Confused about scrub output

[btrfs@iooioio.hadar.uberspace.de]

[-- Attachment #1: Type: text/plain, Size: 2812 bytes --]

Hello there,

I'm trying to figure out what the `btrfs scrub` output means. 
Specifically, I get an error summary that reports "no errors found" and 
then a warning, saying "errors detected during scrubbing, corrected". 
Checking dmesg didn't reveal anything obvious to me.

I'm using Marc Merlin's scrub script [1] but get the same behaviour when 
initiating a scrub directly.

Attached you will find the output of the script (`output.txt`) and a 
filtered (`dmesg | grep -i btrfs`) dmesg log (`dmesg.txt`).

My specs:

```
$ uname -a
Linux venus 5.3.0-40-generic #32~18.04.1-Ubuntu SMP Mon Feb 3 14:05:59 
UTC 2020 x86_64 x86_64 x86_64 GNU/Linux

$ btrfs --version # compiled from source
btrfs-progs v5.4.1

$ sudo btrfs fi show
Label: none  uuid: 7b39ba35-352a-4b76-9f33-79a05c7a1e1b
	Total devices 2 FS bytes used 426.41GiB
	devid    1 size 878.99GiB used 436.06GiB path /dev/sda5
	devid    2 size 894.25GiB used 436.06GiB path /dev/sdb

Label: 'storage'  uuid: cb041791-3931-429d-a249-d9fd4f2ba081
	Total devices 1 FS bytes used 728.32GiB
	devid    1 size 831.51GiB used 738.09GiB path /dev/sdc3

Label: 'snow'  uuid: 1ef734b2-5dc0-4349-8b5b-e4ba46e2f165
	Total devices 1 FS bytes used 761.16GiB
	devid    1 size 931.51GiB used 766.02GiB path 
/dev/mapper/luks-726a8c1f-e8f2-4d59-91d5-a0b52b43cb5e

Label: 'akasha'  uuid: 265cdb90-5b04-42a4-a40b-7ac9d2f9b881
	Total devices 1 FS bytes used 1.86TiB
	devid    1 size 2.73TiB used 1.89TiB path 
/dev/mapper/luks-aff1ccef-a303-470c-9eb3-27fdb97b544d

$ sudo btrfs fi df /
Data, RAID1: total=430.00GiB, used=422.39GiB
System, RAID1: total=64.00MiB, used=96.00KiB
Metadata, RAID1: total=6.00GiB, used=4.00GiB
GlobalReserve, single: total=512.00MiB, used=0.00B

$ sudo btrfs fi df /media/josh/storage
Data, single: total=724.01GiB, used=722.89GiB
System, DUP: total=40.00MiB, used=96.00KiB
Metadata, DUP: total=7.00GiB, used=5.43GiB
GlobalReserve, single: total=512.00MiB, used=0.00B

$ sudo btrfs fi df /media/josh/snow
Data, single: total=760.01GiB, used=759.11GiB
System, DUP: total=8.00MiB, used=96.00KiB
Metadata, DUP: total=3.00GiB, used=2.05GiB
GlobalReserve, single: total=512.00MiB, used=0.00B

$ sudo btrfs fi df /media/josh/akasha
Data, single: total=1.83TiB, used=1.83TiB
System, DUP: total=40.00MiB, used=256.00KiB
Metadata, DUP: total=32.00GiB, used=28.27GiB
GlobalReserve, single: total=512.00MiB, used=0.00B
```

I also tried getting help on Reddit [2] before coming here. Feel free to 
check out that thread.

Would appreciate some insight. Thanks in advance!

Josh

P.S.: This is my first time using a mailing list. I apologize if I'm 
doing this wrong and appreciate pointers on etiquette and best practices.

