Re: ENOSPC while creating snapshot

[Roman Mamedov <rm@romanrm.net>]

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On Fri, 04 Mar 2016 22:10:57 +0100
nexus@smoula.net wrote:

> Hello,
> 
> I'm encountering weird enospc problem while writing to filesystem and 
> creating snapshot at a same time:
> 
> Steps to reproduce:
> 
> start writing data to filesystem:
> 
> # LC_ALL=C dd if=/dev/zero of=/foobar
> dd: writing to '/foobar': No space left on device
> 3003803+0 records in
> 3003802+0 records out
> 1537946624 bytes (1.5 GB, 1.4 GiB) copied, 10.8072 s, 142 MB/s
> 
> in other terminal create read-only (or r/w - results are the same)
> # btrfs sub snap -r / /.snap1
> Create a readonly snapshot of '/' in '//.snap1'
> 
> while creating this snapshot, ENOSPC is produced.
> 
> Interesting is that I can reproduce this on some filesystems but not on 
> others while all of available information are the same. There IS free 
> space in data, metadata and global reserve.
> 
> I've tried balancing and defragmenting with no success. I've tried 
> debugging by using enospc_debug but nothing is written to dmesg or any 
> system log
> 
> Does anybody have a clue what may be the cause?
> 
> # uname -a
> Linux interceptor 4.4.3-1-ARCH #1 SMP PREEMPT Fri Feb 26 15:09:29 CET 
> 2016 x86_64 GNU/Linux

As you use the nodatacow mount option, this seems to be another case of
http://www.spinics.net/lists/linux-btrfs/msg51276.html
http://www.spinics.net/lists/linux-btrfs/msg51819.html

and is fixed by 
https://patchwork.kernel.org/patch/7967161/

Unfortunately the bug is known since the start of the 4.4 series and the patch
is out for 2 months, but it didn't get included into even 4.4.4 released
recently. You have to apply it by yourself and recompile the kernel.

-- 
With respect,
Roman

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