Should I use a thin volume or a thick volume?

All NAS Series, QTS

Thin Volume: Allocate space to Volume when space is needed. For those who value space efficiency or want to protect data with snapshots 

Thick Volume: For those who want to pre-reserve space for specific needs. It will bring a slightly better performance.

Thick volumes are recommended if you are creating multiple volumes but need to guarantee the space for a particular volume. The performance of a thick volume may also be slightly better than a thin volume in some situations. Thick Volume is suitable for applications that require sufficient pre-configured space and services that cannot be interrupted, such as surveillance systems, databases, etc. 

Thin volumes are recommended when you need to create multiple volumes and share the storage pool space efficiently between them. Also, thin volumes are recommended if you are planning to use volume snapshots. When using snapshots with thin volumes, only modifications to existing data on the volume will increase the snapshot used space, the size will be the same as the modified data. 

Note: If Thick Volume is used with snapshots, the storage pool must reserve enough free space for snapshots, otherwise it may run into Read Delete mode and cause operation interruption.

Thick, Thin, and Static Volumes | QTS 5.0.x (qnap.com)