[1] http://marc.merlins.org/linux/scripts/btrfs-scrub
[2] 
https://www.reddit.com/r/btrfs/comments/f089hh/confused_about_scrub_output/

[-- Attachment #2: output.txt --]
[-- Type: text/plain, Size: 2736 bytes --]

<13>Feb 26 00:54:34 root: Quick Metadata and Data Balance of /media/josh/snow (/dev/mapper/luks-726a8c1f-e8f2-4d59-91d5-a0b52b43cb5e)
Done, had to relocate 0 out of 765 chunks
Done, had to relocate 0 out of 765 chunks
Done, had to relocate 0 out of 765 chunks
<13>Feb 26 00:54:39 root: Starting scrub of /media/josh/snow
btrfs scrub start -Bd /media/josh/snow
scrub device /dev/mapper/luks-726a8c1f-e8f2-4d59-91d5-a0b52b43cb5e (id 1) done
Scrub started:    Wed Feb 26 00:54:39 2020
Status:           finished
Duration:         1:52:39
Total to scrub:   766.02GiB
Rate:             115.63MiB/s
Error summary:    no errors found
WARNING: errors detected during scrubbing, corrected
<13>Feb 26 02:47:18 root: Quick Metadata and Data Balance of /media/josh/akasha (/dev/mapper/luks-aff1ccef-a303-470c-9eb3-27fdb97b544d)
Done, had to relocate 0 out of 1910 chunks
Done, had to relocate 0 out of 1910 chunks
Done, had to relocate 0 out of 1910 chunks
<13>Feb 26 02:47:18 root: Starting scrub of /media/josh/akasha
btrfs scrub start -Bd /media/josh/akasha
scrub device /dev/mapper/luks-aff1ccef-a303-470c-9eb3-27fdb97b544d (id 1) done
Scrub started:    Wed Feb 26 02:47:18 2020
Status:           finished
Duration:         3:51:29
Total to scrub:   1.89TiB
Rate:             142.41MiB/s
Error summary:    no errors found
WARNING: errors detected during scrubbing, corrected
<13>Feb 26 06:38:47 root: Quick Metadata and Data Balance of / (/dev/sda5)
Done, had to relocate 0 out of 437 chunks
Done, had to relocate 0 out of 437 chunks
Done, had to relocate 0 out of 437 chunks
<13>Feb 26 06:38:47 root: Starting scrub of /
btrfs scrub start -Bd /
scrub device /dev/sda5 (id 1) done
Scrub started:    Wed Feb 26 06:38:47 2020
Status:           finished
Duration:         0:26:19
Total to scrub:   436.03GiB
Rate:             276.48MiB/s
Error summary:    no errors found
scrub device /dev/sdb (id 2) done
Scrub started:    Wed Feb 26 06:38:47 2020
Status:           finished
Duration:         2:28:14
Total to scrub:   436.03GiB
Rate:             49.08MiB/s
Error summary:    no errors found
WARNING: errors detected during scrubbing, corrected
<13>Feb 26 09:07:01 root: Quick Metadata and Data Balance of /media/josh/storage (/dev/sdc3)
Done, had to relocate 0 out of 734 chunks
Done, had to relocate 0 out of 734 chunks
Done, had to relocate 1 out of 734 chunks
<13>Feb 26 09:07:08 root: Starting scrub of /media/josh/storage
btrfs scrub start -Bd /media/josh/storage
scrub device /dev/sdc3 (id 1) done
Scrub started:    Wed Feb 26 09:07:08 2020
Status:           finished
Duration:         1:20:58
Total to scrub:   738.02GiB
Rate:             154.67MiB/s
Error summary:    no errors found
WARNING: errors detected during scrubbing, corrected

[-- Attachment #3: dmesg.txt --]
[-- Type: text/plain, Size: 4023 bytes --]

