On Sat, Feb 25, 2012 at 09:14:01AM +1030, Jordan Windsor wrote:

I'm running Ubuntu under KVM, with btrfs on the host where the
Qemu/KVM image is stored, the VM was also running at the time. I was
going to check something unrelated in the dmesg output, as I did that
I noticed some errors in it about btrfs here they are:


[ 4294.431807] btrfs: block rsv returned -28
[ 4294.431811] ------------[ cut here ]------------
[ 4294.431831] WARNING: at fs/btrfs/extent-tree.c:5985

This is a warning and it shows up from time to time, accross recent
releases.

This didn't cause any crashes or related issues, also unrelated (I
think?) is that I get very low performance (about 5MBps according to
iostat) in a guest under Qemu/KVM.
3.2.7-1-ARCH x86_64 Arch Linux

Yes, the performance goes down as some pathological should-not-happen
code path is taken. I myself haven't seen this error recently during
testing, but at the time I did, it slowed down the machine for a while,
ie. it's not a inifinite loop. Seems there's a dark corner of
space reservations left for Josef.


david