On Thu, Dec 1, 2016 at 7:10 AM, Stefan Priebe - Profihost AG
[off-list ref] wrote:

Am 01.12.2016 um 14:51 schrieb Hans van Kranenburg:

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On 12/01/2016 09:12 AM, Andrei Borzenkov wrote:

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On Thu, Dec 1, 2016 at 10:49 AM, Stefan Priebe - Profihost AG
[off-list ref] wrote:
...

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Custom 4.4 kernel with patches up to 4.10. But i already tried 4.9-rc7
which does the same.

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# btrfs filesystem show /ssddisk/
Label: none  uuid: a69d2e90-c2ca-4589-9876-234446868adc
        Total devices 1 FS bytes used 305.67GiB
        devid    1 size 500.00GiB used 500.00GiB path /dev/vdb1

# btrfs filesystem usage /ssddisk/
Overall:
    Device size:                 500.00GiB
    Device allocated:            500.00GiB
    Device unallocated:            1.05MiB

Drive is actually fully allocated so if Btrfs needs to create a new
chunk right now, it can't. However,

Yes but there's lot of free space:
    Free (estimated):            193.46GiB      (min: 193.46GiB)

How does this match?

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All three chunk types have quite a bit of unused space in them, so
it's unclear why there's a no space left error.

I remember discussion that balance always tries to pre-allocate one
chunk in advance, and I believe there was patch to correct it but I am
not sure whether it was merged.

http://www.spinics.net/lists/linux-btrfs/msg56772.html

Thanks - still don't understand why that one is not upstream or why it
was reverted. Looks absolutely reasonable to me.

It is upstream and hasn't been reverted.

https://git.kernel.org/cgit/linux/kernel/git/stable/linux-stable.git/tree/fs/btrfs/volumes.c?id=refs/tags/v4.8.11
line 3650

I would try Duncan's idea of using just one filter and seeing what happens:

'btrfs balance start -dusage=1 <mp>'

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With enospc debug it says:
[39193.425682] BTRFS warning (device vdb1): no space to allocate a new
chunk for block group 839941881856
[39193.426033] BTRFS info (device vdb1): 1 enospc errors during balance

It might be nice if this stated what kind of chunk it's trying to allocate.



-- 
Chris Murphy