struggling with a nested flexbox grid

Question

i tried to achieve a grid like pattern with a negative margin based grid system (susy) and failed.

I tried to use flexbox, but i am not sure if its really possible, i thought the best approach would be 2 columns (side A and B) and give the boxes (1) the flex height of 50% of boxes 2, but it somehow doesn't seem to work.

this is as far as i got it working.

.block {
  width: 100%;
  background: grey
}

.column {
    align-content: stretch;
    display: flex;
    flex-flow: column wrap;
    width: 50%;
}

.box_long {
  background-color: pink;
  flex: 0 0 50%;
  padding: 20px;
  border: solid 1px black;
}

.box_small {
  background-color: blue;
  flex: 0 0 25%;
  padding: 20px;
  border: solid 1px black;
}

.box_big {
  background-color: green;
  flex: 0 0 100%;
  padding: 20px;
  border: solid 1px black;
}

<div class="block">
  
  <div class="column">
    <div class="box_long">
    </div>
    <div class="box_big">
    </div>
  </div>

  <div class="column">
    <div class="box_small">
    </div>
    <div class="box_small">
    </div>
    <div class="box_small">
    </div>
    <div class="box_small">
    </div>
    <div class="box_long">
    </div>
  </div>
  
</div>

Answer 1

Is this what you looking for?

* {
  box-sizing: border-box
}

.block {
  background: grey;
  display: flex;
  height: 250px;
}

.column {
  flex: 1;
  display: flex;
  flex-wrap: wrap;
}

.column.col {
  flex-direction: column;
  flex-wrap: nowrap;
}

.column.col div {
  flex: 1;
  border: 1px solid black;
}

.column.row div {
  border: 1px solid black;
  flex-basis: 50%;
  height: 25%;
}

.column.row .box_long {
  height: 50%;
  flex-basis: 100%;
}

.box_long {
  background-color: pink;
}

.box_small {
  background-color: blue;
}

.box_big {
  background-color: green;
}

<div class="block">

  <div class="column col">
    <div class="box_long">
    </div>
    <div class="box_big">
    </div>
  </div>

  <div class="column row">
    <div class="box_small">
    </div>
    <div class="box_small">
    </div>
    <div class="box_small">
    </div>
    <div class="box_small">
    </div>
    <div class="box_long">
    </div>
  </div>

</div>

Answer 2

Here's one method that may work for you:

(No changes to the HTML.)

.block {
  display: flex;
  height: 100vh;
  background: grey;
}

.column {
  display: flex;
  flex-wrap: wrap;
  width: 50%;
}

.box_big {
  flex-basis: 100%;
  height: 70%;
  background-color: chartreuse;
  border: solid 1px black;
}

.box_small {
  flex: 0 0 50%;
  height: 35%;
  background-color: aqua;
  border: solid 1px black;
}

.box_long {
  flex-basis: 100%;
  height: 30%;
  background-color: violet;
  border: solid 1px black;
}

* {
  margin: 0;
  box-sizing: border-box;
}

<div class="block">
  <div class="column">
    <div class="box_long"></div>
    <div class="box_big"></div>
  </div>
  <div class="column">
    <div class="box_small"></div>
    <div class="box_small"></div>
    <div class="box_small"></div>
    <div class="box_small"></div>
    <div class="box_long"></div>
  </div>
</div>

jsFiddle