To improve read performance, ZFS uses system memory as an Adaptive Replacement Cache (ARC). System memory stores the most frequent and recent ZFS pool data used. Layer 2 ARC (L2ARC) is another option to extend the ARC to dedicated disks for dramatic improvement in read speeds verses slower data disks in the pool.
Another OpenZFS performance feature is the ZFS intent log (ZIL). The ZIL writes synchronous transactions to disk in special pre-allocated space so ZFS can confirm that data is on non-volatile storage, providing lower latency for synchronous writes.
Over-provisioning SLOG SSDs is useful for different scenarios. The most useful benefit of over-provisioning is greatly extending SSD life. Over-provisioning an SSD distributes the total number of writes and erases across more flash blocks on the drive.
Seagate provides a thoughtful investigation into over-provisioning SSDs here: https://www.seagate.com/blog/ssd-over-provisioning-benefits-master-ti/.
Some SATA devices are limited to one resize per power cycle. Some BIOS can block resize during boot and require a live power cycle.
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