Basically a junction of 2 or more drives or other vdev shapes.
When created together, with equal capacity, ZFS space-balancing makes a span act like a raid0 stripe.
Basically, the space is added together. That's the important point.
Basically, a stripe occurs where all the devices/vdevs are initialized into the pool simultaneously.
Provided all the devices are of the same size, the stripe behavior will continue regardless of fullness level.
If devices/vdevs are not equally sized, then they will fill mostly equally until one device/vdev is full. The stripe behavior stops at that point and the other device(s)/vdev(s) with more capacity will be used more.
A span occurs where the pool starts with a certain number of devices or vdevs and more devices or vdevs are added later. The newer-added devices start empty. ZFS makes no effort to balance the data in the pool across those devices or vdevs. This is not a design flaw. As more data is copied to the pool ZFS will start filling with the least full device/vdev to a point where the devices/vdevs are roughly equally full. Once all devices/vdevs are roughly equally full, it will start to behave in a stripe-like behavior.
While behaving in a stripe-like manner, here are some pros and cons:
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