Mdthbs

I am having trouble understanding following sentence:

不満を吐き出すだけ吐き出せば、スッキリするだろう。

It is taken from a novel and in this particular scene the characters are complaining about how horrible their trip has been up to this point. The main character is, however, not interested in taking action to better their situation...

According to that I think that this sentence is similar to constructions like 出来るだけ or even やるだけやった.
Therefore, I understand above-stated sentences as something along the lines of
'If they (just) get all of their complaints out, they surely will feel better'

I am highly unsure whether my understanding is correct or a complete miss...

In any case, if somebody could explain meaning and especially the pattern working behind the construction I would be more than a little thankful.

(As English is not my native Language, please turn a blind eye on any language errors)

「不満{ふまん}を吐{は}き出{だ}すだけ吐き出せば、スッキリするだろう。」

There are basically two patterns to this verb-だけ-verb construct.

1) 「Verb in Attributive Form + だけ + Same Verb in any form」

meaning "to [verb] to a degree". That degree is generally fairly limited.

2) 「Verb in Potential Form + だけ + Same Verb in any form」

meaning "to [verb] as much as possible"

"In any form" simply means that the form (of the second verb) changes depending on what you want to say.

「吐き出すだけ吐き出す」 uses pattern #1 above -- "performing the action of 吐き出す to a degree".

This means that you were thinking of the meaning of pattern #2 instead. In other words, you were thinking of the meaning of 「吐き出せるだけ吐き出す」.

Thus the sentence in question means something like:

"If you vent your complaints a little, you will feel better."

When I was passing by a cheap pop-up clothing store a couple of days ago, a clerk was shouting:

「見るだけ見てってくださ～～い！全品{ぜんぴん}お安{やす}くなっておりま～～す！」

That is pattern #1, too. "Please have a look insi~~de!"

Pattern #2 works like below.

「全部食べなくていいよ。食べられるだけ食べて。」

"You don't have to eat all of it. Just eat as much as you can."

You're almost on the nose! This is a construction we use frequently, and while related to できるだけ through the use of だけ, it carries a slightly different meaning due to the use of the plain conjugation (連体形) as opposed to the potential form (可能形).

Take these two examples:

1) 飲めるだけ飲んでみます。

and

2) 飲むだけ飲んでみます。

In 1), the speaker is using だけ exactly how you referenced in the second half of your question.

飲める(だけ) → (All/To the extent/As much as) one can drink

飲んでみます → One will try drinking

So in English, we would translate this to something like:

I'll try drinking as much as I can.

Note the use of "can" as a translation for やれる.

However in 2), the speaker isn't talking about their potential to drink anything; they didn't use the potential form.

If we wanted to analyze this sentence like we did 1), we could say:

飲む(だけ) → (All/To the extent/As much as) one (will) drink

飲んでみます → One will try drinking

So literally:

I'll try drinking as much as I'll drink

It doesn't really sound like the speaker wants to drink anything, does it?

If we wanted a more natural-sounding translation, we could say something like:

I'll give drinking it a shot (but don't expect me to drink all of it)

This to me is the key to understanding やるだけ vs やれるだけ.

With the potential form, we're saying we'll do all we can, but with the plain form, we're only saying we'll do what we do.

Applying this to your example, you give:

不満を吐き出すだけ吐き出せば、スッキリするだろう。

If we instead said:

不満を吐き出せるだけ吐き出せば、スッキリするだろう。

This would carry the meaning:

If they get all of their complaints out, they surely will feel better

However in the plain form, as is, I would translate it closer to:

If they (would) just get their complaints out a little, they'd probably feel better

The "just" and "a little" here are my attempts to show that "they" are probably a bit reluctant to complain.