Create a new version of paste:

root@archiso ~ # uname -r
4.16.5-1-ARCH
root@archiso /mnt # btrfs --version
btrfs-progs v4.16

# Setup a 3 device RAID1.  This example is all on the same disk, but it makes no difference.

root@archiso ~ # mkfs.btrfs --data raid1 --metadata raid1 /dev/vda{1,2,3} -f
btrfs-progs v4.16
See http://btrfs.wiki.kernel.org for more information.

Label:              (null)
UUID:               b765aaea-c99c-4950-a347-354b28e6cc50
Node size:          16384
Sector size:        4096
Filesystem size:    30.00GiB
Block group profiles:
  Data:             RAID1             1.00GiB
  Metadata:         RAID1             1.00GiB
  System:           RAID1             8.00MiB
SSD detected:       no
Incompat features:  extref, skinny-metadata
Number of devices:  3
Devices:
   ID        SIZE  PATH
    1    10.00GiB  /dev/vda1
    2    10.00GiB  /dev/vda2
    3    10.00GiB  /dev/vda3

# Mount using default (zstd) compression.  Lzo has the same behavior, and I'm assuming zlib does too.

root@archiso ~ # mount -o compress /dev/vda1 /mnt
root@archiso ~ # cd /mnt

# Create 1MB file filled with zeros, and by submitting it for defragmentation with compression, force it to be compressed despite being NOCOW and NODATASUM.

root@archiso /mnt # touch zero
root@archiso /mnt # chattr +C zero
root@archiso /mnt # dd if=/dev/zero of=zero bs=1M count=1
1+0 records in
1+0 records out
1048576 bytes (1.0 MB, 1.0 MiB) copied, 0.00568061 s, 185 MB/s
root@archiso /mnt # sync
root@archiso /mnt # filefrag -v zero
Filesystem type is: 9123683e
File size of zero is 1048576 (256 blocks of 4096 bytes)
 ext:     logical_offset:        physical_offset: length:   expected: flags:
   0:        0..     255:     269568..    269823:    256:             last,eof
zero: 1 extent found
root@archiso /mnt # btrfs fi def -c zero

# Confirm the file is now stored as compressed.  Note filefrag correctly shows ending logical offsets, but only understands a single length field which is the uncompressed length, so it can shows a much later ending physical offset.  It just doesn't understand btrfs compression.  Note the starting physical offsets are consecutive, and the 1MB of zeros has been compressed into 8 extents of 4k each (32k total) proving the file was forced to be compressed.

root@archiso /mnt # filefrag -v zero
Filesystem type is: 9123683e
File size of zero is 1048576 (256 blocks of 4096 bytes)
 ext:     logical_offset:        physical_offset: length:   expected: flags:
   0:        0..      31:     269952..    269983:     32:             encoded
   1:       32..      63:     269953..    269984:     32:     269984: encoded
   2:       64..      95:     269954..    269985:     32:     269985: encoded
   3:       96..     127:     269955..    269986:     32:     269986: encoded
   4:      128..     159:     269956..    269987:     32:     269987: encoded
   5:      160..     191:     269957..    269988:     32:     269988: encoded
   6:      192..     223:     269958..    269989:     32:     269989: encoded
   7:      224..     255:     269959..    269990:     32:     269990: last,encoded,eof
zero: 8 extents found

# Verify RAID is fine, and note the file is mirrored on devices 1 & 2, by the data bytes.

root@archiso /mnt # btrfs scrub start -BdR /mnt
scrub device /dev/vda1 (id 1) done
        scrub started at Thu May 17 08:53:54 2018 and finished after 00:00:00
        data_extents_scrubbed: 24
        tree_extents_scrubbed: 7
        data_bytes_scrubbed: 1572864
        tree_bytes_scrubbed: 114688
        read_errors: 0
        csum_errors: 0
        verify_errors: 0
        no_csum: 384
        csum_discards: 0
        super_errors: 0
        malloc_errors: 0
        uncorrectable_errors: 0
        unverified_errors: 0
        corrected_errors: 0
        last_physical: 2169503744
scrub device /dev/vda2 (id 2) done
        scrub started at Thu May 17 08:53:54 2018 and finished after 00:00:00
        data_extents_scrubbed: 24
        tree_extents_scrubbed: 1
        data_bytes_scrubbed: 1572864
        tree_bytes_scrubbed: 16384
        read_errors: 0
        csum_errors: 0
        verify_errors: 0
        no_csum: 384
        csum_discards: 0
        super_errors: 0
        malloc_errors: 0
        uncorrectable_errors: 0
        unverified_errors: 0
        corrected_errors: 0
        last_physical: 1083179008
scrub device /dev/vda3 (id 3) done
        scrub started at Thu May 17 08:53:54 2018 and finished after 00:00:00
        data_extents_scrubbed: 0
        tree_extents_scrubbed: 8
        data_bytes_scrubbed: 0
        tree_bytes_scrubbed: 131072
        read_errors: 0
        csum_errors: 0
        verify_errors: 0
        no_csum: 0
        csum_discards: 0
        super_errors: 0
        malloc_errors: 0
        uncorrectable_errors: 0
        unverified_errors: 0
        corrected_errors: 0
        last_physical: 1083179008

