RAID0 extent sizes?

I've seen it mentioned here that generally data extents are 1G and metadata extents are 256M.

Is that per-drive or per-stripe in the case of RAID0?

That is, if I have data mode raid0 across N drives does the system allocate one 1G extent on each drive making the full stripe allocation N-gigs; or does it allocate 1/Nth(gig) on each drive making the total new allocation 1G?

Does the raid0 have any arity constraints (like how raid1 is always arity-2)?

The 1 GiB (or 256 MiB for metadata) is the allocation unit. So for
striped RAID levels (like 0, 10, 5, 6), the FS will allocate as many as it can across all the available devices, and stripe within those.

Now on to your question -- the stripes within the allocation unit
are 64 KiB in size, so the first 64k goes on the first device, the next 64k on the second device, and so on.

The minimum stripe width (e.g. number of devices) is 2 for RAID-0,
4 for RAID-10, 2 for RAID-5 and 3 for RAID-6.

Hugo.

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[So to check my understanding, and just sticking to RAID-0 data only].

So for RAID-0 data on 5 drives with ample space, the expected outcome of allocating more data space is 5GiB, one 1GiB allocated on each drive.

If one drive is too full (say it was smaller) and didn't have 1G of contiguous space available, the allocation would simply fail.

The net effect is to create an association of allocations, one on each available drive that had "enough space", each of which will contribute exactly 1GiB to the association. So every time the data space allocation expands its going to expand by N-GiB total on an N-drive data=raid0 system.

Since data and metadata are separate you can end up being "out of space" for big files but still be able to create files small enough to fit into the metadata with the inode.

Am I correct?

Correct.

No, it would allocate on the remaining 4 devices instead, with a
total of 4 GiB of space. The allocation in these cases is the maximum feasible, not precisely the number of devices.

Yes.

Not necessarily -- if one device is already full (because it's
smaller), then the number of devices will decrease as appropriate, down to the minimum of 2.

Yes, but this isn't related to the number of devices in striped
RAID allocations.

Partially. :)

Hugo.

Hugo Mills             | "How deep will this sub go?"
hugo@... carfax.org.uk | "Oh, she'll go all the way to the bottom if we don't
http://carfax.org.uk/  | stop her."
PGP: 65E74AC0          |                                                  U571

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