[#LU-8605] Downgrading from 2.8 to 2.5: fsck_namespace_load: fail to load lfsck

After downgrading the system to 2.5 from 2.8 and re-mounting the system got following error while mounting OST
dmesg:

LustreError: 45448:0:(ofd_fs.c:594:ofd_server_data_init()) lustre-OST0000: unsupported read-only filesystem feature(s) 2
LustreError: 45448:0:(obd_config.c:572:class_setup()) setup lustre-OST0000 failed (-22)
LustreError: 45448:0:(obd_config.c:1688:class_config_llog_handler()) MGC10.2.4.47@tcp: cfg command failed: rc = -22
Lustre: cmd=cf003 0:lustre-OST0000 1:dev 2:0 3:f
LustreError: 15b-f: MGC10.2.4.47@tcp: The configuration from log 'lustre-OST0000'failed from the MGS (-22). Make sure this client and the MGS are running compatible versions of Lustre.
LustreError: 15c-8: MGC10.2.4.47@tcp: The configuration from log 'lustre-OST0000' failed (-22). This may be the result of communication errors between this node and the MGS, a bad configuration, or other errors. See the syslog for more information.
LustreError: 45263:0:(obd_mount_server.c:1253:server_start_targets()) failed to start server lustre-OST0000: -22
LustreError: 45263:0:(obd_mount_server.c:1736:server_fill_super()) Unable to start targets: -22
LustreError: 45263:0:(obd_mount_server.c:844:lustre_disconnect_lwp()) lustre-MDT0000-lwp-OST0000: Can't end config log lustre-client.
LustreError: 45263:0:(obd_mount_server.c:1421:server_put_super()) lustre-OST0000: failed to disconnect lwp. (rc=-2)
LustreError: 45263:0:(obd_config.c:619:class_cleanup()) Device 3 not setup
Lustre: server umount lustre-OST0000 complete
LustreError: 45263:0:(obd_mount.c:1319:lustre_fill_super()) Unable to mount /dev/sdb1 (-22)

Steps followed to perform Upgrade/downgrade testing as follows:
1. format and setup the system with 1 MDS(1MDT), 1 OSS(1 OST) and 2 clients with 2.5; create some data
2. shundown the whole system, umount all nodes
3. upgrade the whole system to 2.8; only clear the boot disk, keep data disk untouched
4. remount the whole system, check the data, works fine;
5. shutdown the whole system again, umount all nodes
6. do additional step, remounting the MDS, OSS with abort_recovery option
7. umount the MDS, OSS again
8. downgrade all servers and clients to 2.5 again without touching the data disk
9. mounted MDS. When mounting OSS the above error occurs.

[LU-8605] Downgrading from 2.8 to 2.5: fsck_namespace_load: fail to load lfsck_namespace Created: 13/Sep/16 Updated: 21/Jul/17 Resolved: 13/Sep/16
Status:	Resolved
Project:	Lustre
Component/s:	None
Affects Version/s:	Lustre 2.7.0, Lustre 2.9.0
Fix Version/s:	None

[LU-8605] Downgrading from 2.8 to 2.5: fsck_namespace_load: fail to load lfsck_namespace Created: 13/Sep/16 Updated: 21/Jul/17 Resolved: 13/Sep/16