[    3.477895] Btrfs loaded, crc32c=crc32c-intel
[    3.661750] BTRFS: device label storage devid 1 transid 8377 /dev/sdc3
[    3.661871] BTRFS: device fsid 7b39ba35-352a-4b76-9f33-79a05c7a1e1b devid 2 transid 544500 /dev/sdb
[    3.661993] BTRFS: device fsid 7b39ba35-352a-4b76-9f33-79a05c7a1e1b devid 1 transid 544500 /dev/sda5
[    3.683397] BTRFS info (device sda5): disk space caching is enabled
[    3.683398] BTRFS info (device sda5): has skinny extents
[    3.690708] BTRFS info (device sda5): bdev /dev/sda5 errs: wr 0, rd 0, flush 0, corrupt 5, gen 0
[    4.017156] BTRFS info (device sda5): enabling ssd optimizations
[    5.439798] BTRFS info (device sda5): disk space caching is enabled
[  217.311048] BTRFS info (device sdc3): disk space caching is enabled
[  217.311049] BTRFS info (device sdc3): has skinny extents
[  218.323778] BTRFS: device label snow devid 1 transid 7841 /dev/dm-0
[  218.632814] BTRFS info (device dm-0): disk space caching is enabled
[  218.632815] BTRFS info (device dm-0): has skinny extents
[  219.356281] BTRFS: device label akasha devid 1 transid 8817 /dev/dm-1
[  219.646052] BTRFS info (device dm-1): disk space caching is enabled
[  219.646053] BTRFS info (device dm-1): has skinny extents
[  219.829096] BTRFS info (device dm-1): bdev /dev/mapper/luks-aff1ccef-a303-470c-9eb3-27fdb97b544d errs: wr 0, rd 0, flush 0, corrupt 2, gen 0
[17580.383674] BTRFS info (device dm-0): balance: start -musage=0 -susage=0
[17580.384514] BTRFS info (device dm-0): balance: ended with status: 0
[17581.284867] BTRFS info (device dm-0): balance: start -dusage=0
[17581.286537] BTRFS info (device dm-0): balance: ended with status: 0
[17581.800822] BTRFS info (device dm-0): balance: start -dusage=20
[17581.802535] BTRFS info (device dm-0): balance: ended with status: 0
[17581.977603] BTRFS info (device dm-0): scrub: started on devid 1
[24340.949981] BTRFS info (device dm-0): scrub: finished on devid 1 with status: 0
[24341.076663] BTRFS info (device dm-1): balance: start -musage=0 -susage=0
[24341.080395] BTRFS info (device dm-1): balance: ended with status: 0
[24341.317826] BTRFS info (device dm-1): balance: start -dusage=0
[24341.322250] BTRFS info (device dm-1): balance: ended with status: 0
[24341.551704] BTRFS info (device dm-1): balance: start -dusage=20
[24341.556121] BTRFS info (device dm-1): balance: ended with status: 0
[24341.684055] BTRFS info (device dm-1): scrub: started on devid 1
[38230.250204] BTRFS info (device dm-1): scrub: finished on devid 1 with status: 0
[38230.283518] BTRFS info (device sda5): balance: start -musage=0 -susage=0
[38230.283991] BTRFS info (device sda5): balance: ended with status: 0
[38230.311891] BTRFS info (device sda5): balance: start -dusage=0
[38230.312697] BTRFS info (device sda5): balance: ended with status: 0
[38230.331585] BTRFS info (device sda5): balance: start -dusage=20
[38230.332082] BTRFS info (device sda5): balance: ended with status: 0
[38230.345384] BTRFS info (device sda5): scrub: started on devid 1
[38230.345843] BTRFS info (device sda5): scrub: started on devid 2
[39809.390357] BTRFS info (device sda5): scrub: finished on devid 1 with status: 0
[47124.992836] BTRFS info (device sda5): scrub: finished on devid 2 with status: 0
[47125.372307] BTRFS info (device sdc3): balance: start -musage=0 -susage=0
[47125.373700] BTRFS info (device sdc3): balance: ended with status: 0
[47125.916380] BTRFS info (device sdc3): balance: start -dusage=0
[47125.917923] BTRFS info (device sdc3): balance: ended with status: 0
[47126.258046] BTRFS info (device sdc3): balance: start -dusage=20
[47126.259025] BTRFS info (device sdc3): relocating block group 789397372928 flags data
[47129.814283] BTRFS info (device sdc3): found 28 extents
[47130.826991] BTRFS info (device sdc3): found 28 extents
[47131.508929] BTRFS info (device sdc3): balance: ended with status: 0
[47131.758743] BTRFS info (device sdc3): scrub: started on devid 1
[51990.354577] BTRFS info (device sdc3): scrub: finished on devid 1 with status: 0