I'm trying out RAID5 to understand its space usage. First off, I've 3 devices of 2GB each, in RAID5. Old school RAID5 tells me I've 4GB of usable space. Actual fact: I've about 3.5GB, until it tells me I'm out of space. This is understandable, as Metadata and System took up some space.
Next, I tried device add and remove. My "common sense" tells me, I should be able to remove a device of size equal or smaller than one I added. (isn't it simply move all blocks from old device to new?) So I proceeded to add a 4th device of 2GB, and remove the 2nd device (of 2GB). btrfs device delete tells me I'm out of space. Why? Here are my steps: 01. dd if=/dev/zero of=/root/btrfs-test-1 bs=1G count=2 02. losetup /dev/loop1 /root/btrfs-test-1 03. dd if=/dev/zero of=/root/btrfs-test-2 bs=1G count=2 04. losetup /dev/loop2 /root/btrfs-test-2 05. dd if=/dev/zero of=/root/btrfs-test-3 bs=1G count=2 06. losetup /dev/loop3 /root/btrfs-test-3 07. mkfs.btrfs --data raid5 --metadata raid5 --label testbtrfs2 --nodiscard -f /dev/loop1 /dev/loop2 /dev/loop3 08. mount /dev/loop2 /mnt/b 09. dd if=/dev/zero of=/mnt/b/test1g1 bs=1G count=1 10. dd if=/dev/zero of=/mnt/b/test1g2 bs=1G count=1 11. dd if=/dev/zero of=/mnt/b/test1g3 bs=1G count=1 12. dd if=/dev/zero of=/mnt/b/test512M1 bs=512M count=1 13. dd if=/dev/zero of=/root/btrfs-test-4 bs=1G count=2 14. losetup /dev/loop4 /root/btrfs-test-4 15. btrfs device add --nodiscard -f /dev/loop4 /mnt/b 16. btrfs device delete /dev/loop2 /mnt/b My kernel is 4.0.5-gentoo, btrfs-progs is 4.0.1 from Gentoo. AFTER adding /dev/loop4. As can be seen, /dev/loop4 has lots of space, almost 2GB. # btrfs device usage /mnt/b /dev/loop1, ID: 1 Device size: 2.00GiB Data,single: 8.00MiB Data,RAID5: 1.76GiB Data,RAID5: 10.50MiB Metadata,single: 8.00MiB Metadata,RAID5: 204.75MiB System,single: 4.00MiB System,RAID5: 8.00MiB Unallocated: 0.00B /dev/loop2, ID: 2 Device size: 2.00GiB Data,RAID5: 1.78GiB Data,RAID5: 10.50MiB Metadata,RAID5: 204.75MiB System,RAID5: 8.00MiB Unallocated: 1.00MiB /dev/loop3, ID: 3 Device size: 2.00GiB Data,RAID5: 1.78GiB Data,RAID5: 10.50MiB Metadata,RAID5: 204.75MiB System,RAID5: 8.00MiB Unallocated: 1.00MiB /dev/loop4, ID: 4 Device size: 2.00GiB Data,RAID5: 10.50MiB Data,RAID5: 19.00MiB Unallocated: 1.97GiB -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
