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Here is the paper in question:

Umbra: A Disk-Based System with In-Memory Performance

Section 3.1 covers string handling.

This article (also linked from tfa) explains German strings in more detail.

my tl;dr: after reading the article:

- two 64-bits words representation

- fixed, 32 bits length

- short strings (<12 bytes) are stored in-place

- long strings store a 4 byte prefix in-place + pointer to the rest

- two bits are used as flags in the pointer to further optimize some use-cases

Seems like they missed an opportunity to have a 8 byte version for strings that fit in the 4 byte prefix.