I have a pair of 3TB drives in a btrfs filesystem, and I wish to expand this filesystem by adding two more 2TB drives, using the btrfs device add command. Data in the current filesystem is mirrored in raid1 mode.

I am hoping the resulting filesystem will have 5TB usable capacity with all data in raid1 mode.

But, the official wiki states:

When you have drives with differing sizes and want to use the full capacity of each drive, you have to use the single profile for the data blocks, rather than raid0.

But, this answer suggests that there may have been a patch to use size-mismatched devices to full capacity with full data mirroring:

There is a quasi-round-robin patch in the pipeline to improve on this. Of course, it is still impossible to pair all chunks on different devices if you have a 500GB and a 2TB disk. The patch is intended more for situations like 1 x 1TB + 2 x 500GB where each small disk should prefer to mirror/stripe with the big disk instead of the other small disk.

Which is correct?


According to the wiki you will end up with 5 TB after rebalancing it.

Keep in mind that Btrfs has no triple parity yet.

Take a look at the btrfs disk usage calculator: http://carfax.org.uk/btrfs-usage/


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