[Accepted] Create 2 GiB buffer space when adding a disk

[winnielinnie]

@yorick

After discussion, a good way to implement may be to just make the partitions on drives added to a new vdev 2GB smaller than max, without a dedicated “buffer partition”. Then when replacing or adding to an existing vdev, use the same logic of “2GB smaller than max”, and if that’s smaller than the smallest member in the vdev, then if possible make the partition as large as that member, to allow the replacement to succeed.

No additional UI element is needed. Replacement would still show an error when the drive is genuinely too small; while the slight variations in drive size between revisions or manufacturers is taken care of by this 2GB buffer.

Would you say these graphics explain the process that TrueNAS should do?

* Not drawn to scale

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Legend:

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New disks are purchased. They are all “12-TiB”.

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When a pool/vdev is created, TrueNAS makes a single partition that is the total capacity of the drive minus 2 GiB. Since this is a brand new vdev, the buffer space is maxed out to 2 GiB.

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After some time, one of these drives fail.

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A new “12-TiB” drive is purchased and installed in the server. Even though it is a “12-TiB” drive, its capacity is slightly smaller than the existing drives.

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It can still be used as a replacement drive, since there is enough buffer on the current drives. TrueNAS creates a partition on the replacement drive that is equal in size to the existing partitions. The leftover space is used as a buffer.

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We throw a party to celebrate?