Re: question about creating a raid10

[Stefan K <shadow_7@gmx.net>]

> Btrfs raid10 really should not be called raid10. It sets up the wrong
> user expectation entirely. It's more like raid0+1, except even that is
> deceptive because in theory a legit raid0+1 you can lose multiple
> drives on one side of the mirror (but not both); but with Btrfs raid10
> you really can't lose more than one drive. And therefore it does not
> scale. The probability of downtime increases as drives are added;
> whereas with a real raid10 downtime doesn't change.
WTF?! really, so with btrfs raid10 I can't lose more than one drive? that sucks, that an advantage of raid 10!
and the crazy thing is, thats not documented, not in the manpage nor btrfs wiki and and thats is very important.
thats unbelievable ..


> In your case you're better off with raid0'ing the two drives in each
> enclosure (whether it's a feature of the enclosure or doing it with
> mdadm or LVM). And then using Btrfs raid1 on top of the resulting
> virtual block devices. Or do mdadm/LVM raid10, and format it Btrfs. 
mdadm, lvm.. btrfs is a reason to not use this programms, but since btrfs does not have a 'real' raid10 but a raid01 it does not fit in our use case + I can't configure which disk is in which mirror..

On Wednesday, January 16, 2019 11:15:02 AM CET Chris Murphy wrote:
> On Wed, Jan 16, 2019 at 7:58 AM Stefan K <shadow_7@gmx.net> wrote:
> >
> >  :(
> > that means when one jbod fail its there is no guarantee that it works fine? like in zfs? well that sucks
> > Didn't anyone think to program it that way?
> 
> The mirroring is a function of the block group, not the block device.
> And yes that's part of the intentional design and why it's so
> flexible. A real raid10 isn't as flexible, so to enforce the
> allocation of specific block group stripes to specific block devices
> would add complexity to the allocator while reducing flexibility. It's
> not impossible, it'd just come with caveats like no three device
> raid10 like now; and you'd have to figure out what to do if the user
> adds one new device instead of two at a time, and what if any new
> device isn't the same size as existing devices or if you add two
> devices that aren't the same size. Do you refuse to add such devices?
> What limitations do we run into when rebalancing? It's way more
> complicated.
> 
> Btrfs raid10 really should not be called raid10. It sets up the wrong
> user expectation entirely. It's more like raid0+1, except even that is
> deceptive because in theory a legit raid0+1 you can lose multiple
> drives on one side of the mirror (but not both); but with Btrfs raid10
> you really can't lose more than one drive. And therefore it does not
> scale. The probability of downtime increases as drives are added;
> whereas with a real raid10 downtime doesn't change.
> 
> In your case you're better off with raid0'ing the two drives in each
> enclosure (whether it's a feature of the enclosure or doing it with
> mdadm or LVM). And then using Btrfs raid1 on top of the resulting
> virtual block devices. Or do mdadm/LVM raid10, and format it Btrfs. Or
> yeah, use ZFS.
> 
>