Major HDD performance degradation on btrfs receive

[Nazar Mokrynskyi <nazar@xxxxxxxxxxxxxx>]

I have 2 SSD with BTRFS filesystem (RAID) on them and several 
subvolumes. Each 15 minutes I'm creating read-only snapshot of 
subvolumes /root, /home and /web inside /backup.
After this I'm searching for last common subvolume on /backup_hdd, 
sending difference between latest common snapshot and simply latest 
snapshot to /backup_hdd.
On top of all above there is snapshots rotation, so that /backup 
contains much less snapshots than /backup_hdd.

I'm using this setup for last 7 months or so and this is luckily the 
longest period when I had no problems with BTRFS at all.
However, last 2+ months btrfs receive command loads HDD so much that I 
can't even get list of directories in it.
This happens even if diff between snapshots is really small.
HDD contains 2 filesystems - mentioned BTRFS and ext4 for other files, 
so I can't even play mp3 file from ext4 filesystem while btrfs receive 
is running.
Since I'm running everything each 15 minutes this is a real headache.

My guess is that performance hit might be caused by filesystem 
fragmentation even though there is more than enough empty space. But I'm 
not sure how to properly check this and can't, obviously, run 
defragmentation on read-only subvolumes.

I'll be thankful for anything that might help to identify and resolve 
this issue.

~> uname -a
Linux nazar-pc 4.5.0-rc4-haswell #1 SMP Tue Feb 16 02:09:13 CET 2016 
x86_64 x86_64 x86_64 GNU/Linux

~> btrfs --version
btrfs-progs v4.4

~> sudo btrfs fi show
Label: none  uuid: 5170aca4-061a-4c6c-ab00-bd7fc8ae6030
    Total devices 2 FS bytes used 71.00GiB
    devid    1 size 111.30GiB used 111.30GiB path /dev/sdb2
    devid    2 size 111.30GiB used 111.29GiB path /dev/sdc2

Label: 'Backup'  uuid: 40b8240a-a0a2-4034-ae55-f8558c0343a8
    Total devices 1 FS bytes used 252.54GiB
    devid    1 size 800.00GiB used 266.08GiB path /dev/sda1

~> sudo btrfs fi df /
Data, RAID0: total=214.56GiB, used=69.10GiB
System, RAID1: total=8.00MiB, used=16.00KiB
System, single: total=4.00MiB, used=0.00B
Metadata, RAID1: total=4.00GiB, used=1.87GiB
Metadata, single: total=8.00MiB, used=0.00B
GlobalReserve, single: total=512.00MiB, used=0.00B

~> sudo btrfs fi df /backup_hdd
Data, single: total=245.01GiB, used=243.61GiB
System, DUP: total=32.00MiB, used=48.00KiB
System, single: total=4.00MiB, used=0.00B
Metadata, DUP: total=10.50GiB, used=8.93GiB
Metadata, single: total=8.00MiB, used=0.00B
GlobalReserve, single: total=512.00MiB, used=0.00B

Relevant mount options:
UUID=5170aca4-061a-4c6c-ab00-bd7fc8ae6030    / btrfs        
compress=lzo,noatime,relatime,ssd,subvol=/root    0 1
UUID=5170aca4-061a-4c6c-ab00-bd7fc8ae6030    /home btrfs        
compress=lzo,noatime,relatime,ssd,subvol=/home 0    1
UUID=5170aca4-061a-4c6c-ab00-bd7fc8ae6030    /backup btrfs        
compress=lzo,noatime,relatime,ssd,subvol=/backup 0    1
UUID=5170aca4-061a-4c6c-ab00-bd7fc8ae6030    /web btrfs        
compress=lzo,noatime,relatime,ssd,subvol=/web 0    1
UUID=40b8240a-a0a2-4034-ae55-f8558c0343a8    /backup_hdd btrfs        
compress=lzo,noatime,relatime,noexec 0    1

--
Sincerely, Nazar Mokrynskyi
github.com/nazar-pc
Skype: nazar-pc
Diaspora: nazarpc@xxxxxxxxxxxxxxxxxxxxxxx
Tox: A9D95C9AA5F7A3ED75D83D0292E22ACE84BA40E912185939414475AF28FD2B2A5C8EF5261249

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