From the btrfs gotchas page:
Files with a lot of random writes can become heavily fragmented (10000+ extents) causing trashing on HDDs and excessive multi-second spikes of CPU load on systems with an SSD or large amount a RAM.
I had similar problems as you describe with Virtualbox. The nodatacow option for btrfs did not help in a noticeable way on my system. I tried the auto-defragment option (mentioned as a possible solution for application databases in desktop environments) as well, also without results that would make the behaviour acceptable.
In the end I shrunk my btrfs partion and the Logical Volume it lives in, I created a new LV and formatted it as ext4, and then put the VM disc images that I have (VirtualBox) on that "partition".
OK, here's what I found happening during the periodic balances:
The following process is started on the host:
btrfs balance start -dsweep lt:/dev/md127:7 /data LANG=en_US.UTF-8 PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin DBUS_SESSION_BUS_ADDRESS=unix:path=/var/netatalk/spotlight.ipc TRACKER_USE_CONFIG_FILES=1 TRACKER_USE_LOG_FILES=1 XDG_DATA_HOME=/apps/.xdg/local/share XDG_CONFIG_HOME=/apps/.xdg/config XDG_CACHE_HOME=/apps/.xdg/cache
where /data is my tiered data-volume, /dev/md127 is the SSD array used as buffer/cache.
This process runs until the data from the SSD tier is moved almost completely to the HDD tier - e.g. somewhere along the way I see:
btrfs fi sh /data
Label: '0a44c6bc:data' uuid: ed150b8f-c986-46d0-ada8-45ee219acbac
Total devices 2 FS bytes used 393.14GiB
devid 1 size 7.12TiB used 359.00GiB path /dev/md126
devid 2 size 114.68GiB used 42.06GiB path /dev/md127
and then it goes down until the usage of the SSD tier goes almost to zero.
The strange thing is that so far i was not able to run this command manually.
I still cannot figure out the 'sweep' balance filter.
This is what the -help shows:
# btrfs balance start --help
usage: btrfs balance start [options] <path>
Balance chunks across the devices
Balance and/or convert (change allocation profile of) chunks that
passed all filters in a comma-separated list of filters for a
particular chunk type. If filter list is not given balance all
chunks of that type. In case none of the -d, -m or -s options is
given balance all chunks in a filesystem. This is potentially
long operation and the user is warned before this start, with
a delay to stop it.
-d[filters] act on data chunks
-m[filters] act on metadata chunks
-s[filters] act on system chunks (only under -f)
-v be verbose
-f force reducing of metadata integrity
--full-balance do not print warning and do not delay start
--background|--bg
run the balance as a background process
but this does not explain how it maps to the "lt:/dev/md127:7" part of the command that runs periodically:
btrfs balance start -dsweep lt:/dev/md127:7 /data
What's the meaning here: Run until the /dev/md127 data usage falls below 7% ?!?
Best Answer
This situation can be caused by file fragmentation. You can try to solve it by going to the terminal and then type:
sudo umount /dev/sdxyand then
sudo btrfs filesystem defrag /dev/sdxyIn these commands sdxy is the correct designation of the partition with the problem. For example it might be something like this: 'sda5'. Make sure you check the correct designation before you try anything.