Re: RAID-1 refuses to balance large drive

[Chris Murphy <lists@colorremedies.com>]

On Wed, Mar 23, 2016 at 1:10 PM, Brad Templeton <bradtem@gmail.com> wrote:
> It is Ubuntu wily, which is 4.2 and btrfs-progs 0.4.  I will upgrade to
> Xenial in April but probably not before, I don't have days to spend on
> this.   Is there a fairly safe ppa to pull 4.4 or 4.5?

I'm not sure.


 In olden days, I
> would patch and build my kernels from source but I just don't have time
> for all the long-term sysadmin burden that creates any more.
>
> Also, I presume if this is a bug, it's in btrfsprogs, though the new one
> presumably needs a newer kernel too.

No you can mix and match progs and kernel versions. You just don't get
new features if you don't have a new kernel.

But the issue is the balance code is all in the kernel. It's activated
by user space tools but it's all actually done by kernel code.



> I am surprised to hear it said that having the mixed sizes is an odd
> case.

Not odd as in wrong, just uncommon compared to other arrangements being tested.

>  That was actually one of the more compelling features of btrfs
> that made me switch from mdadm, lvm and the rest.   I presumed most
> people were the same. You need more space, you go out and buy a new
> drive and of course the new drive is bigger than the old drives you
> bought because they always get bigger.

Of course and I'm not saying it shouldn't work. The central problem
here is we don't even know what the problem really is; we only know
the manifestation of the problem isn't the desired or expected
outcome. And how to find out the cause is different than how to fix
it.



> Under mdadm the bigger drive
> still helped, because it replaced at smaller drive, the one that was
> holding the RAID back, but you didn't get to use all the big drive until
> a year later when you had upgraded them all.  In the meantime you used
> the extra space in other RAIDs.  (For example, a raid-5 plus a raid-1 on
> the 2 bigger drives) Or you used the extra space as non-RAID space, ie.
> space for static stuff that has offline backups.  In fact, most of my
> storage is of that class (photo archives, reciprocal backups of other
> systems) where RAID is not needed.
>
> So the long story is, I think most home users are likely to always have
> different sizes and want their FS to treat it well.

Yes of course. And at the expense of getting a frownie face....

"Btrfs is under heavy development, and is not suitable for
any uses other than benchmarking and review."
https://www.kernel.org/doc/Documentation/filesystems/btrfs.txt

Despite that disclosure, what you're describing is not what I'd expect
and not what I've previously experienced. But I haven't had three
different sized drives, and they weren't particularly full, and I
don't know if you started with three from the outset at mkfs time or
if this is the result of two drives with a third added on later, etc.
So the nature of file systems is actually really complicated and it's
normal for there to be regressions - and maybe this is a regression,
hard to say with available information.



> Since 6TB is a relatively new size, I wonder if that plays a role.  More
> than 4TB of free space to balance into, could that confuse it?

Seems unlikely.


-- 
Chris Murphy