That'd be the ZFS ARC cache, you can adjust the usage, but as another member said it's probably 74% of the available ZFS (RAM) cache.
ZFS has 2 caching tiers.
- ARC: Uses upto X% of available RAM (I think unRAID has this set as 1/8th of the total system RAM, you can adjust it)
- L2ARC: This can be enabled to act as a secondary (larger, but slower) cache, usually placed on an NVME/SSD drive -- as items fall out of the ARC cache, they can end up in L2ARC if enabled.
- These are considered volatile. Not a big issue with read workloads, but worth noting for write heavy workloads. ZFS groups transactions into a memory buffer and then writes to disk. So you want some form of battery backup, by default I think the write buffer fires every ~5 seconds. Again, not a concern if you're doing low-writes and heavy-reads (like a media server).
So as you read data it gets loaded into the ARC cache and stays until it's evicted. If you have L2ARC enabled, it would then move to that. This is why ZFS is really good at read access / heavy read workloads, because after the first I/O hits, it's stays in hot memory.
JorgeB gave a great answer. Once you're past the Pool/Vdev config, you have datasets which appear as just normal folders, but are a collection within the larger pool. You can also nest datasets within datasets if you want to get granular with your snapshots.
For example, on my system I have 4x4TB drives configured into 1 ZFS pool with 2 vdev mirror groups of 2 drives (~7TB of usable space).
- A few side notes, LZ4 compression is really good, and there's been some tests that show it can perform faster (I/O wise) when enabled.
- If you're worried about redundancy, Vdev mirrors are the way to go. They can be a tad more friendly. See: https://jrs-s.net/2015/02/06/zfs-you-should-use-mirror-vdevs-not-raidz/
- To the end-user, this just looks like a folder (zfspool) of folders.
You can get an idea for the flexibility of this in the screenshots below, using the Sanoid plugin to handle scheduling of snapshots of sub datasets for individual docker/VM's running. Interface is from the ZFS Master plugin. I imagine a lot of these plugin features will eventually make their way into native unRAID at some point, since ZFS already provides this stuff - it's just the interface.