Re: Replacing disk strange (buggy?) behaviour - RAID1

[Jonah Sabean <me@jse.io>]

Thanks for the reply, it's appreciated!

> Mounting raid1 btrfs writable in degraded mode creates chunks with
> single profile. This is long standing issue. What is rather surprising
> that you apparently have chunk size 819GiB which is suspiciously close
> to 10% of 8TiB. btrfs indeed limits chunk size to 10% of total space,
> but it should not exceed 10GiB. Could it be specific Ubuntu issue?
>
> So when you wrote data in degraded mode it had to allocate new chunk
> with "single" profile.

That's strange as I didn't write actual data to the disk during that
time. Perhaps Ubuntu wrote some hidden file or something to it, I have
no idea, but I didn't interact with the filesystem beyond doing the
replace after mounting it. Still... 10% of 8TiB may be what's
happening and if so that's really strange... and massive. I'd don't
think it was a single chunk equal to 10% though, as when I converted
it to raid1, I specified the soft filter with `-dconvert=raid1,soft`,
and the resulting output once it was complete was:

Done, had to relocate 825 out of 1648 chunks

So the hypothesis that it was one large chunk doesn't make hold up to
me knowing that output. I thus assume the chunks were all 1GiB given
that many. Unfortunately I didn't save the output of the balance with
`-dusage=0`, but I do recall it being in the 800s as well.

> > Why didn't it free
> > those up as it replaced the missing disk and duplicated the data in
> > RAID1?
>
> Device replacement restored mirrored data (chunks with "raid1" profile)
> on the new device. It had no reasons to touch chunks with "single"
> profile because from btrfs point of view these chunks never had any data
> on replaced device so there is nothing to write there.
>
> > Shouldn't it all be RAID1 once it's complete,
> No. btrfs replace restores content of missing device. It is not
> replacement for profile conversion.

Makes sense, knowing that I would expect that. I just have no idea why
it allocated 800GiB, especially since I didn't write anything to the
single disk during the convert process, much less ~800GiB.

> > Everything was RAID1 all up until the disk replacement, so it clearly
> > did this during the `btrfs replace` process.
>
> No, it did it during degraded writable mount.
>
> > Did I do this wrong, or
> > is there a bug?
>
> There is misfeature that btrfs creates "single" chunks during degraded
> mount. Ideally it should create degraded raid1 chunks.

Hmm... would be nice to see this then. Is there a patch for it,
assuming it's planned?

Thanks,
-Jonah