# Calculate the btrfs logical extent number, for the first file extent, starting at physical offset 269952 of 4k blocks.

root@archiso /mnt # echo $[269952*4096]
1105723392

# Read mirrored copies of the disk data (so, in compressed form) for the first file extent.  Note btrfs-map-logical takes its last argument as a device in the volume, so it can be done offline, not a mountpoint, but that does NOT specify which mirrored copy will be written to the file as "-c" does that.

root@archiso /mnt # btrfs-map-logical -l 1105723392 -b 4096 -o /root/1105723392.begin.copy1 -c 1 /dev/vda1
mirror 1 logical 1105723392 physical 11010048 device /dev/vda2
mirror 2 logical 1105723392 physical 1097334784 device /dev/vda1
root@archiso /mnt # btrfs-map-logical -l 1105723392 -b 4096 -o /root/1105723392.begin.copy2 -c 2 /dev/vda1
mirror 1 logical 1105723392 physical 11010048 device /dev/vda2
mirror 2 logical 1105723392 physical 1097334784 device /dev/vda1

# As expected, the mirrored copies are the same.  Take a brief look at the compressed form.

root@archiso /mnt # diff --brief /root/1105723392.begin.copy*
root@archiso /mnt # xxd /root/1105723392.copy1 | head -n 5
00000000: 785e ecd0 010d 0000 00c2 a0f7 4f6d 0f07  x^..........Om..
00000010: 1128 0c18 3060 c080 0103 060c 1830 60c0  .(..0`.......0`.
00000020: 8081 f781 0100 00ff ffec d081 0c00 0000  ................
00000030: c020 7feb 7b7c 2114 9f01 0306 0c18 3060  . ..{|!.......0`
00000040: c080 0103 060c 1830 f01f 0800 00ff ffec  .......0........
root@archiso /mnt # xxd /root/1105723392.copy2 | head -n 5
00000000: 785e ecd0 010d 0000 00c2 a0f7 4f6d 0f07  x^..........Om..
00000010: 1128 0c18 3060 c080 0103 060c 1830 60c0  .(..0`.......0`.
00000020: 8081 f781 0100 00ff ffec d081 0c00 0000  ................
00000030: c020 7feb 7b7c 2114 9f01 0306 0c18 3060  . ..{|!.......0`
00000040: c080 0103 060c 1830 f01f 0800 00ff ffec  .......0........

# Simulate device 1 failure and replacement.  Shouldn't have data loss as long as we get the replacement in without another drive failing.

root@archiso /mnt # cd
root@archiso ~ # umount /mnt
root@archiso ~ # dd if=/dev/zero of=/dev/vda1 bs=1M
dd: error writing '/dev/vda1': No space left on device
10241+0 records in
10240+0 records out
10737418240 bytes (11 GB, 10 GiB) copied, 22.4585 s, 478 MB/s
dd if=/dev/zero of=/dev/vda1 bs=1M  0.01s user 6.50s system 28% cpu 22.460 total
1 root@archiso ~ # sync                                                                                                                      :(

# Make sure check passes.  Space cache warnings are documented as being able to be ignored, and btrfs easily sidesteps the problem and fixes them.  They are usually indicative of something weird having happened, and we had a complete drive fail.

root@archiso ~ # btrfs check /dev/vda2
Checking filesystem on /dev/vda2
UUID: b765aaea-c99c-4950-a347-354b28e6cc50
checking extents
checking free space cache
failed to load free space cache for block group 30408704
failed to load free space cache for block group 1104150528
checking fs roots
checking csums
checking root refs
found 688128 bytes used, no error found
total csum bytes: 0
total tree bytes: 131072
total fs tree bytes: 32768
total extent tree bytes: 16384
btree space waste bytes: 121695
file data blocks allocated: 557056
 referenced 1572864

# Mount the volume degraded.  Mounting with or without compression makes no difference to how replace will misbehave.

root@archiso ~ # mount -o compress,degraded /dev/vda2 /mnt

# Make sure scrub passes.  (Not that there's any files with checksums.)  Note all data bytes are on device 2, since 1 is missing.

