For doing storage system configuration changes (e.g. adding new MDTs and OSTs to the filesystem) it is desirable to allow blocking modifications to the filesystem using the "lctl barrier_freeze" command. However, this also freezes the MGT device, preventing new MDT and OST devices from registering themselves with the MGS.

It would be desirable to have an option like "lctl barrier_freeze --nomgs" (similar to the "nomgs" option for mkfs.lustre and mount.lustre) to freeze all of the storage targets (MDT, OST) but exclude the MGT device. Currently, if barrier_freeze is used the MDT/OST addition fails with the following error on the MGS:

Lustre: MGS: the system is in barrier, refuse the connection from MDT es01a-MDT0007 temporary

and the following on the MDS server:

LustreError: 15f-b: es01a-MDT0007: cannot register this server with the MGS: rc = -16. Is the MGS running?
LustreError: 41629:0:(obd_mount_server.c:2061:server_fill_super()) Unable to start targets: -16
LustreError: 41629:0:(obd_mount_server.c:1641:server_put_super()) no obd es01a-MDT0007
LustreError: 41629:0:(obd_mount_server.c:133:server_deregister_mount()) es01a-MDT0007 not registered
Lustre: server umount es01a-MDT0007 complete

For testing, this could use a modified version of test_46b in patch https://review.whamcloud.com/53300 that adds an "lctl barrier_freeze --nomgs ..." command before the new MDT/OST devices are added to the filesystem, and then un-freezes the filesystem after the OSTs appear on the client (e.g. in "lfs df").