Based on looking at how identically sized, empty, qcow2 files grow when they're added to a Btrfs volume, the 1GiB Btrfs chunk or allocation unit, doesn't have an immediate physical allocation. It's more of a virtual thing, but it has a physical manifestation.
Single profile, 5 disks: As data is copied, one drive has one chunk allocated to it, and data is copied into that chunk and thus into one qcow2 file until the qcow2 file is about 1GiB in size. Then it stops growing and another qcow2 file starts to grow, again up to 1GiB in size. Until all qcow2s are 1GiB. Now when everyone is identical, they actually aren't, chances are one of them has some little bit of extra metadata so the allocator is going to pick the block device with the most free space next, which is how this can affect uneven sized devices. For raid0,5,6 I'm not sure if my interpretation is correct. But what I see is, at the time the chunk is allocated, the block devices with sufficient free space belong to it; and grow in 64KB increments. e.g. 5 qcow2's in a Btrfs data raid0 will grow to ~1GiB in size each as I copy 5GiB of data to the volume. Since I used raid1 metadata in all cases, the qcow2's are a bit uneven in practice. If I then add a 6th qcow2, I don't immediately notice it grow. I *think* it's because the most recent chunk is still only writing to the block devices available at the time that chunk was created; shortly though I start seeing that 6th qcow2 grow. This suggests that this volume has 5 strip (device) chunks; and a 6 strip chunks. *sigh* for what it's worth, Btrfs chunk is not the same thing as mdadm chunk. The mdadm chunk is a strip, since that's what SNIA's dictionary calls it. A stripe is strip x numdevices. So if you have a 5 device raid0, that's 5 strips of 64KB each, or a stripe size of 320KB meaning it takes a file of at least 320KB to write to all 5 disks at the same time. And I side note that in the latest Phoronix raid tests, Btrfs is kicking ass compared to most everything else. Chris Murphy -- 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