root@archiso ~ # btrfs scrub start -BdR /mnt
WARNING: device 1 not present
scrub device /dev/vda1 (id 1) canceled
        scrub started at Thu May 17 08:56:59 2018 and was aborted after 00:00:00
        data_extents_scrubbed: 0
        tree_extents_scrubbed: 0
        data_bytes_scrubbed: 0
        tree_bytes_scrubbed: 0
        read_errors: 0
        csum_errors: 0
        verify_errors: 0
        no_csum: 0
        csum_discards: 0
        super_errors: 0
        malloc_errors: 0
        uncorrectable_errors: 0
        unverified_errors: 0
        corrected_errors: 0
        last_physical: 0
scrub device /dev/vda2 (id 2) done
        scrub started at Thu May 17 08:56:59 2018 and finished after 00:00:00
        data_extents_scrubbed: 16
        tree_extents_scrubbed: 1
        data_bytes_scrubbed: 557056
        tree_bytes_scrubbed: 16384
        read_errors: 0
        csum_errors: 0
        verify_errors: 0
        no_csum: 136
        csum_discards: 0
        super_errors: 0
        malloc_errors: 0
        uncorrectable_errors: 0
        unverified_errors: 0
        corrected_errors: 0
        last_physical: 1385168896
scrub device /dev/vda3 (id 3) done
        scrub started at Thu May 17 08:56:59 2018 and finished after 00:00:00
        data_extents_scrubbed: 0
        tree_extents_scrubbed: 8
        data_bytes_scrubbed: 0
        tree_bytes_scrubbed: 131072
        read_errors: 0
        csum_errors: 0
        verify_errors: 0
        no_csum: 0
        csum_discards: 0
        super_errors: 0
        malloc_errors: 0
        uncorrectable_errors: 0
        unverified_errors: 0
        corrected_errors: 0
        last_physical: 1385168896

# Check our file's integrity is intact in uncompressed form.

root@archiso ~ # xxd /mnt/zero | head -n 5
00000000: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000010: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000020: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000030: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000040: 0000 0000 0000 0000 0000 0000 0000 0000  ................

# Check our file's integrity is intact in its on disk (compressed) form.  Note since device 1 is missing, copy 1 is now on device 2, where before it was on device 1.

root@archiso ~ # btrfs-map-logical -l 1105723392 -b 4096 -o /root/1105723392.degraded.copy1 -c 1 /dev/vda2
mirror 1 logical 1105723392 physical 11010048 device /dev/vda2
mirror 2 logical 1105723392 physical 1097334784 device (null)
root@archiso ~ # xxd /root/1105723392.degraded.copy1| head -n 5
00000000: 785e ecd0 010d 0000 00c2 a0f7 4f6d 0f07  x^..........Om..
00000010: 1128 0c18 3060 c080 0103 060c 1830 60c0  .(..0`.......0`.
00000020: 8081 f781 0100 00ff ffec d081 0c00 0000  ................
00000030: c020 7feb 7b7c 2114 9f01 0306 0c18 3060  . ..{|!.......0`
00000040: c080 0103 060c 1830 f01f 0800 00ff ffec  .......0........
root@archiso ~ # diff --brief /root/1105723392.{begin,degraded}.copy1

# Check our file's integrity, in that it can still be read and uncompressed.

root@archiso ~ # cat /mnt/zero > /dev/null

# Replace of (missing) device 1 with our new device 1.

root@archiso ~ # btrfs replace start -B 1 /dev/vda1 /mnt
root@archiso ~ # btrfs replace status /mnt
Started on 17.May 08:58:14, finished on 17.May 08:58:14, 0 write errs, 0 uncorr. read errs

# Unmount the volume and make sure btrfs check passes

root@archiso ~ # umount /mnt
root@archiso ~ # btrfs check /dev/vda1
Checking filesystem on /dev/vda1
UUID: b765aaea-c99c-4950-a347-354b28e6cc50
checking extents
checking free space cache
checking fs roots
checking csums
checking root refs
found 753664 bytes used, no error found
total csum bytes: 0
total tree bytes: 131072
total fs tree bytes: 32768
total extent tree bytes: 16384
btree space waste bytes: 121191
file data blocks allocated: 622592
 referenced 1638400

# Mount the volume.

root@archiso ~ # mount -o compress /dev/vda1 /mnt

# Make sure scrub passes.  (Still no files with checksums.)  Note data bytes are now on devices 1 and 2.  Also note no_csum used to be 136 and is now 152, there's an extra data_extent, 2 extra tree_extents, extra data and tree bytes, and last physical is later than before our drive failure.  This surprises me, but I have no idea if it's important or somehow expected.

