Before upgrade,the system configuration is one MDS(1.8.7-RHEL5), two OSS(1.8.7-RHEL5 and 2.1.0-RHEL6) and two clients(1.8.7-RHEL5 and 1.8.7-RHEL6), then upgrade servers and clients to 2.1.55-RHEL6, check the system, works fine. Then do the downgrade, MDS downgrade to 2.1.0-RHEL6, OSS downgrade to 1.8.7-RHEL5 and 2.1.0-RHEL6, clients downgrade to 2.1.0-RHEL5 and 2.1.0-RHEL6

Got LBUG when checking the dir/files in the OST pool. MDS console log please see the attached.