btrfs filesystem show takes a long time

[kdave]

the command btrfs fi show takes too much time:

time btrfs fi show              
Label: none  uuid: 513dc574-e8bc-4336-b181-00d1e9782c1c
Total devices 2 FS bytes used 2.34GiB
devid    1 size 927.79GiB used 4.03GiB path /dev/sdv2
devid    2 size 927.79GiB used 4.03GiB path /dev/sdar2

real    12m59.763s
user    0m0.008s
sys     0m0.044s

time btrfs fi show
Label: none  uuid: 513dc574-e8bc-4336-b181-00d1e9782c1c
Total devices 2 FS bytes used 2.34GiB
devid    1 size 927.79GiB used 4.03GiB path /dev/sdv2
devid    2 size 927.79GiB used 4.03GiB path /dev/sdar2

real    6m22.498s
user    0m0.012s
sys     0m0.024s

time btrfs fi show
Label: none  uuid: 513dc574-e8bc-4336-b181-00d1e9782c1c
Total devices 2 FS bytes used 2.34GiB
devid    1 size 927.79GiB used 4.03GiB path /dev/sdv2
devid    2 size 927.79GiB used 4.03GiB path /dev/sdar2

real    6m19.796s
user    0m0.012s
sys     0m0.024s

Maybe its related to that I've some harddisk in my system:

ls /dev/disk/by-path/ |grep -v part |wc -l
44

This is also a known Debian Bug #891717

-- System Information:
Debian Release: 9.3
  APT prefers stable-updates
  APT policy: (500, 'stable-updates'), (500, 'stable')
Architecture: amd64 (x86_64)

Kernel: Linux 4.9.0-5-amd64 (SMP w/20 CPU cores)
Locale: LANG=en_US.UTF-8, LC_CTYPE=en_US.UTF-8 (charmap=UTF-8), LANGUAGE=en_US:en (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: systemd (via /run/systemd/system)

Versions of packages btrfs-progs depends on:

ii  e2fslibs    1.43.4-2
ii  libblkid1   2.29.2-1
ii  libc6       2.24-11+deb9u1
ii  libcomerr2  1.43.4-2
ii  liblzo2-2   2.08-1.2+b2
ii  libuuid1    2.29.2-1
ii  zlib1g      1:1.2.8.dfsg-5

[1n5aN1aC]

I'd like to add that I also have this 'issue' but in my case, it finds and prints all my btrfs filesystems nearly instantly. Only after that does it sit there waiting for 30+ seconds before finally returning.

btrfs fi show /mnt/data returns nearly instantly.

[colttt]

a new update:

time strace -ir -o btrfs_out_2.log btrfs fi show
Label: none  uuid: 24be286b-ece6-4481-aa48-af255e96e5bd
        Total devices 2 FS bytes used 1.21GiB
        devid    1 size 219.84GiB used 5.03GiB path /dev/sdb2
        devid    2 size 219.84GiB used 5.03GiB path /dev/sde2


real    0m32.085s
user    0m0.248s
sys     0m1.416s

and the strace output:
btrfs_out_2.log

[colttt]

ok an other update, it happens also with kernel 4.19 and btrfs-tools 4.17

time strace -ir -o btrfs_out_4.19_2.log btrfs fi show
Label: none  uuid: 24be286b-ece6-4481-aa48-af255e96e5bd
        Total devices 2 FS bytes used 1.79GiB
        devid    1 size 219.84GiB used 4.03GiB path /dev/sdb2
        devid    2 size 219.84GiB used 4.03GiB path /dev/sde2


real    0m34.065s
user    0m0.356s
sys     0m1.717s

sometimes it works within 3-4s, I can't see a pattern.

lsb_release -a
No LSB modules are available.
Distributor ID: Debian
Description:    Debian GNU/Linux 9.6 (stretch)
Release:        9.6
Codename:       stretch
###
uname -a
Linux tani 4.19.0-0.bpo.1-amd64 #1 SMP Debian 4.19.12-1~bpo9+1 (2018-12-30) x86_64 GNU/Linux
###
ii  btrfs-progs                         4.17-1~bpo9+1+b1               amd64        Checksumming Copy on Write Filesystem utilities

and here the strace output
btrfs_out_4.19_2.log