root@archiso ~ # btrfs scrub start -BdR /mnt
scrub device /dev/vda1 (id 1) done
        scrub started at Thu May 17 09:00:29 2018 and finished after 00:00:00
        data_extents_scrubbed: 17
        tree_extents_scrubbed: 7
        data_bytes_scrubbed: 622592
        tree_bytes_scrubbed: 114688
        read_errors: 0
        csum_errors: 0
        verify_errors: 0
        no_csum: 152
        csum_discards: 0
        super_errors: 0
        malloc_errors: 0
        uncorrectable_errors: 0
        unverified_errors: 0
        corrected_errors: 0
        last_physical: 2169503744
scrub device /dev/vda2 (id 2) done
        scrub started at Thu May 17 09:00:29 2018 and finished after 00:00:00
        data_extents_scrubbed: 17
        tree_extents_scrubbed: 1
        data_bytes_scrubbed: 622592
        tree_bytes_scrubbed: 16384
        read_errors: 0
        csum_errors: 0
        verify_errors: 0
        no_csum: 152
        csum_discards: 0
        super_errors: 0
        malloc_errors: 0
        uncorrectable_errors: 0
        unverified_errors: 0
        corrected_errors: 0
        last_physical: 1418723328
scrub device /dev/vda3 (id 3) done
        scrub started at Thu May 17 09:00:29 2018 and finished after 00:00:00
        data_extents_scrubbed: 0
        tree_extents_scrubbed: 8
        data_bytes_scrubbed: 0
        tree_bytes_scrubbed: 131072
        read_errors: 0
        csum_errors: 0
        verify_errors: 0
        no_csum: 0
        csum_discards: 0
        super_errors: 0
        malloc_errors: 0
        uncorrectable_errors: 0
        unverified_errors: 0
        corrected_errors: 0
        last_physical: 1418723328

# Go straight to checking our file's integrity by checking its on disk (compressed) form.  Note copy 1 is now on device 2, and copy 2 is now on device 1.
# Oh, no!  These shouldn't differ!  We have mirrored copies that are different, and the data that is actually read off the disk will depend on which device btrfs goes to.

root@archiso ~ # btrfs-map-logical -l 1105723392 -b 4096 -o /root/1105723392.replaced.copy1 -c 1 /dev/vda1
mirror 1 logical 1105723392 physical 11010048 device /dev/vda2
mirror 2 logical 1105723392 physical 1097334784 device /dev/vda1
root@archiso ~ # btrfs-map-logical -l 1105723392 -b 4096 -o /root/1105723392.replaced.copy2 -c 2 /dev/vda1
mirror 1 logical 1105723392 physical 11010048 device /dev/vda2
mirror 2 logical 1105723392 physical 1097334784 device /dev/vda1
root@archiso ~ # diff --brief /root/1105723392.replaced.copy*
Files /root/1105723392.replaced.copy1 and /root/1105723392.replaced.copy2 differ

# Copy 1 is proper, which is now the one on device 2.  But, copy 2 on device 1 (the drive we just added as a replacement) contains uncompressed data.  Writing all 1 bits to the file at the beginning instead of 0's, copy 2 shows all 'FF' bytes here.

1 root@archiso ~ # xxd /root/1105723392.replaced.copy1 | head -n 5                                                                           :(
00000000: 785e ecd0 010d 0000 00c2 a0f7 4f6d 0f07  x^..........Om..
00000010: 1128 0c18 3060 c080 0103 060c 1830 60c0  .(..0`.......0`.
00000020: 8081 f781 0100 00ff ffec d081 0c00 0000  ................
00000030: c020 7feb 7b7c 2114 9f01 0306 0c18 3060  . ..{|!.......0`
00000040: c080 0103 060c 1830 f01f 0800 00ff ffec  .......0........
root@archiso ~ # xxd /root/1105723392.replaced.copy2 | head -n 5
00000000: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000010: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000020: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000030: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000040: 0000 0000 0000 0000 0000 0000 0000 0000  ................

# Reading and uncompressing the file has undefined behavior.  If btrfs goes to the disk with compressed data, mirror 1 (device 2), like it did in this example, user has hidden file corruption.

root@archiso ~ # cat /mnt/zero
root@archiso ~ # umount /mnt

# To force it to go to the disk with uncompressed data, let's simulate a failure on device 2, so the only mirror it has left is on device 1.

root@archiso ~ # dd if=/dev/zero of=/dev/vda2 bs=1M
dd: error writing '/dev/vda2': No space left on device
10241+0 records in
10240+0 records out
10737418240 bytes (11 GB, 10 GiB) copied, 21.3278 s, 503 MB/s
1 root@archiso ~ # mount -o compress,degraded /dev/vda1 /mnt                                                                                 :(

# What happens here depends on the already uncompressed data, and how the decompression algorithm handles it.  All zero's like this example seems to always give an Input/output error.  Other data has been seen to cause random kernel memory corruption and likely bring the system down with it without a helpful oops or stack trace pointing to the culprit.

root@archiso ~ # cat /mnt/zero
cat: /mnt/zero: Input/output error