:21:25Z")
Hi @Gregstrq,
the `"a"` really means `append`. Arrays are written immediately after assignment
and it is currently not possible to edit them after that.
This is a limitation of JLD2 (lack of implementation). In principle it might be possible. (Try editing a JLD2 file using the HDF5 libary)
---
### Author:  [@Gregstrq](https://discourse.julialang.org/u/Gregstrq)
#### Post date: [January 21, 2022, 1:24pm UTC](https://discourse.julialang.org/t/jld2-triggering-the-actual-writes-to-file/74981/3 "2022-01-21T13:24:01Z")
Do you know when the writes are triggered in the case of HDF5?
---
### Author:  [@JonasIsensee](https://discourse.julialang.org/u/JonasIsensee)
#### Post date: [January 21, 2022, 3:34pm UTC](https://discourse.julialang.org/t/jld2-triggering-the-actual-writes-to-file/74981/4 "2022-01-21T15:34:28Z")
This usually also happens immediately.
[https://juliaio.github.io/HDF5.jl/stable/#Memory-mapping](https://juliaio.github.io/HDF5.jl/stable/#Memory-mapping)