Jon Smirl sent me a link describing new ZFS feature – data deduplication.
This is a technique which allows to store multiple data objects in the same place when their content is the same, thus effectively saving the space. There are three levels of data deduplication – files (objects actually), blocks and bytes. Every level allows to store single entity for the multiple identical objects, like single block for several equal data blocks or byte range and so on. ZFS supports block deduplication.
This feature existed effectively from the beginning in the elliptics network distributed hash table storage, but it has two levels of data deduplication: object and transaction. Well, actually we have transaction only, but maximum transaction size can be limited to some large enough block (like megabytes or more, or can be unlimited if needed), so if object is smaller than that, it will be deduplicated automatically.
Which basically means that if multiple users write the same content into the storage and use the same ID, no new storage space will be used, instead transaction log for the selected object will be updated to show that two external objects refer to given transaction.
Depending on transaction size it may have a negative impact, in particular when transaction size is smaller than log entry, it will be actually a waste of space, but transactions are required for the log-strucutred filesystem and to implement things like snapshots and update history. By default log entry size equals to 56 bytes, so it should not be a problem in the common case.