Mgs hold on the mgs_open_llog as resources on llog, and each resource will...

  Mgs hold on the mgs_open_llog as resources on llog, and each resource will be locked by
ldlm_llog.
      Each different version llog for one filesystem/pool will have same mgs_open_llog, and
different version_desc(or llog patch), which descripe llog cmds the client/mds should run to keep up with current llog version.

      Also, mgs_open_llog will contain current updated llog , and the llog patch just contain the
specail llog_hdr_bitmap , which set valid bits to the llog cmds need to run for the client hold the old
llog version.