Second extend of second lvmraid mirror does not sync

[Steve Dodd]

This is a weird corner case. Extending an lvmraid(7) type1 mirror for the second time seems to result in the mirror legs not getting synced, *if* there is another type1 mirror in the vg. This reliably reproduces for me:

# quickly fill two 10G files with random data
openssl enc -aes-256-ctr -pass pass:"$(dd if=/dev/urandom bs=128 count=1 2>/dev/null | base64)" -nosalt < /dev/zero | dd bs=$((1024*1024*1024)) count=10 of=pv1.img iflag=fullblock
openssl enc -aes-256-ctr -pass pass:"$(dd if=/dev/urandom bs=128 count=1 2>/dev/null | base64)" -nosalt < /dev/zero | dd bs=$((1024*1024*1024)) count=10 of=pv2.img iflag=fullblock

# change loop devices if you have loads of snaps in use
losetup /dev/loop10 pv1.img
losetup /dev/loop11 pv2.img
pvcreate /dev/loop10
pvcreate /dev/loop11
vgcreate testvg /dev/loop10 /dev/loop11

lvcreate --type raid1 -L2G -n test testvg
watch lvs -o +raid_sync_action,sync_percent,raid_mismatch_count testvg

# wait for sync

lvcreate --type raid1 -L2G -n test2 testvg
watch lvs -o +raid_sync_action,sync_percent,raid_mismatch_count testvg

# wait for sync

# the following will sync OK, observe kernel message for output from md subsys noting time taken
#
lvextend -L+2G testvg/test2
watch lvs -o +raid_sync_action,sync_percent,raid_mismatch_count testvg

# wait for sync

# the following will FAIL to sync, the sync will seem to complete instantly, e.g:
# Feb 02 15:22:50 asr-host kernel: md: resync of RAID array mdX
# Feb 02 15:22:50 asr-host kernel: md: mdX: resync done.
#
lvextend -L+2G testvg/test2

lvchange --syncaction check testvg/test2
watch lvs -o +raid_sync_action,sync_percent,raid_mismatch_count testvg

# observe error count

This may cause administrator alarm unnecessarily ... :/

For some reason the precise sizes with which the LVs are created, and are then extended by, do appear to matter.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: lvm2 2.02.176-4.1ubuntu3
ProcVersionSignature: Ubuntu 4.15.0-43.46-generic 4.15.18
Uname: Linux 4.15.0-43-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.5
Architecture: amd64
Date: Sat Feb 2 15:33:16 2019
ProcEnviron:
 TERM=screen
 PATH=(custom, no user)
 LANG=en_GB.UTF-8
 SHELL=/bin/bash
SourcePackage: lvm2
UpgradeStatus: No upgrade log present (probably fresh install)
mtime.conffile..etc.lvm.lvm.conf: 2018-07-22T18:30:15.470358

[steved424 (steved424-redhat-bugs)]

Description of problem:

Extending an lvmraid(7) type1 mirror for the second time seems to result in the mirror legs not getting synced, *if* there is another type1 mirror in the vg.

Version-Release number of selected component (if applicable):

2.02.176 (4.1ubuntu3)

How reproducible:

Seems to be reliably reproducible here on Ubuntu 18.04 at least.

Steps to Reproduce:

# quickly fill two 10G files with random data
openssl enc -aes-256-ctr -pass pass:"$(dd if=/dev/urandom bs=128 count=1 2>/dev/null | base64)" -nosalt < /dev/zero | dd bs=$((1024*1024*1024)) count=10 of=pv1.img iflag=fullblock
openssl enc -aes-256-ctr -pass pass:"$(dd if=/dev/urandom bs=128 count=1 2>/dev/null | base64)" -nosalt < /dev/zero | dd bs=$((1024*1024*1024)) count=10 of=pv2.img iflag=fullblock

# change loop devices if you have loads of snaps in use
losetup /dev/loop10 pv1.img
losetup /dev/loop11 pv2.img
pvcreate /dev/loop10
pvcreate /dev/loop11
vgcreate testvg /dev/loop10 /dev/loop11

lvcreate --type raid1 -L2G -n test testvg
watch lvs -o +raid_sync_action,sync_percent,raid_mismatch_count testvg

# wait for sync

lvcreate --type raid1 -L2G -n test2 testvg
watch lvs -o +raid_sync_action,sync_percent,raid_mismatch_count testvg

# wait for sync

# the following will sync OK, observe kernel message for output from md subsys noting time taken
#
lvextend -L+2G testvg/test2
watch lvs -o +raid_sync_action,sync_percent,raid_mismatch_count testvg

# wait for sync

# the following will FAIL to sync, the sync will seem to complete instantly, e.g:
# Feb 02 15:22:50 asr-host kernel: md: resync of RAID array mdX
# Feb 02 15:22:50 asr-host kernel: md: mdX: resync done.
#
lvextend -L+2G testvg/test2

lvchange --syncaction check testvg/test2
watch lvs -o +raid_sync_action,sync_percent,raid_mismatch_count testvg

# observe error count

Actual results:

The sync after the final lvextend completes instantly, and a subsequent lvchange --syncaction check reports a high number for raid_mismatch_count

Expected results:

The sync after the final lvextend should take at least a few seconds, and a subsequent lvchange --syncaction check should not report any errors for raid_mismatch_count (unless the underlying hardware has failed.)

Additional info:

Launchpad bug: https://bugs.launchpad.net/ubuntu/+source/lvm2/+bug/1814389